Monday, December 13, 2021

2021 Season Summary

The 2021 Atlantic hurricane season was quite active, with a total of

21 cyclones attaining tropical depression status,
21 cyclones attaining tropical storm status,
7 cyclones attaining hurricane status, and
4 cyclones attaining major hurricane status.

Before the beginning of the season, I predicted that there would be

17 cyclones attaining tropical depression status,
16 cyclones attaining tropical storm status,
8 cyclones attaining hurricane status,
5 cyclones attaining major hurricane status.

The average numbers of tropical storms, hurricanes and major hurricanes (over the 30 year period 1991-2020) were 14.4, 7.2, and 3.2, respectively. This puts the 2021 season a bit above average overall, with a much higher than usual number of named storms but near average numbers of hurricanes and major hurricanes. Likewise, my predictions underestimated the number of named storms, but were close on hurricanes and major hurricanes. The 21 named storms were the third-most recorded for an Atlantic season at the time, behind 2020 and 2005. The Accumulated Cyclone Energy (ACE) value for the 2021 season (which accounts for duration and intensity of storms as well as number) was roughly 145, once again above normal but not by a huge margin. The exact figure will likely shift slightly in post-season analysis.

A bevy of short-lived, weak systems inflated the total number for the season, especially early on. There were three named storms in June for the first time since 1968. Hurricane Elsa set a record for earliest fifth named storm when it was named on July 1. However, the end of the season was surprisingly quiet. With the exception of meandering Tropical Storm Wanda, which spent a week in early November moving around the north Atlantic, there was no activity from October 4 onward. This is quite unusual: on average, one would expect around 3 named storms and at least 1 hurricane during that time. Even stranger, the entire world experienced a lack of strong cyclones during October and November 2021. To explain this behavior, we'll examine a few factors.

At the beginning of the season, an ongoing La Niña event was on the wane, and indeed it officially ended over the summer. However, equatorial Pacific sea temperature anomalies remained negative (consistent with La Niña) and even began to decrease again by late in the year. The relevant regions of the Pacific and their temperature anomalies throughout the year are illustrated above. Overall, the weak La Niña that occurred was consistent with pre-season predictions. It is correlated with lower wind shear in the Atlantic and thus consistent with the somewhat above normal activity observed. Likewise, the tropical north Atlantic had fairly warm ocean temperatures, which helped tropical cyclones to form and intensify. However, this doesn't explain why the Atlantic shut down in October.

2021 featured a particularly strong west African monsoon; note the high precipitation over Africa around 10° N or so between June and August in the map above. This is signficant because tropical waves, the precursors to most Atlantic tropical cyclones, form over Africa and move westward into the Atlantic. Indeed, in contrast to 2020, a larger proportion of storms originated in the tropical Atlantic (see the map of storm tracks below):

Every year, the opportunity for cyclone formation off of Africa closes in early October, when the wave axis shifts toward the equator and east Atlantic temperatures start to get cooler. The center of activity usually then shifts to the Caribbean, but the Caribbean saw an unusual amount of dry sinking air late in the year, stifling any formation. This is again in stark contrast to 2020, when the west Caribbean saw its most active period in recorded history from October through November. These factors help to indicate why the 2021 season ended earlier than expected.

In my predictions, I put the Caribbean islands and the Gulf of Mexico at highest risk, with the U.S. east coast and central America at lower risk. These predictions were fairly good: Hurricane Elsa, Tropical Storm Fred, Hurricane Grace, and Hurricane Ida impacted the Caribbean Islands, while Ida and Hurricane Nicholas had the greatest Gulf impacts (though Grace also hit the Yucatan and the southwest Gulf). The Caribbean impacts might have been greater still had some of the above storms not tracked directly over the greater Antilles. No landfalls occurred in central America and only the extremely unusual Henri had any significant impact on the U.S. east coast as a tropical cyclone.

Some other notable facts or records from 2021 include:
  • Tropical Storm Ana's formation on May 22 made 2021 the seventh consecutive season with a named storm before June 1
  • Elsa was the easternmost forming early July hurricane in the tropics since 1933; it also had the highest forward speed of any known Atlantic hurricane in the tropics
  • Grace's remnants reformed as a tropical storm in the eastern Pacific and were named Marty
  • Henri was the slowest-moving cyclone ever recorded in the U.S. northeast; this led to unusual flooding impacts in the region
  • Ida tied 2020's Hurricane Laura for highest winds of a landfalling Louisiana hurricane (150 mph)
Finally, Hurricane Sam was the season's strongest storm, with winds of 155 mph and a central pressure of 929 mb. It was a long-lived and powerful storm, clocking in at over 50 ACE just on its own and holding major hurricane status for 7.75 consecutive days. Despite this, it went out to sea and didn't significantly impact any landmasses.

The 2021 season was quite active, though with many weak systems. The vast majority of the season's damage and loss of life were due to Hurricane Ida, which made landfall in Louisiana as a category 4 hurricane.


Tuesday, November 2, 2021

Tropical Storm Wanda (2021)

Storm Active: October 30-November 7

On October 25, an early-season nor'easter formed off the United States east coast. The low deepened rapidly over the next day and brought hurricane-force wind gusts to coastal Massachusetts, but it was a non-tropical system. The system meandered near the coast for another day before slowly retreating eastward over the open Atlantic. It remained an intense low for the next several days, but was still attached to a frontal boundary. The low moved turned southeastward on the 27th and pursued that heading for a few days until it encountered warmer waters. Late on the 30th, the cyclone was classified as Subtropical Storm Wanda; it had more central convection and a tighter windfield, but was still interacting with nearby fronts.

After formation, Wanda slowed down and meandered across the open central Atlantic. Meanwhile, the cyclone was separating further from the nearby upper-level low. It transitioned into a fully tropical storm on November 1.

Wanda turned northward and made some steady progress in that direction over the next few days, following a trough to its north. The convective structure remained somewhat subtropical in appearance, with curved bands north and west of the center, but little in the way of deep central convection. However, on November 4, a ridge built to the north of Wanda and turned it back southeastward, in the general direction of the Azores.

By November 6, Wanda began to feel the pull of a powerful cyclone over the north Atlantic. It turned back to the northeast and began to accelerate, passing northwest of the Azores. The cyclone encountered high shear and colder waters and transitioned to an extratropical cyclone on the 7th.

The above image shows Wanda on November 3 over the open Atlantic.

Wanda took a meandering track over the north Atlantic, beginning as an extratropical low near the Eastern seaboard and ending past the Azores.

Wednesday, September 29, 2021

Tropical Storm Victor (2021)

Storm Active: September 29-October 4

On September 27, a tropical wave moved off of Africa into the eastern Atlantic. It was located on the eastern edge of a monsoonal trough of low pressure, while another tropical wave was situated on the western end. For a few days, these disturbances battled it out, but the eastern one ultimately became dominant and organized into Tropical Depression Twenty on September 29. It was located in quite favorable conditions, but its large size kept strengthening gradual. The depression became Tropical Storm Victor later that day. It was the second earliest formation of a twentieth named storm on record, behind only Vicky of the previous year, which formed on September 14. Victor was named at an unusually low latitude for the Atlantic at 8.3° N.

Despite forming close to the equator, the storm gained latitude steadily as its trajectory bent from west-northwest to northwest due to the influence of an upper-level low in the subtropics eating away at the ridge that usually steers such storms west. Meanwhile, Victor managed to consolidate a little and strengthen into a high-end tropical storm with maximum winds of 65 mph and a central pressure of 997 mb. However, on October 1 it moved into the sphere of influence of the aforementioned upper-level low, which began to impart strong wind shear. At the same time, Victor reached a drier airmass. The storm steadily weakened from there on out.

The storm became a tropical depression on October 2, but intermittent convective bursts allowed it to remain a tropical cyclone for a while after that. Wind shear degraded the circulation gradually and Victor at last degenerated into a trough of low pressure on October 4.

The above image shows Victor at peak intensity, when it was already dealing with increasing shear and dry air intrusion.

During most of September, the steering pattern over the tropical Atlantic allowed tropical cyclones to escape northward, out to sea. Victor followed this pattern, avoiding land areas.

Saturday, September 25, 2021

Subtropical Storm Teresa (2021)

Storm Active: September 24-25

On September 23, a surface trough formed in conjunction with an upper-level low that was producing a huge area of scattered thunderstorm activity in an arc east and south of Bermuda. The system consolidated some over the following day as it moved generally northward and a well-defined surface center appeared north of Bermuda. The island experienced only minor impacts as the disturbance passed near it. There was some deep convection too, though it was generally located north and east of the center of circulaton. The storm was non-frontal. However, it was still located under an upper-level low and furthermore did not look much like a conventional tropical storm: thus, it was classified Subtropical Storm Teresa. Teresa was the second earliest nineteenth named storm (typically, "T" storm) on record, after only Teddy of 2020.

The storm's genesis was not well-anticipated, and it only had a small window for intensification, as a powerful extratropical low was forming near New England. Teresa was heading northwest initially, and then turned north and then northeast as it became caught up in the low. Increasing shear overwhelmed the storm and it became post-tropical on September 25. The low lost its identity soon after.

Teresa's satellite presentation looks distinctly subtropical, with little convection near the center and an asymmetric appearance.

Teresa added to the list of short-lived cyclones of the 2021 season.

Wednesday, September 22, 2021

Hurricane Sam (2021)

Storm Active: September 22-October 5

A tropical wave exited Africa into the Atlantic on September 19. Finding favorable conditions, it began to organize, and a spin was soon evident on satellite imagery. It stayed pretty far south, missed Cabo Verde, and moved west over the next few days. By the 21st, a low had formed, but the disturbance didn't have the closed circulation center necessary to be called a tropical cyclone. One more day was enough for the system to become Tropical Depression Eighteen.

It took another day or so for the new tropical cyclone to start intensifying, but after that it was off to the races. On September 23, it strengthened into Tropical Storm Sam, which was the second earliest an eighteenth named storm had ever formed on record in the Atlantic (Sally of 2020 was the record-holder). Sam had a small core, which allowed it to quickly vault to hurricane strength on the 24th. There was little wind shear in the system's vicinity, and waters were plenty warm to support development; the moderately dry mid-level air around the storm was unable to penetrate it and slow down the intensification process. By the afternoon of September 25, Sam reached category 4 intensity.

After having moved west for a few days, Sam had slowed down and turned west-northwest under the influence of a weak subtropical ridge. Indeed, the ridge was so weak that Sam's forward motion was unusually slow for a hurricane in its location. The storm was still small enough that cold water upwelling didn't present a significant issue even with the lower forward speed. The cyclone had a small, symmetric eye surrounded by extremely cold cloud tops and managed to reach a peak intensity of 155 mph winds and a central pressure of 929 mb during the afternoon of September 26. Right afterward, the eye contracted and dry air forced its way into the center from the east during an eyewall replacement, causing sudden rapid weakening. This brought Sam down to category 3 by the 27th.

As the storm continued northwest, it fought back, restrengthening to category 4 the next day. Its structure was quite different, with a larger, less symmetric eye, and a larger radius of maximum winds. On September 29, it made its closest approach to the Lesser Antilles, passing well to the northeast. The next day, Sam begin to speed up and turn north as it rounded the subtropical ridge. Remarkably, the storm continued intensifying, reaching its secondary peak intensity of 150 mph winds and a central pressure of 934 mb early on October 1.

Overnight, Sam made its closest approach to Bermuda to the south-southeast; the island was far enough away to be spared significant impacts. Gradual weakening commenced as the storm recurved, but its tropical storm force windfield continued to expand. Early on October 3, Sam finally lost major hurricane strength after being category 3 or above for 7.75 consecutive days, joining only a small handful of Atlantic hurricanes to achieve this feat. Later that day, as it moved northeast toward the north wall of the Gulf stream, the eye cleared out again and became symmetric, with well-defined concentric eyewalls. This resulted in some restrengthening that evening near 40° N! The storm's structure was exceptional for that latitude. At last, on October 4, Sam encountered much colder water and began extratropical transition. As the inner core decayed, Sam weakened to a category 1. The storm became a hurricane-force extratropical low early on October 5 and merged with another low over the far north Atlantic soon after.

The above image shows Sam at its peak intensity on September 26.

Sam's long track as a major hurricane made it one of the largest Accumulated Cyclone Energy (ACE) producers recorded. Despite its longevity, Sam didn't affect any landmasses except Bermuda, to which it only delivered a glancing blow.

Monday, September 20, 2021

Tropical Storm Rose (2021)

Storm Active: September 19-23

The assembly line of tropical cyclones continued as another tropical wave began to organize in the east Atlantic around September 18. Before long, it was named Tropical Storm Rose southwest of Cabo Verde. Only 2005 and 2020 had an "R" storm (or seventeenth named storm) form earlier in the season than Rose did. At the time of its formation, the high pressure ridge usually located between Bermuda and the Azores had retreated quite far east, leaving Rose an opportunity to move north-northwest. The storm had a day or two in favorable atmospheric conditions and over fairly warm water, and it strengthened modestly.

Just north of the tropics, however, Rose found drier air, higher shear, and cooler water beginning September 21. These snuffed out the storm's thunderstorm activity, leaving it a naked swirl by the 22nd. Rose weakened to a tropical depression that day. Soon after, it degenerated into a remnant low.

The above image shows Rose over the open east Atlantic.

Due to a weakness in the subtropical ridge, Rose turned north just west of Cabo Verde and didn't spend much time at a tropical cyclone.

Saturday, September 18, 2021

Tropical Storm Peter (2021)

Storm Active: September 18-22

A tropical wave entered the Atlantic in mid-September and encountered some favorable conditions for development; already by September 15, after about a day over water, there was a well-defined center of circulation. However, thunderstorm activity was limited over the next few days as it moved west-northwest. Convection increased some as the system moved nearer to the Lesser Antillies on the 17th, but it took until the 18th for a true closed surface low to appear. At that time, it was designated Tropical Depression Sixteen. Soon after that, it strengthened into Tropical Storm Peter.

Peter was approaching a major threat to its survival: a powerful upper-atmospheric trough was bringing high wind shear to much of the western Atlantic, and conditions became hostile for the new tropical storm as it continued west-northwestward. It managed to strengthen a little more the shear out of the west stripped the center of thunderstorm coverage on September 19. Flare-ups continued over the next day though, when the center passed just north of the Leeward Islands, too far away to cause much in the way of rain.

The system followed a typical curving path toward the north after that. Relentless shear brought about Peter's steady decline starting on September 20. The next day, it weakened to a tropical depression, and the following day, a remnant low. The remnants showed some signs of organization a few days later and were briefly monitored for redevelopment south of Bermuda. However, ex-Peter did not manage to regenerate into a tropical cyclone. Soon, it was absorbed by a larger low to the north.

The above image shows the sheared Tropical Storm Peter shortly after being named.

After crossing the tropical Atlantic as a tropical wave, Peter spent only a few days as a tropical cyclone because of hostile upper-level winds.

Tropical Storm Odette (2021)

Storm Active: September 17-18

Around mid-September, there was an area of disturbed weather just northeast of the Bahamas associated with an upper-level trough. Gradually the vorticity worked down to surface level as the system moved generally north-northwestward over the following days. Shear was fairly high in the area, but the low managed to develop enough convection to be named Tropical Storm Odette off the U.S. mid-Atlantic coast on September 17.

Though it formed pretty close to land, it was already moving out to the northeast. Moreover, it was already beginning extratropical transition! The thunderstorm activity was well east of the center of circulation, and this "center" was in fact multiple low-level swirls orbiting one another. The poorly-organized cyclone degraded further as shear increased, but nevertheless strengthened due to interaction with an approaching front. Just a day after being named, Odette became post-tropical. The low moved out over the open north Atlantic and stalled southeast of Atlantic Canada for a few days. Though it was monitored for redevelopment, ex-Odette did not regenerate into a tropical cyclone. Within a few more days, it was swept away by the next trough entering the Atlantic.

The above image shows Odette shortly after formation. At least three vortices are visible, with the deep convection displaced east of the center.

Odette was only tropical for a day (see circular dots), but the post-tropical low spent several days meandering the north Atlantic after becoming post-tropical (triangular points).

Sunday, September 12, 2021

Hurricane Nicholas (2021)

Storm Active: September 12-16

During early September, another tropical wave crossed the Atlantic, this one passing through the tropics without incident until it reached the southwestern Caribbean Sea. While the wave moved over central America on September 9, the northern portion pursued a more northwestward course toward the Bay of Campeche. There it merged with a trough, producing a large area of disturbed weather. Southwesterly wind shear made development slow, but the system managed to organize enough to be designated Tropical Storm Nicholas during the morning of September 12.

Nicholas initially moved northward, but turned a little toward the right on the 13th. Though that was the storm's general trajectory, the evolution of the interior was complex. The cyclone reformed to the north and east a few distinct times that day, and at points there were multiple vortices evident on radar, rotating one another cylonically. Despite the lack of a persistent center, Nicholas deepened significantly over the warm waters of the western Gulf of Mexico and reached hurricane strength very close to the coast of Texas that night (attaining a minimum pressure of 988 mb).

Just before landfall, Nicholas turned northeast and slowed down, postponing when it crossed the coastline into the early morning hours of September 14. At the time, it had its greatest peak winds of 75 mph. The storm quickly weakened inland as it moved east-northeast. Nevertheless, it was a prolific rain producer, with storm totals exceeding a foot of rain in parts of Texas and Louisiana. That night, it weakened to a tropical depression. Nicholas lingered near the Louisiana coast for several more days, decaying gradually, but still adding to its rain totals in the south. It became post-tropical on September 16, and finally dissipated on the 17th.

The above infrared satellite imagery shows Hurricane Nicholas just before landfall in Texas very early on September 14.

Nicholas's slow forward motion over the Gulf coast elevated rainfall amounts in Louisiana.

Saturday, September 11, 2021

Tropical Storm Mindy (2021)

Storm Active: September 8-9

A broad low pressure system formed in the extreme southwestern Caribbean near the beginning of September. Due to land interaction and strong upper-level winds, it was disorganized at first as it drifted over Nicaragua and then farther northwest over the following several days. It didn't have the chance to do much more than produce scattered downpours until it cleared the Yucatan Peninsula on September 5. Even then, atmospheric conditions weren't all that favorable as it slowly turned north and then northeast over the Gulf of Mexico. When the disturbance finally did spin up, it was in a hurry. On September 8, a closed circulation suddenly developed off the coast of the Florida panhandle. The system was named Tropical Storm Mindy.

In the few hours it had over water, Mindy developed an impressive core on radar. Winds were estimated to reach 45 mph when the cyclone made landfall that night. The storm weakened to a tropical depression over southern Georgia the next morning and turned east-northeast. Though it emerged back over water by the afternoon of the 9th, conditions were not favorable for restrengthening. This was due to a nearby front and the outflow of Hurricane Larry. Mindy became post-tropical that night and dissipated entirely soon after.

The above image shows Tropical Storm Mindy just after formation and just before landfall in Florida.

Mindy was another short-lived storm which strengthened right before landfall in the U.S.

Wednesday, September 1, 2021

Hurricane Larry (2021)

Storm Active: August 31-September 11

An intense tropical wave moved westward across Africa near the end of August, splashing down off the coast of Guinea on the 30th of that month. Unlike some of its predecessors, this system organized almost immediately and was desginated Tropical Depression Twelve southeast of Cabo Verde during the evening of August 31. The storm was large, but strengthened remarkably quickly. Curved convective bands soon surrounded the center and what appeared to be a dry slot in fact was incorporated into the circulation as a proto-eye. The depression became Tropical Storm Larry and didn't stop there. Rapid intensification brought it to the verge of hurricane strength less than a day later. Early on September 2, it was upgraded to a hurricane.

After that point, it took some time for Larry's internal structure to catch up with its impressive outer bands. Over the next day, an eyewall tried to form multiple times, though none of the attempts were quite closed all the way around. Furthermore, the storm had multiple concentric eyewalls at times, complicating the strengthening process. Gradual strengthening continued nevertheless and Larry became a category 2 on September 3. A final burst brought the storm to major hurricane status overnight. From there, it remained a category 3 for several days. uring this stretch, Larry also reached its peak intensity of 125 mph winds and a minimum central pressure of 955 mb. Structural changes continued, however: around September 6, Larry became an annular hurricane. This type of hurricane has a large symmetric eye and little in the way of banding features. They are also more resilient to changes in the surrounding environment.

Indeed, the enviroment around Larry did change some. It slowed down and turned northwest during the same period, and the slower motion over led to more cold water upwelling beneath the storm as it gained latitude. Waters in the area of the subtropical Atlantic it traversed were just as warm or warmer as those it had encountered before, but the high ocean temperatures did not extend as deep, leaving Larry susceptible to churning cold water up under itself. This didn't slow it down too much though, perhaps again due to the annular structure. The storm weakened back to a category 2 only on September 8th as some dry air disrupted the center, but it was larger than ever and was producing rough surf all along the Atlantic coast of north America.

The storm turned the corner and made its closest approach to Bermuda on September 9 as a strong category 1, around 170 miles east-northeast of the island. Despite the center being far away, Bermuda still experienced some gale conditions. From there, the storm turned north-northeast and accelerated as hurricanes usually do in the mid-latitudes. On the 10th, it crossed north of the Gulf stream and encountered much colder ocean temperatures. Despite that, it only weakened a little and maintained an impressive inner core as it slammed into Newfoundland that night. By then, its forward speed was over 45 mph, so hurricane conditions arrived and departed in only a few hours. Remarkably, Larry was still tropical as it entered the Labrador sea, and remained a hurricane north of 50° N. It finally became extratropical on the 11th at a latitude of 54° N.

Soon after, it merged with another extratropical low. Larry's journey northward, along with its interaction with a mid-latitude trough, had the effect of transporting a huge amount of warm air and moisture poleward. As a result, the last impacts of ex-Larry were pretty unusual: on September 12, the combined low moved off the coast of Greenland, causing a huge snowstorm that dumped several feet of snow in areas of southeastern Greenland.

The above image shows major hurricane Larry over the open Atlantic. The lack of outer banding features and large eye are typical features of an annular hurricane.

Larry was the first tropical cyclone to strike Newfoundland as a hurricane since Igor in 2010.

Sunday, August 29, 2021

Tropical Storm Julian (2021)

Storm Active: August 28-29

Another tropical wave was crossing the Atlantic in mid-August when a broad area of low pressure formed along it. Atmospheric conditions didn't support development, however, and the disturbance tracked slowly northwest for several days. The Northern Atlantic Oscillation (NAO) was in negative phase, meaning that the Bermuda-Azores high pressure system was weaker than usual. This gave the system a window to gain latitude before reaching the Caribbean islands. Because it did so, it encountered cooler waters for a while, and organization was again put on hold. Around August 27, the system was at the latitude of Bermuda (though well to the east), and turned eastward. Finally, late on August 28, after tracking across the Atlantic for more than ten days, the disturbance was classified Tropical Depression Eleven.

By that time, the cyclone was already accelerating northeastward in the mid-latitude flow, pushed onward by an approaching cold front. Interaction with this front also drove some intensification and the system became Tropical Storm Julian on August 29. Though it was the eleventh cyclone of the season, it took the "J" name because it reached tropical storm strength while Tropical Depression Ten was still below this threshold. The cyclone reached an intensity of 60 mph winds and a pressure of 995 mb that evening just before transitioning to an extratropical low. It continued across the north Atlantic for a few more days before being absorbed.

Julian had the typical appearance of a cyclone moving northeastward in the subtropics, with an asymmetric, "comma-like" satellite presentation.

A vast majority of the track above shows the system that became Julian before and after its short stint as a tropical cyclone (the four circular dots in the middle).

Tropical Storm Kate (2021)

Storm Active: August 27-September 1

On August 22, an impressive tropical wave entered the Atlantic. It tracked westward at a fairly low latitude and slowly organized. A few days later, it was starting to contend with higher wind shear, making conditions less favorable for tropical cyclone development. Nevertheless, a well-defined low pressure center appeared by early on August 27. Later that day, convection was organized enough to designate the system Tropical Depression Ten several hundred miles east of the Lesser Antilles.

A weakness in the subtropical ridge, expanded by soon-to-be Tropical Storm Julian, drew the new depression generally poleward, and the center was intermittently exposed on the western edge of the convection by northwesterly shear. The shear gradually increased over the next few days as Ten followed its meandering northward path. The storm managed to intensify just enough to be named Tropical Storm Kate on August 30 (though it was the tenth tropical depression of the season, the eleventh snagged the name Julian one day sooner).

Even though wind shear declined from that point, water temperatures were not especially warm and Kate also was encountering a drier air mass. The upper-level winds out of the west were replaced by ones out of the north, bringing even drier air over the center of circulation. It's also the case that a storm moving north is affected more adversely by upper-level winds opposite to the direction of motion (out of the north) than toward the direction of motion (out of the south). This combination led to Kate weakening back to a tropical depression, and then opening up into a trough entirely on September 1.

Kate was a sheared tropical cyclone all its life, as the above photo illustrates. Periodic bursts of convection were immediately displaced southeast of the center.

The beginning of Kate's track shows it as a tropical wave across the Atlantic (triangular points). Once it became a tropical cyclone, the system turned sharply north into a weakness in the subtropical ridge.

Friday, August 27, 2021

Hurricane Ida (2021)

Storm Active: August 26-September 1

Around August 23, a tropical wave that had entered the Atlantic basin back in mid-August began to generate some concentrated thunderstorm activity as it crossed into the Caribbean Sea. Most of the vorticity associated with the system was quite far south, though, and still embedded in the intertropical convergence zone. On the 25th, a broad low formed along the wave axis. In part due to the influence of an upper-level trough over the northwest Caribbean, the system consolidated farther north, culminating in Tropical Depression Nine the next day. At that point, it was located just southwest of Jamiaca and was lifting to the north-northwest.

Initially, most of the rain was displaced north and east of the low-level circulation, causing considerable flooding in Jamaica. The storm was over very warm water and in an environment of high relative humidity, though, so it wasted no time in organizing. By that afternoon, it was upgraded to Tropical Storm Ida. Overnight, vigorous convection began a trend of rapid intensification, which continued through August 27. By the time its center was baring down on western Cuba that afternoon, Ida was already a hurricane. The system first crossed over Isla de la Juventud, before making landfall in mainland Cuba that evening as a category 1. Land interaction barely slowed down the storm, and before long it had continued northwest into the Gulf of Mexico.

Conditions in the Gulf were nearly ideal: wind shear died down even more, the atmosphere was moist, and Ida's path took it directly over a warm eddy that had some of the highest oceanic heat content in the Atlantic basin. Intensification was at first slow and steady as the cyclone cleared out an eye on the 28th and became a category 2 hurricane. During the final day of its approach to the Gulf coast, however, Ida exploded. The minimum central pressure was observed to drop from 985 mb to 929 mb in just 24 hours, bringing the hurricane to category 4 strength with top sustained winds of 150 mph. The storm had also grown significantly and carried an enormous storm surge into southeast Louisiana upon landfall during the early afternoon of August 29. These top winds matched those of Laura a year earlier, tying the record for the highest recorded in a Louisiana landfall.

The storm brought hurricane-force winds well inland as it turned north and weakened. Early on August 30, it was downgraded to a tropical storm and crossed into Mississippi. It became a tropical depression and turned northeastward that evening. Even after bringing 10+ inches of rain to a large swath of Louisiana and Mississippi, Ida caused more tremendous rainfall on its journey toward the Atlantic. An interaction between it and an approaching cold front led to the cyclone's post-tropical transition on September 1, but more importantly led to enhanced precipitation over parts of the mid-Atlantic and widespread flooding. The next day, Ida's remnants moved over Atlantic Canada before dissipating.

The above image shows Ida at peak intensity as a strong category 4 hurricane just before landfall in Louisiana.

Ida's track took it directly across areas of high oceanic heat content in the northern Gulf of Mexico; this contributed to the rapid intensification just before landfall.

Sunday, August 15, 2021

Hurricane Henri (2021)

Storm Active: August 15-23

On August 14, a non-tropical low developed north-northeast of Bermuda in the subtropical Atlantic. It developed surprisingly quickly and had an impressive, if small, satellite signature by the next afternoon. The low was navigating around a high pressure ridge to its west, which was located over the U.S. east coast. This clockwise flow unusually pushed the system south, bringing it just east of Bermuda late on August 15. At that time, it was designated Tropical Depression Eight. Despite its proximity to the island, Eight was so small that Bermuda experience little more than showers.

Anomalously warm waters allowed the system to intensify and become Tropical Storm Henri by the afternoon of August 16. The storm continued its loop around Bermuda and turned west-southwest on the 17th. Curved bands near the center became better defined throughout that day and the storm steadily strengthened. Hints of an eye appeared on satellite imagery on August 18, bringing Henri to the verge of hurricane strength. However, wind shear out of the north increased that evening, squashing the cyclone's cloud shield and halting intensification. The storm proved resilient since it had access to warm ocean temperatures and ample moist air, and only weakened slightly over the next day in the face of 25 knots of shear.

Meanwhile, it continued westward along the 30°N parallel. As the shear abated during the afternoon of August 19, Henri's structure began to improve again. The next morning, it began a long-anticipated turn toward the north as the ridge steering it collapsed and was replaced by a trough along the eastern seaboard. The next morning, Henri strengthened into a hurricane as it accelerated north-northeastward. An eye even opened up briefly on August 21, though it was a bit ragged, and most deep convection was south and east of the center.

Normally, a cyclone in Henri's position would have recurved out to sea, but the setup in this case was quite different. A ridge was building to the system's east, while simultaneously an elongated upper-level low over the mid-Atlantic exerted a pull toward the west. This allowed Henri to defy climatology and continue northward toward land. While located over the Gulf stream, the storm deepened a little more, reaching its peak intensity of 75 mph winds and a minimum pressure of 986 mb early on August 22. The storm began to slow down upon nearing the coast, yet another unusual feature of its track: tropical cyclones that do manage to hit the northeast are typically accelerated to forward speeds of over 30 mph by mid-latitude jet streams. Henri, in contrast, was hemmed in by the ridge and continued to swirl around the upper-level low, turning left around landfall.

Colder waters weakened the storm down to a strong tropical storm before the center made landfall along the coast of Rhode Island that afternoon. Along with some minor storm surge, Henri was primarily a rainmaker. It looped slowly west, weakened to a tropical depression, and stalled over New York state before turning east on August 23. The storm finally became post-tropical that afternoon. What was left of Henri accelerated eastward and moved out to sea the next day.

The above image shows Henri as a minimal hurricane moving north toward New England on August 22.

It's hard to overstate how odd Henri's track was: the cyclone even originated in the subtropics east of Bermuda. For a storm with such an origin to loop back west and hit North America was unprecedented.

Hurricane Grace (2021)

Storm Active: August 13-21

On August 9, a tropical wave entered the Atlantic and moved quickly westward. From the start, the system produced an impressive area of thunderstorms, but its quick motion slowed down development. Four days later, it became organized enough to be designated Tropical Depression Seven well east of the Lesser Antilles. The storm covered ground rapidly, and was approaching the Windward Islands by the next day when it was upgraded to Tropical Storm Grace. Despite the upgrade, aircraft reconaissance indicated that the cyclone had serious structural issues: the low-level center was very ill-defined and difficult to even identify. In fact, sometime around August 14, Grace might have opened up into a trough of low pressure.

Nevertheless, the system was maintained as a tropical cyclone through the 14th, though it did weaken to a depression south of Puerto Rico. Disorganized thunderstorms extended from north of Puerto Rico to well south of the Dominican Republic by the next day along the former tropical wave axis. There were several areas of vorticity, but Grace consolidated some toward the southern end that evening. On August 16, the center was better defined and Grace clipped the souternmost point of the Dominican Republic, still as a tropical depression. Heavy rainfall in Hispaniola led to the risk of mudslides and flooding.

Land interaction lessened from that point and Grace slowly became a bit better organized as the strong subtropical ridge continued to push it just north of west. The center passed just south of Haiti that night, thankfully sparing the country the heaviest rainfall. Early on August 17, the storm regained tropical storm strength just in time for a direct hit on the island of Jamaica. The small country was not enough to significantly disrupt the cyclone, and in fact it strengthened gradually and grew in size that day. By the time it emerged off the west coast of Jamaica, Grace was a strong tropical storm.

Now over open water, the storm could tap into the highest oceanic heat content anywhere in the Atlantic. Relative humidity levels near Grace were only moderate (in the 50-60% range), which slowed the storm's intensification with occasional dry air intrusions. Otherwise, there was little to stop its strengthening and the cyclone was upgraded to a hurricane on August 18. The storm reached its first peak intensity of 80 mph winds and a pressure of 986 mb overnight before making landfall in the northern Yucatan peninsula before sunrise on August 19. The storm weakened over land and became a tropical storm late that morning, but maintained a vigorous circulation and emerged as a still strong tropical storm into the Bay of Campeche that evening.

Atmospheric conditions were better in the southern Gulf of Mexico than they had been over the Caribbean, with ample moisture in the air. It didn't take long for Grace to take advantage. By the morning of August 20, it had regained hurricane strength and extremely deep convection began firing north and east of the center. This managed to wrap all the way around by the afternoon and the cyclone rapidly intensified. Remarkably, it went from a category 1 to a high-end category 3 by late that night, becoming the first major hurricane of the 2021 season. An eye feature began to clear out just before Grace made its final landfall in Mexico very early in the morning on August 21. At landfall, Grace had an estimated peak intensity of 125 mph sustained winds and a minimum central pressure of 962 mb. This was the highest windspeed in a recorded landfall along the Mexican Gulf coast south of Tampico, surpassing Karl of 2010.

After moving inland, the mountainous terrain of Mexico quickly weakened the cyclone, bringing it to a tropical storm by that afternoon. Grace dissipated entirely by that evening after bringing heavy precipitation even as far as Mexico City. The remnants continued into the eastern Pacific ocean and ultimately reformed into a tropical storm early on August 23. Since the circulation had dissipated and reformed, however, the cyclone received a new name: Tropical Storm Marty. Marty persisted for only a few days before becoming post-tropical.

The above image is an infrared satellite view of Grace on the night of its final landfall in Mexico as a major hurricane.

A strong subtropical ridge kept Grace moving on a steady trajectory just north of west for almost its entire existence.

Wednesday, August 11, 2021

Tropical Storm Fred (2021)

Storm Active: August 10-14, 15-18

Around August 1, a tropical wave left Africa and entered the Atlantic. The wave did not develop much over the following 6 days, as conditions were not favorable and there was another wave closeby to the east. A few hundred miles east of the Windward Islands, the disturbance won out over its neighbor and developed a broad low-pressure center. By August 9, the system was located just north of Barbados. It moved west-northwestward into the Caribbean soon after. The circulation was quite impressive on satellite imagery: it had well-defined banding features and a clear spin. Nevertheless, it wasn't until that night that a center of circulation formed and it was upgraded to Tropical Storm Fred south of Puerto Rico.

Fred was dealing with dry air to its west, but its immediate problem was land interaction. After missing Puerto Rico to the south, the storm hit Hispaniola directly on the 11th, passing directly over the center of the island by that evening. The circulation was significantly disrupted and Fred weakened to a tropical depression. Though the cyclone was back over water overnight, there was some wind shear out of the west and it left most of its thunderstorm activity behind. It took most of the day on August 12 for Fred to slowly recover as it paralleled the northern coast of Cuba.

More land soon interrupted this reorganization, as Fred moved over central Cuba the next day. In fact, the low-level circulation became impossible to locate by that evening. Sometime soon after, Fred degenerated into a trough of low pressure and ceased to be a tropical cyclone temprorarily. However, heavy rains were still widespread across the region, so this changed the overall imapcts little. The system's remnants moved west-northwestward into the Gulf of Mexico on August 14 and began to organize again. The wind shear was a bit more favorable and ocean temperatures were warm as ex-Fred rounded the edge of the subtropical ridge and moved north-northwestward well west of Florida. On August 15, the circulation closed off and the system regained tropical storm status.

At first, the center was exposed on the western edge of the convection, but a semicircular core blossomed that evening and Fred began to strengthen. This trend continued through the morning of August 16 and Fred turned just east of north. By that time, the storm had a classic "comma" appearance, with a large curved band on the east side and a dry slot just southeast of the center. Some dry air intrusion via this slot capped Fred's intensity, but it still managed to become a strong tropical storm with top sustained winds of 65 mph before its final landfall in the Florida panhandle that afternoon. After landfall, the cyclone weakened steadily but brought a wide swath of heavy rain to the U.S. southeast and up the Appalachians. On August 17, Fred weakened to a tropical depression as it accelerated north-northeastward.

The storm continued to bring severe weather northward and eventually transitioned into a post-tropical storm over West Virginia the next day. Even after becoming post-tropical, ex-Fred brought severe weather into New England. It finally dissipated around August 19.

The above image shows Fred at peak intensity just before landfall in the Florida panhandle.

Fred's intensity was kept in check by land interaction for most of its lifetime.

Thursday, July 1, 2021

Hurricane Elsa (2021)

Storm Active: June 30-July 9

On June 27, a vigorous tropical wave left the African coastline. It stuck to the low-latitudes away from any Saharan dry air and quickly moved westward. By the 29th, it had developed a broad low pressure center and was showing signs of organization. Things really ramped up the next day when spiral banding features became evident, though satellite data indicated that there was still only a sharp trough at surface level (winds flowed around a line oriented north to south, rather than around a center of circulation). Moist equatorial inflow from the southwest generated some more spin and helped the system reach tropical depression status that night.

By the morning of July 1, the cyclone was producing gale force winds and was upgraded to Tropical Storm Elsa. Remarkably, Elsa broke the record for earliest fifth named storm (or "E" storm) that had been set just one year previously by 2020's Tropical Storm Edouard on July 6 of that year. It was still early in the summer and the tropical trade winds that blow east to west were very strong: Elsa reached a remarkable forward speed of 29 mph toward the west-northwest that afternoon. Such forward speeds are rare in the tropics, as opposed to the mid-latitudes where they are more common. Typically, this speed would make it hard for thunderstorm activity to "keep up" with the system's center, but otherwise favorable conditions allowed Elsa to intensify (albeit gradually) throughout the day.

Overnight, a circular central dense overcast blossomed and the storm's core became much better defined. This allowed a faster burst of strengthening that continued into July 2 and brought Elsa to hurricane strength that morning. The center passed just south of the island of Barbados and crossed the Windwards into the Caribbean sea later that morning. Elsa became a hurricane farther east in the tropics than any other recorded storm so early in the calendar year since 1933. It also was the fastest moving hurricane ever recorded in the tropics, at one point reaching 31 mph. However, after reaching a peak intensity of 85 mph winds and a pressure of 991 mb that afternoon, the storm's rapid forward speed finally caught up with it: the center of circulation outrun the associated thunderstorm activity and convection collapsed.

As a result, some weakening occurred overnight and into the morning of July 3, putting Elsa at high-end tropical storm strength. It passed well south of Puerto Rico and the Dominican Republic, and paralleled the southern coast of Haiti that afternoon. Though it stayed offshore, torrential rainfall impacted the southeastern peninsula. The cyclone approached the western edge of the steering ridge that evening and finally slowed down, allowing new deep convection to develop over the center of circulation. And yet, Elsa continued to baffle: even as the satellite presentation improved, aircraft reconnaissance indicated that central pressure continued to increase, rising to 1009 mb on July 4. Such a pressure is more typical of a tropical depression than a strong tropical storm!

Regardless, more rain was in store as the storm passed just north of Jamaica that day. Elsa then slowed even more and turned northwest toward central Cuba. The system remained fairly disorganized but held nearly steady in intensity up to landfall in Cuba on July 5. Fortunately, impacts in the country were not that severe. That evening, the center emerged into the Gulf of Mexico. Being back over water allowed the cyclone to again build a core, but it was slow going; wind shear out of the southwest kept the low-level circulation exposed and the thunderstorm activity primarily to the northeast. Nevertheless, slow strengthening occurred as Elsa moved north toward Florida on the 6th.

Satellite imagery during that afternoon indicated that the shear vector was now from the south - a bit more favorable for intensification as the center was moving northward. Hints of an eye appeared on radar and Elsa regained hurricane strength that evening. By that time, the outer bands were affecting western Florida, but the compact core meant that most impacts stayed offshore. Overnight, dry air invaded the core and the system weakened again to a tropical storm. It still had maximum sustained winds of 65 mph, however, when it made landfall in northwestern Florida late in the morning of July 7.

Elsa brought moderate flooding to the southeast as it began to curve northeastward, passing inland into Georgia. At first, it weakened over land, but it maintained an impressive circulation. It remained a tropical storm and even restrengthened some as it passed over the Carolinas on July 8. By the evening, flooding rains had spread northward into the mid-Atlantic states. The cyclone's center exited the coast briefly overnight before making further landfalls in New England during the morning of July 9. By that time, it was losing tropical characteristics as it continued to accelerate northeastward. At last, Elsa completed its long journey and became post-tropical that afternoon. Its remnants passed through Atlantic Canada and ultimately dissipated over the north Atlantic.

The above image shows Elsa near peak intensity passing over the Windward Islands.

Elsa had impacts all the way across the Caribbean and all the way up the eastern coastline of North America.

Monday, June 28, 2021

Tropical Storm Danny (2021)

Storm Active: June 28-29

Around June 26, a surface trough well southeast of Bermuda was producing some scattered thunderstorm activity as it moved rather quickly west-northwestward. Ocean waters beneath the disturbance weren't especially warm, nor the atmosphere too moist, but some gradual organization occurred nevertheless. Early on the 28th, it was apparent that a well-defined center of circulation had developed, and the system was classified Tropical Depression Four. At the time, it was a very small, sheared system off the South Carolina coastline.

The depression's track was a little unusual: a strong high was situated over the eastern seaboard, which kept Four moving quickly west-northwestward toward land. The system had a rather high pressure reading of 1013 mb due to ambient high pressures in the surrounding area and this shallowness also contributed to its fast forward motion in the strong low-level flow. In any case, warm waters near the coast allowed the storm to strengthen a bit around landfall and it became Tropical Storm Danny. It made landfall during the evening of the 28th a little north of the Georgia-South Carolina border. Because the cyclone was small, heavy rain and tropical storm force winds were confined to a small region.

After landfall, the storm deterioriated rapidly. It weakened to a tropical depression overnight and dissipated the next morning over central Georgia.

The above image shows the tiny Tropical Storm Danny just before landfall in South Carolina.

Danny was another short-lived tropical cyclone because it formed so close to land.

Saturday, June 19, 2021

Tropical Storm Claudette (2021)

Storm Active: June 19-21

Around June 12, a broad area of low pressure associated loosely with the central American gyre (CAG) to the south formed in the southern Bay of Campeche. It was producing occasional bursts of thunderstorm activity and some spin was evident on satellite imagery, but the size of the system and its proximity to land inhibited tropical cyclone development. The low barely moved for the next several days. It finally made some northward progress starting on June 17 toward a weakness in the subtropical ridge. It was quite disorganized though, with the only convection a band northeast of the ill-defined center.

The disturbance changed little the next day as it approached the northern coastline of the Gulf of Mexico, though aircraft and satellite measurements indicated that it possessed sustained wind speeds to gale force. Heavy rain swept across southeastern Louisiana as the system approached. It was only early on the 19th, as the center was moving over land, that it finally became organized enough to be named Tropical Storm Claudette, with peak winds of 45 mph. However, the classification of the system changed its impacts little; flooding was the primary concern as it moved inland and turned northeast, crossing into Mississippi. Claudette weakened to a tropical depression later on June 19.

An approaching cold front steered the system east-northeast across the southeast United States, bringing scattered downpours and gusty winds with it. The next day, as Claudette approached the Atlantic, the proximity to water fueled some convective redevelopment and the cyclone began to strengthen again. It became a tropical storm again on June 21 near the coast. Forunately, it was fast-moving, and was swept out along the mid-latitude westerlies in short order. After passing over the Gulf stream, it encountered cooler waters. The center was still not well-defined, and Claudette lost its identity as it sped away from land that evening.

As with many storms forming from CAG's, Claudette was a rather messy cyclone; it never looked completely tropical. 2020's Tropical Storm Cristobal was similar in this regard, and was another June CAG storm. Such systems develop most often in June or October/November.

Unusually, Claudette spent more time over land as a tropical cyclone than it did over water!

Monday, June 14, 2021

Tropical Storm Bill (2021)

Storm Active: June 14-15

On June 13, a non-tropical low pressure center developed just off the coast of the southeastern United States in association with a stalled warm front. It moved northeastward a little farther from land and encountered the hot waters of the Gulf stream; sea surface temperature anomalies were extremely high just off of the North Carolina. By the morning of June 14, a burst of convection had covered the low. It was designated Tropical Depression Two shortly after.

The system strengthened steadily during the next day despite moderate wind shear. As a result it was upgraded to Tropical Storm Bill. It also accelerated significantly on its northeastward path. By the afternoon of the 15th, Bill had reached its peak intensity of 60 mph winds, but was already beginning extratropical transition. The central thundestorm activity was displaced from the center by strong upper-level winds and the cyclone became extratropical that evening.

The above image shows Bill as a tropical storm on June 15.

Bill formed near land, but rapidly moved out to sea during its brief stint as a tropical cyclone.

Sunday, May 23, 2021

Tropical Storm Ana (2021)

Storm Active: May 22-23

Around May 20, a low pressure system developed east of Bermuda. At first, it was non-tropical, an elongated low with an attached frontal boundary on the southeastern side. However, it acquired some tropical characteristics as it moved generally in a counterclockwise loop over the next day or two. Ocean temperatures were below the usual threshold for supporting tropical development, but some cool air aloft drove enough instability for thundestorm activity to pop up near the low's center by early on May 22. The system was already producing gale force winds by this point. It had a small radius of maximum winds, a characteristic indicative of a tropical cyclone, but it was embedded in an upper-level low. Because of the latter, it was named Subtropical Storm Ana that morning, the first named storm of the 2021 Atlantic hurricane season. With Ana's formation, 2021 marked the seventh consecutive season with a storm forming before the traditional start date of June 1.

Later that day, Ana's center came within 175 miles (280 km) of Bermuda to the northeast, but the central area of thunderstorms was so small that the island received little more than showers and gusty winds. That evening, Ana began a more definite movement toward the northeast as it felt the flow of an upper-level trough exiting Atlantic Canada. This took the small storm away from the safe haven of low wind shear under its upper-level low. Related to this, Ana transitioned to a fully tropical storm by early on the 23rd. This didn't do much to alter the storm's fate; moving northeast over even colder waters and harsh shear, it weakened to a tropical depression and then degenerated to a remnant low that evening.

The image above shows Ana as a subtropical storm shortly after naming.

Ana was a small and short-lived cyclone with no land impacts.

Monday, May 17, 2021

Professor Quibb's Picks – 2021

My personal prediction for the 2021 North Atlantic Hurricane season (written May 16, 2021) is as follows:

17 cyclones attaining tropical depression status,
16 cyclones attaining tropical storm status,
8 cyclones attaining hurricane status,
5 cyclones attaining major hurricane status.

I predict that the 2021 season will feature above-average activity, though not at the pace of the record-breaking 2020 season. Every decade, the NOAA revises the 30-year averages for number of cyclones to reflect new data and better track climate change; the 1991-2020 averages were 14.4 tropical storms, 7.2 hurricanes, and 3.2 major hurricanes, up from 12.1 tropical storms, 6.4 hurricanes, and 2.7 major hurricanes in the 1981-2010 period. My forecast therefore exceeds this new average, but not by a great deal.

In making this prediction, I first consider the El Niño Southern Oscillation (ENSO) index, which measures sea surface temperature anomalies in the equitorial Pacific ocean. Warmer than normal temperatures (El Niño) correlate to decreased Atlantic activity (and increased Pacific activity) and cooler than normal temperatures (La Niña) the opposite. A La Niña event, the strongest in nearly a decade, is currently ongoing and began in the latter half of 2020, contributing to the frenzy of activity of that season.
Nevertheless, recent data indicate that the La Niña is waning and models generally show this should continue into the summer (see the above graph - the vertical axis indicates the relevant temperature anomaly). However, the ensemble average still indicates ENSO neutral to negative conditions. Overall, I predict lingering La Niña effects will still boost activity this year.

In the same vein, most of the 21st century has been in the positive phase of the theorized Atlantic Multidecadal Oscillation, which has led to elevated sea surface temperatures, and, generally, more hurricanes. It's hard to disentangle such long-term climate cycles from modern anthropogenic global warming, and the headline regarding ocean temperatures is largely the same as the last few years: the Atlantic, Caribbean, and Gulf of Mexico will be warm, even by recent standards. The largest anomalies are likely once again to be in the subtropical Atlantic.

A few other factors that influence tropical cyclone formation are relative humidity of the atmosphere and wind shear. A moist atmosphere allows nascent tropical disturbances to develop thunderstorm activity and grow. Wind shear refers to a change in wind direction and speed between the lower levels and upper levels of the atmosphere; higher values of wind shear hamper tropical cyclones because they prevent them from becoming or remaining vertically stacked. Long-term models can give at least some sense of what average conditions to expect during peak hurricane season (see above: the top figure shows expected precipitation anomalies for August-October 2021 and the bottom zonal wind shear anomalies for the same period). Using these and a few other factors, I'll give a finer analysis of the risks by region. My estimates are on a scale from 1 (least risk) to 5 (most risk).

U.S. East Coast: 3
A mixed bag of conditions leaves the U.S. east coast with middling risk. Indications are that the summer will be wet in this region, with ample heat and moisture for tropical cyclones to form and strengthen. On the other hand, the Bermuda high looks a bit weaker than usual given the neutral to negative ENSO index, suggesting that hurricane tracks might veer east out to sea. With neutral ENSO more likely late in the year, expect fall fronts to reduce east coast risk by the end of September; if there are landfalls here, it will be in the front end of the season.

Yucatan Peninsula and Central America: 2
After a devastating 2020, these areas will (hopefully) experience much less hurricane activity this year. Preciptation forecasts (see above) anticipate a drier west Caribbean, and while wind shear will be lower than average in most of the basin, the same is not true for the eastern Caribbean, where threats to the Yucatan and Central America could form. If hurricanes do affect this region, I'd expect it to be in October and November; rapid intensification episodes close to land are the primary risk.

Caribbean Islands: 5
Even in the most active season in history last year, tropical cyclone activity in the main development region in the tropics between the Windwards and Africa was lackluster. Things will probably be different this year. The dusty Saharan Air Layer (SAL), and its suffocating effect on east Atlantic tropical waves, looks to be less prominent than usual. Furthermore, shear is low, temperatures are warm, and precipitation anomalies are at least around neutral over the tropical Atlantic. This could open the door for some long-track hurricanes à la 2017. The biggest question mark is whether these will avoid land or not, but everywhere from the Lesser Antilles to the Bahamas should be on high alert.

Gulf of Mexico: 4
Though maybe a little dry, the Gulf may have the highest sea surface temperature anomalies outside the subtropical Atlantic this summer. Storms forming near the Bahamas and homegrown hurricanes in the Gulf are both likely to occur at sometime in the year, putting this region at above-average risk.

Overall, I expect the 2021 Atlantic hurricane season to be above-average, though not exceptionally so. Nevertheless, this is just an amateur forecast. Individuals in hurricane-prone areas should always have emergency measures in place. For more on hurricane safety sources, see here. Remember, devastating storms can occur even in otherwise quiet seasons.


Thursday, May 6, 2021

Hurricane Names List – 2021

The name list for tropical cyclones forming in the North Atlantic basin for the year 2021 is as follows:


This list is the same as the list for the 2015 season, with the exception of Elsa and Julian, which replaced Erika and Joaquin, respectively, after those names were retired.

Friday, January 1, 2021

Sting Jets

Early on October 15, 1987, an innocuous low-pressure system was moving across the Bay of Biscay off the west coast of France. Within one day, it became one of the most strongest windstorms in European history. Poorly anticipated, the storm produced hurricane-force sustained winds for hours over portions of Great Britain and France as well as absurdly strong gusts. The highest measured during the storm was 135 mph, corresponding to category 4 on the hurricane Saffir-Simpson scale (the cyclone was not tropical, however, so the word "hurricane" did not apply). Subsequently known simply as the "Great Storm of 1987," it prompted further study of the mechanisms by which extratropical storms produce extreme winds.

The above satellite image shows the Great Storm of 1987 with a long frontal "tail" extending all the way down to the Canary Islands.

By the time of the Great Storm, the overall genesis process for extratropical cyclones was well understood. The energy for extratropical cyclone formation ultimately derives from temperature differences: cold air from the polar regions meets warm air from the subtropics, usually between 30 and 60 degrees latitude north and south. At these interfaces, there are differences in air pressure at the same altitude in the atmosphere since cold air is denser than warm. This instability provides the energy to drive cyclone formation.

Under the right circumstances, small perturbations in the flow along a boundary of air masses can trigger the formation of a low-pressure system, as indicated above. The cyclonically rotating boundaries between warm and cold air become the warm and cold fronts that control weather in the mid-latitudes. Note that all diagrams, including that above, are the correct orientations in the Northern Hemisphere: the directions of spin would be reversed south of the equator. Our concern in this post is investigating where the strongest surface winds occur in these extratropical storms.

The above schematic shows major low-level winds associated with an extratropical cyclone at different stages of development. These are typical of a rapidly developing and strong storm, which is assumed to be moving northeast. As the storm ramps up, the dominant feature is the mild and wet Warm Conveyor Belt (WCB). This feeds the center a supply of moisture; indeed, nearly all the precipitation occurs ahead (east) of the advancing cold front boundary. Windy conditions can accompany the WCB, but they are not usually too extreme.

In the wake of the cold front comes the chilly and dry Cold Conveyor Belt (CCB). Most intense in a mature storm, this feature often packs stronger winds than the WCB, though they occur after precipitation has passed. Both of these are large-scale, well-understood features, but could not account for the unusually strong surface winds observed in some rapidly intensifying storms. It is the third feature above that fills in the gap: the so-called Sting Jet (SJ).

Named for the "sting at the end of the tail", the sting jet occurs near the very tip of the cloud head, where the bent-back cold front in the diagram above ends. This feature occurs most commonly in cyclones that explosively intensify or "bomb cyclones". The technical definition for this is a pressure drop of 24 mb or more in a period of 24 hours. As shown in the diagram below, just east of this "tail" of the cloud structure, instability causes dry air to descend from high in the atmosphere. Below this is the sting jet. It is a smaller feature compared to the CCB and WCB, about 100 km wide instead of several hundred. This conveyor belt of air is pushed toward the ground by the intruding dry air above.

Typically, friction with the land (or ocean) keeps winds near the surface lower than the strongest winds a few thousand feet above sea level. However, the descending site jet can transport these strong winds quickly to the surface. Moreover, the sting jet comes just head of the CCB (written CJ in the above picture) out of the south or southwest. In a cyclone moving northeast, these winds align with the storm's direction of motion, boosting them even higher. The result: localized but extremely intense wind gusts at the ground, associated with little to no precipitation.

Nearly all documented examples of sting jets are associated with north Atlantic storms impacting Europe. Since the Great Storm of 1987, roughly a dozen more examples have been positively identified. Satellite data indicate that further events likely occur over water where surface observations are sparse. Few studies have investigated the occurrence of sting jets elsewhere around the world, but explosive intensification of extratropical cyclones also occurs in the northwest Pacific and near Antarctica. Fortunately, comparable events in these regions have far fewer human impacts.

A thorough survey of the causes of sting jets is beyond the scope of this post; however, our understanding of this phenomenon is far from complete. Computer models struggle to resolve the feature, especially its tendency to "fan out" in to many small jets near the surface. As a result, predicting these events is still difficult. There is a lot on the line: the Great Storm of 1987 killed 22 people and caused billions in damages. Hopefully, future advances in advance warning will avert the worst impacts of these powerful storms.