Friday, December 15, 2023

2023 Season Summary

The 2023 Atlantic hurricane season was above average in activity, with a total of

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

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

15 cyclones attaining tropical depression status,
14 cyclones attaining tropical storm status,
8 cyclones attaining hurricane status, and
4 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. The eventual numbers for 2023 were well above my prediction for the total number of named storms, though my forecasts for hurricanes and major hurricanes were actually slight overestimates. The (preliminary) Accumulated Cyclone Energy value for the 2023 season is 146, solidly above average. This measure of activity accounts both for strength and duration of tropical cyclones. The exact value sometimes shifts when post-season analysis is complete. Overall, the 2023 season was a mixed bag, well above average in some indicators and near to slightly below in some others. This reflected the unusual conditions that made predicting 2023's outcome particularly tricky.

The 2023 season took place during a fairly strong El Niño event, meaning that equatorial Pacific sea surface temperatures were well above recent normals (the anomalies in different regions are shown in the diagram above). There's a well-established correlation between these anomalies and weather trends throughout the rest of the world, including stronger shear over the Atlantic ocean and a weaker Bermuda high. These tend to led to suppressed hurricane activity and more storms curving out to sea, respectively.

Though several cyclones this season struggled with wind shear, the overall wind shear pattern was not the typical one for an El Niño (see the figure above). Indeed, most of the main development region of the Atlantic basin had below-normal zonal wind shear, where zonal means the component of wind shear in the longitudinal direction. The following diagram illustrates just how unusual this combination of low wind shear and El Niño is, relative to the historical record.

The primary reason that El Niño did not behave normally was the incredibly warm sea surface temperatures in the tropical Atlantic. These warm waters were the result of a combination of anthropogenic global warming and a spring of very weak trade winds, which left the surface portion of the ocean unmixed with cooler waters below. This helps to explain the large number of named storms that formed in 2023. Fortunately for many land areas, however, the other common effect of El Niño did manifest: the Atlantic subtropical ridge was extremely weak all season, allowing many storms to recurve out to sea without affecting land. This included the season's strongest storm, category 5 Hurricane Lee. In fact, 2023 had the lowest death toll and damages from tropical cyclones dating back at least to the 2015 season.

2023's activity also came in intense bursts. The month of June was quite active with three named storms, including two tropical storm formations (Bret and Cindy) in the tropical Atlantic, which is the first such occurrence on record. After that, only one storm formed between June 26 and August 19! The floodgates opened after that, however, with the period from August 20 and September 28 seeing the formation of a whopping 13 named storms (a new record, beating 2020), and 5 hurricanes (tying a record last set in 2012). November, the last month of hurricane season, saw no formations at all.

In my region-by-region predictions, I forecasted that the Caribbean islands would be at high risk this season, and that the coast of the Gulf of Mexico and U.S. east coast would be at relatively low risk. These predictions were reasonably accurate, since the majority of tropical cyclones which affected land this year did so in the Lesser Antilles. Of course, Hurricane Idalia's category 3 landfall in the big bend region of Florida was the one major exception. Idalia caused significant impacts, but they were smaller in magnitude compared to other major hurricane impacts in the region in previous years due to the landfall location and the relatively small radius of maximum winds. Some other notable facts or records from 2023 include:
  • Though it wasn't operationally identified at the time, a unnamed subtropical cyclone formed in the northwestern Atlantic on January 16, and became the strongest tropical or subtropical Atlantic cyclone ever recorded in January
  • Hurricane Franklin's minimum pressure of 926 mb was the lowest ever recorded in a tropical cyclone that far north in the open Atlantic
  • Tropical Storm Philippe claimed the unusual record of longest-lasting Atlantic tropical cyclone with a peak intensity of under 70 mph

The 2023 season was unpredictable from start to finish, but ultimately had relatively mild impacts compared to many other recent years.


Monday, October 23, 2023

Tropical Depression Twenty-One (2023)

Storm Active: October 23-24

Around October 20, a broad area of low pressure in the far southwestern Caribbean sea began to organize. It drifted very slowly westward and became more concentrated, all the while bringing heavy rains to the neighboring central American countries. During the afternoon of October 23, it was designated Tropical Depression Twenty-One. There was no time for the system to develop any more, however, because it made landfall that very night in central Nicaragua. The main impact was heavy rainfall. Twenty-One dissipated by the next morning.

The image above shows Twenty-One shortly after formation.

Twenty-One only became a tropical cyclone less than 12 hours before landfall in Nicaragua.

Thursday, October 19, 2023

Hurricane Tammy (2023)

Storm Active: October 18-26,27-29

Around October 10, yet another late season tropical wave entered the eastern tropical Atlantic. The system gained organization as it moved westward and appeared to be close to tropical depression status on October 14. However, no well-defined center formed, and conditions became a little less favorable over the next few days. Thunderstorm activity in association with the broad low became more diffuse, and it took even more time for it to regroup. It wasn't until October 18 that the system finally was upgraded to Tropical Storm Tammy. At that time, it was 1000 kilometers east of the Windward Islands.

Tammy developed a fairly impressive central dense overcast and had good inflow on satellite imagery, but the vortex also showed clear tilt and was elongated northwest to southeast. This limited significant strengthening in the short term, but aircraft reconnaissace did find that Tammy was producing sustained winds of around 60 mph on October 19, making it a fairly strong tropical storm. The next day, the cyclone reorganized somewhat and developed a very small core. This led to a short burst of intensification and Tammy was a category 1 hurricane late that morning. The storm gradually turned toward the northwest as it approached the Lesser Antilles. Its center passed just to the east of Martinique and Dominica on the morning of October 21. Though scattered heavy rainfall was widespread across the nearby islands, the small windfield of Tammy kept hurricane-force winds off of land for the most part.

After a bit more strengthening, Tammy reached its first peak intensity of 85 mph winds and a pressure of 988 mb later on the 21st. The center made a direct landfall in Barbuda that evening, bringing severe impacts to the small island. After that, the storm moved gradually away from the Caribbean islands, though rainfall lingered from the outer bands through October 22. The storm continued to round the periphery of the subtropical ridge and turned north. It maintained category 1 strength and continued to exhibit a small but powerful area of central convection.

In the subtropics, Tammy turned northeast and encountered a more diffluent upper-level environment. This began another period of strengthening late on October 24 and an eye appeared on satellite imagery the next morning. Tammy soon reached its peak intensity as a category 2 hurricane with 105 mph winds and a minimum pressure of 965 mb. Later in the day, stronger shear set in as the storm began to interact with a nearby front and weakening began. The storm underwent a rather quick extratropical transition early on October 26. Little more than a day later, though, ex-Tammy separated from the front and became a tropical cyclone again. At that point, it was estimated to be a strong tropical storm in intensity.

The storm remained very compact, with deep convection occurring only very close to the center. Therefore, even though it was rather close to Bermuda, the island did not receive any severe impacts. It moved south of east away from Bermuda after that and gradually weakened due to dry air around it. Warm ocean waters could not counteract the unfavorable atmospheric environment and Tammy lost deep convection by late on October 28. It became post-tropical for the final time early on the 29th.

The image above shows Hurricane Tammy at peak intensity as a category 2 on October 25.

Tammy's primary impacts were to the Leeward Islands.

Friday, October 13, 2023

Tropical Storm Sean (2023)

Storm Active: October 10-15

On October 6, a late-season tropical wave entered the Atlantic. By October, the Intertropical Convergence Zone retreats southward toward the equator, so tropical waves tend to form at lower latitudes. This system was no exception; it tracked westward over the next couple of days at around 7.5° N, passing well to the south of Cabo Verde. Though the wave was producing widespread thunderstorms, it took time to spin up. Late on October 10, it had acquired enough organization to be designated Tropical Depression Nineteen. The next morning, it was upgraded to Tropical Storm Sean.

A gap in the ridge to Sean's northwest allowed it to turn right and gain some latitude over the next few days. Due to wind shear, the storm remained disorganized. It even weakened to a tropical depression late on October 11 before regaining tropical storm status the next morning when a larger area of convection developed on the east side of the circulation.

Shear over the system relaxed somewhat after that, but Sean faced the new obstacle of very dry air aloft. This proved to be a more potent adversary for the cyclone; it weakened and grew increasingly shallow on the 14th, returning to tropical depression status. Intermittent convective bursts allowed the storm to retain its status as a tropical cyclone into the 15th, but they became steadily less organized. The decaying storm also turned back toward the west in the low-level flow. Late that day, the system became a remnant low. Not long after, the remnants dissipated east of the Leeward Islands.

The image above shows Sean as a disorganized tropical storm on October 13.

Sean did not affect any land areas.

Saturday, September 30, 2023

Tropical Storm Rina (2023)

Storm Active: September 28-October 1

On September 23, a fairly late season tropical wave entered the Atlantic. It moved westward at a low latitude, passing well south of Cabo Verde and crossing the tropical Atlantic at a steady pace. By the 26th, an area of low pressure had developed along the wave, but the last push to tropical storm status was slow-going due to the size of the disturbance and the fact that Tropical Storm Philippe was close by to the northwest. Around the same time, the system turned rather sharply toward the north-northwest as it began a binary interaction with Philippe. A closed circulation became evident on September 28 and the disturbance was named Tropical Storm Rina.

The storm was over warm water but faced strong westerly wind shear and some disruption from the interaction with Philippe, which was only a little over 500 miles away to the west. Therefore, convection was confined primarily to the south and east of Rina's center. Due to the Fujiwhara effect, Philippe and Rina began to orbit each other cyclonically, which meant that Rina picked up speed and turned toward the west-northwest over the next couple of days. On September 30, the storm turned poleward and separated from the binary interaction, weakening as it did so. The strong shear over Rina did not lessen, and the center lost definition on October 1. By late that evening, it had degenerated into a remnant low. What was left of Rina dissipated soon after.

The image above shows Tropical Storm Rina early on September 29. The eastern part of Tropical Storm Philippe is visible at left.

Rina was a short-lived tropical storm that did not affect land.

Saturday, September 23, 2023

Tropical Storm Philippe (2023)

Storm Active: September 23-October 6

Around September 19, another tropical wave entered the Atlantic. It moved west-northwest for the next few days and displayed some impressive thunderstorm activity, though it lacked a well-defined center. The system acquired enough organized to be classified Tropical Depression Seventeen on September 23, when it was roughly halfway between the African coastline and the Lesser Antilles. At the time of formation, the low-level center was rather west of the deepest convection, having outrun the rest of the system. Most of the mid-level spin was similarly displaced east of the center.

Despite battling wind shear, the system's satellite presentation improved steadily and it strengthened into Tropical Storm Philippe shortly after formation, with gradual intensification occurring thereafter. The center of the storm reformed northeast of its previous position on September 24, bringing the low-level circulation closer to the convective canopy. Over the next few days, wind shear increased some more, exposing the center again and slightly weakening Philippe. Sporadic bursts of thunderstorm activity continued to generate gale-force winds well east or southeast of the center, but the cyclone was very poorly organized as it continued generally westward. It turned northwest on September 27, but this motion was short-lived. In fact, Philippe was barely a tropical storm by the 28th: there appeared to be multiple centers of vorticity stretching east to west and the overall circulation was highly elongated.

Complicating the storm's situation further was newly formed Tropical Storm Rina to the east. The two storms influenced each other and began to orbit cyclonically around a common center due to the Fujiwhara effect. On Philippe's part, it slowed dramatically and took an unusual dip toward the southwest beginning late on the 28th and continuing through September 30. During that time, the center again became better defined and the storm deepened a little as a more solid central dense overcast became established.

The influence of Rina waned on October 1 as the distance between the cyclones increased. A mid-level ridge northeast of the storm became the dominant steering feature of Philippe's relatively shallow vortex, and it moved generally west-northwestward the next couple of days. The convection had become more vigorous, but was still displaced south and east of the center by shear for the most part. During the evening of October 2, the center of the storm made landfall in the island of Barbuda. Due to Philippe's structure, however, heavy rains spread into most of the Leeward Islands in the wake of the center's passing.

For the next day, the storm's center continued northwestward, passing north of the remaining Caribbean islands in its path, but the main area of convection moved almost due west into the northeastern Caribbean. This meant that heavy rainfall continued on the islands, but the cyclone itself lost organization as the separation between center and thunderstorm activity became greater and greater. By that night, the circulation was so ill-defined that Philippe was barely a tropical storm. New convection ultimately developed somewhat nearer the shallow vortex as the system turned north. Some of the activity was the result of the interaction of Philippe with a frontal boundary well north of the center, though. Regardless of its origin, this part of the storm swept over Bermuda by October 5 and brought tropical storm conditions there.

The next morning, Philippe's center was absorbed into the larger frontal system and it became post-tropical. The remnant low interacted with another non-tropical low to its west and the combined system ultimately deepened and swept into the upper northeast U.S. and Atlantic Canada by October 8.

The image above shows Philippe on September 27. The center is nearly exposed at the cloud cover's western edge. The storm struggled with wind shear nearly all of its existence.

Philippe was a meandering, long-lived, and hard-to-predict tropical storm. It has the strange distinction of being the first known Atlantic tropical cyclone to last as long as it did (13 days) without achieving maximum winds of more than 50 mph.

Tropical Storm Ophelia (2023)

Storm Active: September 22-23

Around September 18, a frontal boundary moved off the U.S. east coast over the Atlantic ocean. The southern end of the front stalled not too far from shore. A non-tropical low formed along that southern edge, just north of the Bahamas, on September 21. The low moved generally north or north-northwest and deepened rather quickly over the next day. During the afternoon of September 22, an area of deep convection developed on the western side of the storm, spiraling north and east away from the center. Though the system was asymmetric, satellite and aircraft reconaissance measurements indicated it had transitioned to a tropical storm. Therefore, it was named Tropical Storm Ophelia.

Ophelia was in an environment of rather strong shear and suffered from the aforementioned asymmetry. Despite this, it managed to strengthen some more as it approached land. The cyclone made landfall in North Carolina early on September 23 at its peak intensity of 70 mph winds and a pressure of 981 mb. The large system slowly pushed inland that day and gradually weakened, but brought a large area of storm surge, heavy rain, and tropical storm force winds to the mid-Atlantic. It weakened to a tropical depression that evening and became post-tropical shortly thereafter while located over southeastern Virginia. What was left of Ophelia turned north and then east, bringing more steady rain to the northeast as it weakened.

The above image shows Ophelia a few hours after becoming a tropical storm on September 22.

At the time Ophelia made landfall in North Carolina, the large storm's impacts were already being felt over an area that extended much further north.

Sunday, September 17, 2023

Hurricane Nigel (2023)

Storm Active: September 15-21

Around a week into September, another tropical wave entered the Atlantic. Several days later, it merged with another tropical disturbance to its east. This consolidation took some time, but gradual organization continued and ultimately culminated in the formation of Tropical Depression Fifteen on September 15. The circulation remained rather broad, but conditions were favorable for intensification. The depression strengthened into Tropical Storm Nigel late on the 16th. The storm moved steadily on a northwest path across the central Atlantic under the influence of a subtropical ridge.

Early on September 18, Nigel became a hurricane. The structure underwent an interesting evolution over the next day, with a very large eye opening up by the 19th. Initially, though, the convection around the eye was quite thin, especially on the north side. The eyewall became more solid throughout the day and Nigel intensified further to a category 2 hurricane. That night, the cyclone reached its peak intensity of 100 mph maximum sustained winds and a minimum pressure of 971 mb. Around the same time, it turned north, rounding the edge of the subtropical ridge.

On September 20, Nigel began to accelerate northeast as it was swept up by the mid-latitude westerlies. This soon brought the hurricane to cooler waters and a higher shear environment, causing it to gradually weaken. The remaining deep convection was displaced from the center the following day and the system transitioned to post-tropical in the evening.

The image above shows Nigel near peak intensity on September 19. Its eye was unusually large relative to the storm's total size, a feature more common in hurricanes in the subtropics.

Nigel did not directly affect any land areas as a tropical cyclone.

Thursday, September 7, 2023

Hurricane Margot (2023)

Storm Active: September 7-17

On September 5, another tropical wave entered the eastern Atlantic. It moved west-northwest and developed fairly quickly as it passed over Cabo Verde. In fact, it was still bringing rain to the westernmost islands of Cabo Verde when it was classified Tropical Depression Fourteen during the morning of September 7. Not long after that, it was upgraded to Tropical Storm Margot.

Margot faced a mixed-bag of conditions. It was moving over fairly warm ocean waters and faced only moderate wind shear. However, it was surrounded by a fairly dry environment, so it took time for the storm to develop deep convection. On September 8, the center became exposed to the southwest of the strongest thunderstorms. Nevertheless, Margot eventually managed to organize and began gradually strengthening on September 9. A more substantial inner core appeared the next day as the storm turned toward the north, following a weakness in the subtropical ridge.

By September 11, Margot was a well-organized storm with a partial eye and impressive outflow to the north. This brought it to hurricane strength. Margot had its ups and downs over the next couple of days as it chugged northward, with some dry air in the circulation but an inner core and eye occasionally visible on satellite imagery. The net effect of all this was a little more gradual intensification. Margot reached a peak intensity as a high-end category 1 hurricane with 90 mph winds and a central pressure of 970 mb on September 13.

After that point, the storm's core became a little more diffuse, but its windfield expanded as it gained latitude. On September 14, Margot turned right in the face of a ridge blocking its path, and started a slow clockwise loop west of the Azores. During the loop, the storm underwent a gradual decay as dry air eroded its thunderstorm activity. It weakened to a tropical storm on the 15th and lost nearly all convection by early on September 17. At that time, it became post-tropical.

The above image shows Margot at a hurricane on September 12.

Margot's only land impacts as a tropical cyclone were to Cabo Verde right after its formation as a tropical depression on September 7.

Wednesday, September 6, 2023

Hurricane Lee (2023)

Storm Active: September 5-16

On September 1, a strong tropical wave moved over the Atlantic from the west coast of Africa, producing a large area of vigorous thunderstorms near the coast as it did so. The system steadily gained organization as it moved westward at a fairly fast pace. A low pressure center formed on September 4 and the circulation became well-defined enough to designate the system Tropical Depression Thirteen on September 5. Very high sea surface temperatures in the tropical Atlantic allowed the storm to intensify quickly from that point as it moved west-northwestward. It became Tropical Storm Lee six hours after formation and developed prominant spiral banding almost immediately. Some slight northeasterly shear was not enough to prevent it from becoming a category 1 hurricane by the evening of September 6.

Conditions became even more favorable on September 7 and Lee's structure improved rapidly. The nascent eyewall, which was open to the west in the morning, closed off quite quickly after that and the stage was set for the hurricane to deepen at an incredible pace. The eye cleared out and became symmetric by the evening and Lee strengthened to a category 4 hurricane. That evening, Lee reached its peak intensity as a powerful category 5 hurricane, with maximum winds of 165 mph and a central pressure of 926 mb. These winds were the highest recorded in any Atlantic hurricane since Dorian of 2019. Shortly after that, wind shear disrupted the center of Lee and it began a weakening trend on September 8. The eye clouded over and the central dense overcast eroded, particularly on the west side. This brought the storm back down to a category 3 as it made its closest approach to the northeasternmost Caribbean islands.

Lee maintained its steady west-northwest heading, though its forward speed gradually slowed. The weakening halted on September 10, by which time the storm was a category 2. Its structure subsequently improved, though not to the level of its original burst of intensification. Lee regained major hurricane status that day. Further increases in winds were precluded by the storm's inner core organization, which consisted of a very large outer eyewall and a decaying partial inner eyewall. However, even though the winds did not increase further, the hurricane-force wind field had expanded significantly and continued to grow over the next few days.

By September 11, the storm had turned northwest and slowed further. Its path brought it well northeast of the Bahamas, though the large circulation caused large swells and rip currents to the Caribbean islands and U.S. east coast. Lee also traveled over the cool wake in ocean temperatures left behind by Franklin and exacerbated that issue by upwelling cool waters with its slow forward motion. As a result, a slow weakening began on September 13. The storm turned toward the north that day under the influence of an approaching trough. On September 14, Lee dropped to category 1 intensity and made its closest approach to Bermuda, passing well to the west. Regardless, the storm's large windfield brought a sustained period of tropical storm conditions to the island.

On the 15th, the cyclone became significantly less tropical in appearance, with dry air invading the southern semicircle and displacing convection northward. That evening, tropical storm conditions began across a wide area of coastal New England and Atlantic Canada. Early on September 16, Lee completed extratropical transition. Nevertheless, post-tropical Lee still brought winds to near hurricane force to Nova Scotia when it made landfall in the easternmost part of the province later that day. The center of the storm crossed the Bay of Fundy and made another landfall in New Brunswick very early on September 17. Weakening more quickly, ex-Lee moved over the Gulf of St. Lawrence and finally over Newfoundland on the way out to sea.

The image above shows Lee as a category 5 hurricane. Just after this satellite imagery, mid-level shear from the southwest disrupted the center and weakened the storm. The beginnings of this disruption are already apparent in the image.

Lee did not made landfall as a tropical cyclone but did have direct impacts on Atlantic Canada after becoming post-tropical.