Sunday, December 18, 2022

2022 Season Summary

The 2022 Atlantic hurricane season was close to average in overall activity, with a total of

16 cyclones attaining tropical depression status,
14 cyclones attaining tropical storm status,
8 cyclones attaining hurricane status, and
2 cyclones attaining major hurricane status.

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

20 cyclones attaining tropical depression status,
19 cyclones attaining tropical storm status,
9 cyclones attaining hurricane status, and
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. The eventual numbers for 2022 were quite close to this average, though with realtively few major hurricanes. The (preliminary) Accumulated Cyclone Energy value for the 2022 season was 95, a little below average but well-within "near normal" range. This measure of activity accounts both for strength and duration of tropical cyclones. The exact value sometimes shifts when post-season analysis is complete. This year, my predictions were significantly above the outcome, except in the hurricanes category where it was fairly close. Other official forecasts, such as those by the NOAA and by Colorado State University, were similarly above the mark. So what happened?
As predicted before the season, La Niña conditions prevailed throughout the summer and fall. This meant that equatorial Pacific sea surface temperatures were below normal, as the above diagram indicates for different zones. Such a strong La Niña would usually correlate to increased Atlantic activity, and was a major factor in pre-season forecasts. Atlantic ocean temperatures in hurricane formation zones also ran above normal, so we'll have to look elsewhere for an explanation of the underperformance.

Of particular note is the fact that no tropical cyclones formed during the month of August. This was the first time this had occurred since 1997, and the first time ever that this was observed to happen during a La Niña, dating back to the mid-20th century when detailed observations of the El Niño cycle began. Further, the only storms which formed prior to August were three short-lived tropical storms. The first hurricane was Danielle, which reached hurricane status on September 3, becoming the latest first hurricane in the Atlantic since 2013. In contrast, September through November was an unusually busy period, more consistent with climatological patterns.
The main culprits for the August quiet period were elevated wind shear and dry air. The map above charts the cumulative anomalies in relative humidity in the mid-levels of the atmosphere over the period August 1-22, 2022, vs. 30-year averages. The main development region in the tropical Atlantic east of the Caribbean was the epicenter of a dry air spell which lasted most of the month. Saharan dust was a major contributor to the dry air event. It prevented tropical waves from developing convective activity, which in turn would allow them to build vorticity, gain latitude, and detach from the Intertropical Convergence Zone (ITCZ). When tropical waves reached the Caribbean, another obstacle awaited them.

The first three weeks of August also saw well above average wind shear in the Caribbean sea. Wind shear measures the difference in winds at different heights in the atmosphere; a greater difference in winds prevents atmospheric vortices from the low and mid-levels aligning vertically, or "stacking". This is a necessary ingredient for tropical cyclones to form and strengthen. In comparison to my pre-season prediction, this wind shear event was especially surprising, since I had cited models which indicated wind shear would be below average for the June-July-August period. It's not clear why the model forecast was poor, but the immediate sources of the shear were systems called tropical upper-trospheric troughs (TUTTs). These are large areas of low pressure in the higher levels of the atmosphere, in this case usually located over the central Atlantic northeast of the Caribbean. These features generate counterclockwise flow, which runs opposite to the low-level east-to-west trade winds in the tropics. This combination amounts to high wind shear.

In my region-by-region predictions, I put all sectors of the Atlantic basin at higher-than-normal risk, but had only the Gulf of Mexico at highest risk. Unfortunately, this part of the forecast verified. The strongest winds and largest impacts of 2022 came from Hurricane Ian, which made landfall along the Florida Gulf coast as a top-end category 4 hurricane. Elswhere, Hurricane Fiona was the most impactful; it primarily affected Purto Rico and neighoring islands, as well as Atlantic Canada.

Some other notable facts or records from 2022 include:
  • The 2022 season ended a streak of six consecutive seasons in which a tropical cyclone had formed prior to June 1
  • Tropical Storm Bonnie and Hurricane Julia both crossed from the Atlantic to the Pacific basins and retained their names; this was the first time this had happened twice on record
  • When Fiona made landfall as an extratropical cyclone in Nova Scotia, it broke the record for lowest barometric pressure ever reported in Canada; the new record was 931.6 mb
  • Martin and Nicole were hurricanes simultaneously, which was the third time this had occurred in November, after 1932 and 2001
  • Nicole was the first Florida hurricane landfall in November since Hurricane Kate in 1985
The headline numbers for 2022 tell the story of an average season, but hide several surprising twists. This year is a good reminder that long-term hurricane season forecasts have a long way to go!


Monday, November 7, 2022

Hurricane Nicole (2022)

Storm Active: November 7-11

Around November 5, a huge area of disturbed weather developed across the eastern Caribbean and neighboring areas of the Atlantic. The system was broad and disorganized, but brought several inches of rain to Puerto Rico and Hispaniola as the strongest storms passed northward through that area over the following day. A low pressure center appeared on November 6 northwest of Puerto Rico, but it was non-tropical in nature at first. This slowly changed as scattered convection developed near the center, and the system was designated Subtropical Storm Nicole early on November 7.

At first, the cyclone moved gradually northwest, but a ridge began to build north of Nicole, turning it toward the west the next day. Meanwhile, despite periodic intrusions of dry air into the center, the storm strengthened steadily. The pressure gradient between the system and the ridge drove some intense winds north of the center; as a result, Nicole had a very large radius of gale force winds, even as the core tightened. The cyclone transitioned to a fully tropical storm on November 8. It actually turned south of west that night and approached hurricane strength as it neared the Bahamas. The storm owed its continued intensification to anomalously warm waters in the western Atlantic; these waters allowed it to reach category 1 hurricane status on November 9. By then, Nicole was close to landfall in eastern Florida. The system didn't look much like a typical hurricane, as it lacked a traditional central dense overcast, but this didn't stop it from bringing strong winds and heavy rain to much of the Florida peninsula.

The center officially moved ashore very early on November 10. In doing so, it became just the third hurricane on record to make landfall in Florida in November, and the first since Hurricane Kate in 1985. Soon after, it weakened to a tropical storm. The ridge to the storm's north moved out toward the east, allowing Nicole to begin a northward turn. It nevertheless emerged briefly over the Gulf of Mexico. It stayed very close to the west coast of Florida though, and soon moved ashore again along the Gulf coast. An approaching front picked up Nicole at that point and began to accelerate it toward the northeast. The storm weakened to a tropical depression that night, and became post-tropical over the southeast U.S. on November 11. It brought a quick hit of flooding rains to regions all along the Appalachians over the next day or so.

Nicole was an unusual hurricane on satellite imagery, with little convection near the center. The above image shows it at landfall on November 10.

Both Nicole's formation point and its westward track were rare for a November cyclone, allowing it to join a small handful of U.S. landfalling hurricanes in the month.

Wednesday, November 2, 2022

Hurricane Martin (2022)

Storm Active: November 1-3

On October 25, a trough of low pressure formed in the western Atlantic, extending from near the central Caribbean islands to near the U.S. East coast. The system was producing a large area of disorganized thunderstorm activity. A low formed along the trough around October 28, but it was non-tropical in nature. The system flirted with tropical status, but didn't quite have enough convective activity to be classified a tropical cyclone. The low passed near Bermuda on October 30, but strong upper-level winds prevented further development. After moving further northeastward, the system found a pocket of favorable conditions and began to strengthen and develop a warm core. On November 1, it became Tropical Storm Martin around 550 miles east-northeast of Bermuda.

The storm had a curved-band appearance on satellite imagery, with most convection extending west and north of the center. Sea surface temperatures weren't particularly warm, but a cold upper atmosphere generated enough temperature gradient for plentiful instability. In addition, a favorable jet interaction helped to drive intensification in Martin. On November 2, the storm accelerated northeastward as it felt the mid-latitude southwesterly flow, and developed a small eye on satellite imagery; as a result, Martin was upgraded to a hurricane that day. Since Martin and Lisa were hurricanes simultaneously, this became the third time in recorded history that two Atlantic hurricanes coexisted in November, after 1932 and 2001.

The cyclone intensified further on November 3, reaching peak winds of 85 mph. However, it was also swiftly becoming extratropical, with the inner core becoming less defined, but the radius of gale force winds ballooning quickly outward as the low deepened. Martin's transition to an extratropical storm on November 3 was extreme in many ways in fact: by the time it became post-tropical, it was absolutely rocketing northeastward at almost 60 mph. Tropical storm force winds extended across a diameter of over 1000 miles, stretching across a majority of the Atlantic around 50 ° N latitude. Finally, though Martin achieved a minimum pressure of 960 mb as a tropical cyclone, it deepened to a low of 932 mb after transition.

After the low's peak in strength on November 4, it began to slowly wind down and move more slowly to the east. Ex-Martin ultimately dissipated northwest of Ireland a few more days after that.

The image above shows Martin after strengthening to a hurricane on November 2.

Martin did not directly affect any land areas, but became a potent extratropical storm with hurricane force winds over the far northern Atlantic.

Monday, October 31, 2022

Hurricane Lisa (2022)

Storm Active: October 31-November 5

During the last week of October, an area of disturbed weather formed in the southeastern Caribbean sea. It moved generally west-northwestward over the next few days, and upper-level winds gradually became more favorable for development. Around October 30, a broad low pressure center formed west of the main convective mass, with several small vortices rotating around a poorly-defined circulation. At that point, it was not yet organized enough to be declared a tropical cyclone. The next day, however, more significant thunderstorm activity popped up closer to the center swirl and the system was classified Tropical Storm Lisa.

At first, Lisa's circulation was not well-stacked, which slowed strengthening in the short term. The steering pattern near Lisa was quite simple: a ridge to its north kept it on a consistent west-northwest track over the next few days. On November 1, the storm passed just north of Honduras, but since it had a small circulation, impacts to that country were minor. By that time, Lisa's circulation had become better defined, but pockets of dry air prevented it from rapidly intensifying. Nevertheless, winds increased steadily, and Lisa ultimately became a hurricane early on November 2.

The hurricane strengthened right up to landfall, reaching a peak intensity of 85 mph winds and a central pressure of 990 mb before moving into Belize that afternoon, bringing strong winds and storm surge to a relatively small area of the coast. The cyclone remained intense, however, as it pushed inland. It was downgraded to a tropical storm that night and to a tropical depression on November 3. Lisa moved west-northwestward and emerged into the Bay of Campeche the next evening. The storm did get a boost in thunderstorm activity once it was back over water, but restrengthening was cut short on November 4 by a sharp increase in upper-level winds. The convection was quickly stripped away from the center of circulation, and Lisa meandered a little longer before dissipating on November 5.

The above image shows Lisa as a category 1 hurricane, right before landfall in Belize.

Lisa was a small cyclone and its effects were mainly felt very close to the forecast track in Belize and southeastern Mexico.

Tuesday, October 11, 2022

Tropical Storm Karl (2022)

Storm Active: October 11-15

On October 10, a disturbance belonging to the same large area of vorticity as the remnants of Hurricane Julia entered the Bay of Campeche. It found a pocket of favorable conditions for development and organized rather quickly over the next day. A large area of convection blossomed near the nascent center of circulation on the 11th and the system was classified Tropical Storm Karl. At the time of naming, the storm was drifting northeastward under weak steering currents, but was far enough from any coastline that it was not impacting land at the moment.

Karl had marginally favorable conditions over the following day and strengthened some, reaching peak winds of 60 mph on October 12. Late that day, a few things changed: first, an area of high pressure moved in over the areas of Mexico west of Karl, setting up an anti-cyclonic steering flow. Since Karl was on the eastern side of that flow, it made a sharp turn and began to move generally southeastward. Second, stronger upper-level winds began to encroach from the northwest, bringing dry air with them. As a result, both the tropical storm and thunderstorm activity were pushed southeastward, and Karl fluctuated in intensity over the next few days as convection was stripped away from the center and then reformed.

By October 14, steady weakening was occurring as the center approached the coast of Mexico along the southern Bay of Campeche. Most of Karl's moisture had been pushed over land, and was causing heavy rain there. The system weakened to a tropical depression as shear overwhelmed it and it became post-tropical early on the 15th just before official landfall in Mexico.

The image above shows Tropical Storm Karl on October 12. Already, the effect of upper-level winds is evident: Karl's center is nearly exposed on the northwest side.

Karl meandered around the Bay of Campeche during its time as a tropical cyclone; impacts were primarily confined to heavy rains along the regions of Mexico south and east of the storm track.

Saturday, October 8, 2022

Hurricane Julia (2022)

Storm Active: October 6-9 (left Atlantic on October 9)

The system that became Hurricane Julia originated as a tropical wave which entered the Atlantic near the end of September. It took a low latitude route across the basin but began to show some spin on satellite imagery by around October 3 as it was approaching the southern Windward islands. On October 4, a surface low formed and was easily visible as a naked swirl of clouds on satellite imagery; all thunderstorm activity was displace southeast of the center. This small vortex was ejected westward and dissipated, while the broad mid-level circulation took a more southerly route. As a result, the disturbance had significant land interaction with the northern coast of Venezuela, which slowed its development. That did not stop the system from being a prodigious rainmaker in that country, however.

Tropical Depression Thirteen formed at last late on October 6, when the center was still located near the coast of Venezuela. A ridge of high pressure was steering the depression quickly westward and also keeping it from gaining latitude. On the 7th, it strengthened into Tropical Storm Julia. Julia had some initial trouble with maintaining a well-defined center of circulation and some shear out of the northeast kept the northern semicircle bare but for some scattered spiral bands. Nevertheless, the warm waters of the Caribbean fueled steady intensification.

The storm veered a little south of west on October 8 and found a pocket of more favorable atmospheric conditions, allowing it to intensify into a hurricane. Extremely intense convection blossomed around the center that night as Julia approached landfall in Nicaragua. The cyclone reached a peak intensity of 85 mph winds and a pressure of 982 mb before making landfall very early in the morning on October 9. The storm brought very high rainfall rates to Nicaragua, but was also still moving quickly, so the rainfall was not prolonged. By the afternoon, the center of Julia emerged into the eastern Pacific, still maintaining tropical storm strength. It was the second storm after Bonnie to maintain tropical storm strength crossing from the Atlantic to the east Pacific; this was the first year that two such events were recorded. Julia maintained the same name as an eastern Pacific tropical storm.

It turned west-northwest and generally moved parallel to the coast of central America, bringing additional precipitation to El Salvador and Guatemala over the next day. Though it was over water, atmospheric conditions and proximity to land prevented the system from recovering, and Julia weakened into a tropical depression; it dissipated near the coast of Guatemala on October 10.

The above image shows Hurricane Julia at landfall very early on October 9.

Julia was the second storm of the year to cross from the Atlantic to the Pacific.

Tropical Depression Twelve (2022)

Storm Active: October 4-6

Near the end of September, a vigorous tropical wave left Africa and moved over the Atlantic. It passed near Cabo Verde a day or so later. The wave was producing significant thunderstorm activity, but took a few more days to organize. Tropical Depression Twelve eventually formed on October 4, not too far west of Cabo Verde. Much like its predecessor, Tropical Depression Eleven, Twelve was moving northwestward toward unfavorable atmospheric conditions by the time it was classified.

The next day, southwesterly shear began to rip convection away from the center, and the environment was only getting harsher. Late on October 6, the depression was downgraded to a remnant low, without having been named. This low dissipated shortly thereafter.

In the above image, the center of circulation of Tropical Depression Twelve is visible as a naked swirl to the west of any thunderstorm activity. This was due to wind shear.

Twelve did not affect any land areas as a tropical cyclone.

Thursday, September 29, 2022

Tropical Depression Eleven (2022)

Storm Active: September 28-29

Around September 21, a small tropical wave a few hundred miles west of the African coast began to produce scattered thunderstorms. Late September is near the end of "Cape Verde season", when cyclones typcially develop from tropical waves in the Atlantic between Africa and the Caribbean islands. At this tail end of the period, steering currents are typically much weaker than in late spring, so tropical waves move more slowly. This disturbance was an extreme case: it meandered around the central tropical Atlantic for a whole week, moving only very slowly the whole time. Toward the end of the week, a persistent area of convection developed, but conditions were only marginally favorable and it took a long time to acquire the organization necessary to be a tropical cyclone. At last, on September 28, it became Tropical Depression Eleven.

Shortly after formation, Eleven managed to start moving a little faster toward the north toward an upper-level trough. Proximity to this trough would be its undoing, however, for wind shear increased the further north it went. After little over a day of being a tropical cyclone, the depression had its thunderstorm activity stripped away toward the northeast by the strong shear and it became a remnant low. This low dissipated very soon after that.

Tropical Depression Eleven was a small and short-lived storm, which did not encounter conditions favorable enough for it to strengthen.

Eleven's slow meandering track as a distrubance and brief tenure as a tropical cyclone did not take it near any land areas.

Saturday, September 24, 2022

Tropical Storm Hermine (2022)

Storm Active: September 23-25

A powerful tropical wave located over Senegal in west Africa entered the Atlantic on September 22 and immediately began to organize. It became Tropical Depression Ten the very next day. Very unusually, it moved north-northwest after hitting the ocean, responding to a weakness in the ridge to its north. That night, it strengthened into Tropical Storm Hermine, stealing the "H" name from Tropical Depression Nine, which only became a tropical storm a little later and therefore was assigned the name Ian.

Hermine managed to produce deep convection to the north-northeast of the center through early on the 24th, but as it moved north conditions turned hostile quickly. Cool ocean waters and strong upper-level winds stripped the center of thunderstorm activity by the afternoon. Since these winds pushed the moisture associated with Hermine northeast, the storm indirectly brought scattered heavy rains to Mauritania, Western Sahara, and the Canary Islands. These regions have a desert climate and it was very rare for them to be affected, even indirectly, by a tropical cyclone. Hermine weakened to a depression that evening. The next day, the shallow system turned back toward the west and became post-tropical. Its remnants dissipated shortly therafter.

The above image shows Hermine just after formation on September 23. The western coast of Africa is visible at right.

Hermine took an unusual northward path in the far eastern Atlantic and brought precipitation to northwest Africa and the Canary Islands, which are visible above and to the right of the storm track.

Hurricane Ian (2022)

Storm Active: September 23-30

A tropical wave moved over the Atlantic around the middle of September. It took a quite southerly track and had little in the way of organized thunderstorm activity before reaching the Caribbean on September 21. Though Caribbean waters were warm, strong upper-level winds from the outflow of the enormous Hurricane Fiona hindered the system from consolidating. Despite this, a well-defined surface low formed on the 22nd. At first, there wasn't enough convection near it to classify as a tropical cyclone, but it became Tropical Depression Nine early on September 23. Only late that day when Fiona was approaching Canada did the associated upper-level winds finally die down enough for the depression to strengthen more easily. It was named Tropical Storm Ian that night. The "I" name was assigned to this storm because in between being named a tropical depression and a tropical storm, a new depression (Ten) formed and took the name Hermine.

Wind shear had diminished, but Ian had internal structural issues to overcome. The low- and mid-level circulations were not vertically stacked, preventing organization for a day or so. Meanwhile, the storm moved westward, passing well south of Jamaica. After the vortices had become better aligned on September 25, the system still had trouble developing deep convection due to dry air inside in the circulation. Ultimately, the favorable conditions of the western Caribbean allowed Ian to overcome this obstacle as well and begin rapid intensification.

This trend brought the storm to hurricane strength by early on September 26 as it gradually curved toward the north. Ian managed to reach category 2 by that night without having a complete eyewall; rather, the core still consisted of spiral bands. Another burst of intensification occurred just before landfall in western Cuba overnight. At landfall, it had maximum winds of 125 mph (category 3 intensity) and a minimum central pressure of 947 mb. The storm's passage northward over land did weaken it, but the inner structure actually improved, if anything. When the center emerged over the Gulf of Mexico during the morning of the 27th, it almost immediately cleared out a large eye. By this time, Ian was feeling the influence of a potent trough of low pressure over the eastern United States and turned very gradually east of north in its southwesterly flow.

The hurricane underwent an eyewall replacement cycle that afternoon. In this process, a secondary eyewall forms outside the first, ultimately contracting and replacing it. Ian's maximum strength did not increase during this process, but its radius of tropical storm and hurricane-force winds increased. A new, larger eye began to emerge that evening. Soon, the hurricane was intensifying again and became a category 4. This trend unforunately continued almost until landfall. Ian ultimately reached a peak strength of 155 mph winds with a central pressure of 936 mb during the morning of September 28. A few hours later, the storm made landfall along the western coast of Florida at only the slightly reduced intensity of 150 mph winds and a pressure of 940 mb.

The imapcts to the landfall region were devastating; the slow-moving storm brought significant storm surge just south of the landfall point and extreme rainfall totals generally north of the center. Ian moved northeast across the state for the next day or so and weakened down to a tropical storm before emerging into the Atlantic ocean early on the 29th. By that time, the cyclone was no longer fully tropical; its interaction with a trough had lent it some frontal features. The windfield was large and less concentrated at the center, and similarly for rainfall. Nevertheless, access to the energy of the Gulf stream did help Ian to regain some strength and it became a hurricane again later that day.

Ian made its final landfall in South Carolina during the afternoon of September 30 as a category 1 hurricane with peak winds of 85 mph and a central pressure of 977 mb. Shortly after landfall, the storm lost what remained of its warm core and transitioned to a post-tropical system. The remnants weakened rapidly and merged with another disturbance over the mid-Atlantic the next day. The moisture from Ian contributed to a prolonged rain event for the coastal northeastern United States over the following several days.

The above image shows Ian at peak intensity a few hours before landfall in Florida.

During the latter part of Ian's track, it consistently deviated to the east of most forecasts due to the difficulty of predicting its interaction with a trough over the eastern United States.