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.


Friday, December 18, 2020

2020 Season Summary

The 2020 Atlantic hurricane season was extremely active, with a total of

31 cyclones attaining tropical depression status,
30 cyclones attaining tropical storm status,
14* cyclones attaining hurricane status, and
7* cyclones attaining major hurricane status.

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

20 cyclones attaining tropical depression status,
18 cyclones attaining tropical storm status,
9 cyclones attaining hurricane status, and
5 cyclones attaining major hurricane status.

*The operational number of hurricanes was 13 and major hurricanes 6, but Gamma was upgraded to a hurricane and Zeta to a major hurricane in post-season analysis.

The average numbers of named storms, hurricanes, and major hurricanes for an Atlantic hurricane season (over the 30 year period 1981-2010) are 12.1, 6.4, and 2.7, respectively. The unprecedented activity of the 2020 season vastly exceeded both these averages and my preditions, above average though they were. With an astonishing 30 named storms, 2020 surpassed 2005's record of 28. Its 14 hurricanes became second-most on record (behind only 2005 with 15), and its 7 major hurricanes tied 2005 for the most ever recorded. 2020 was the second season to use the Greek alphabet to name cyclones after exhausting the 21 names on the ordinary list, and the first to use the Greek letters "Eta", "Theta", and "Iota". The Accumulated Cyclone Energy (ACE) value for the 2020 season (which accounts for duration and intensity of storms as well as number) was around 180, in the "hyperactive" range.

2020 also set a smattering of other activity records. For the third named storm and the fifth onward, each formation was the earliest for that number storm on record. The season had five hurricanes of at least category 4 strength, tying a record. A total of ten storms were named in September, surpassing the previous busiest Septembers of 2002 and 2010, each with eight. This busy month included three storms being (operationally) named the same day: on September 18, Tropical Storm Wilfred, Subtropical Storm Alpha, and Tropical Storm Beta all received names. This was only the second time in the hurricane database that such an event occurred, after August 15, 1893. Note, however, that the post-season analysis on Alpha found that it had in fact formed one day earlier, on September 17. 2020 was the first time three major hurricanes had formed in the month of October. November also had three named storms (tied for most), and two major hurricanes (a first).

Illustrating just how active September was, the above image shows the Atlantic on September 14, when there were five (!) cyclones existing simultaneously. Hurricane Sally is visible, strengthening in the Gulf of Mexico, Hurricane Paulette is off of the east coast of North America, a weakening Tropical Depression Rene is a sheared blob east-southeast of Paulette, Tropical Storm Teddy is strengthening in the tropical Atlantic halfway between the Lesser Antilles and Africa, and a recently formed Tropical Storm Vicky is northeast of Teddy. As if that were not enough, the tropical wave that gave rise to the short-lived Tropical Storm Wilfred can be seen in the bottom-right corner emerging from Africa, as can the disturbance in the Gulf of Mexico southwest of Sally that ulimately became Tropical Storm Beta. That's seven systems in one picture!

There are several factors that contributed to such an active hurricane season. As predicted in pre-season outlooks, a significant La NiƱa event developed during the summer, shown by the above graphs of sea surface temperature anomalies in different areas of the equatorial Pacific. These relatively cool temperatures are correlated with increased hurricane activity on the other side of the Americas, and this was certainly the case in 2020. A subtler, longer-term cycle also may have contributed to a record season: the Atlantic Multi-Decadal Oscillation (AMO) maintained its positive phase. This index is computed from Atlantic sea-surface temperatures, which were warm even relative to an overall warming planet. This in turn led to a stronger African monsoon and increased rainfall across the tropical Atlantic. As the name suggests, the AMO varies only slowly over dozens of years. Indeed, the five seasons 2016-2020 were all above average. Each also had at least 1 category five hurricane, the first such five-year period on record: Matthew in 2016, Irma and Maria in 2017, Michael in 2018, Dorian and Lorenzo in 2019, and Iota in 2020.

2020 also saw many, many instances of rapid intensification, as shown in the table above. Ten cyclones satisfied the technical criteria for rapid intensification, namely a 25 kt (30 mph) increase of maximum winds in a span of 24 hours. This tied the record set in 1995. Even more incredible were the top bouts of strengthening on the list: Delta, Eta, and Iota all gained at least 60 knots (70 mph) in 24 hours, which had only been observed a handful of times previously. This points to a final noteworthy aspect of 2020's activity: the absurdly ideal conditions in the western Caribbean sea during October and November. Record low vertical wind shear and high oceanic heat content extending well below the surface led to three of the season's seven major hurricanes developing there in those two months, as well as five of the ten intensfication episodes listed above. Tragically, these favorable conditions led to the season's worst impacts in Nicaragua and Honduras, which experienced a never before seen two consecutive category 4 landfalls in one week (Eta and Iota). The latter hurricane was the latest forming category 5 on record in the Atlantic basin (it achieved category 5 on November 16, surpassing the 1932 Cuba hurricane's record of November 6-9).

The above graphic (click to enlarge) illuminates yet another metric by which 2020 exceeded all previously known seasons: continental United States landfalls. A record twelve tropical storms and six hurricanes (the latter tying a record) hit the United States, putting the entire Gulf and east caosts under cyclone-related watches and warnings at some point during the year. The epicenter of this barrage was Lousiana, which had five landfalls, three of which were hurricanes and two major hurricanes. The strongest was the devastating Laura, which had the highest winds of a known landfalling Louisiana hurricane since 1856. Even so, the impact of these storms wasn't as great as it could have been, since relatively sparsely populated areas were affected.

Some other notable facts or records from the 2020 Atlantic hurricane season include:
  • Tropical Storm Arthur was a pre-season tropical cyclone that formed on May 14, marking the record sixth consecutive season in which a storm formed before the official start of the season on June 1
  • When Tropical Storm Bertha formed on May 27, it was the first time since 2012 that there were multiple pre-season storms
  • Hurricane Paulette first became a depression on September 6, and transitioned into a remnant low for the last time on September 19; in this interval, a remarkable seven named storms formed in the Atlantic: Rene, Sally, Teddy, Vicky, Beta, Wilfred, and Alpha
  • The diameter of tropical storm force winds of Hurricane Teddy just before its extratropical transition was 850 miles, breaking the top 5 for Atlantic hurricanes at the time
  • Subtropical Storm Alpha became the easternmost-forming Atlantic named storm on record when it formed near Portugal on September 18; that same day, it became the first named storm to make landfall in that country
  • Tropical Storm Theta traversed the farthest east of any November Atlantic tropical cyclone on record, ending up east of 20 °E before dissipating.
The 2020 Atlantic hurricane season was the busiest on record in a number of categories, featuring a great number of deadly and destructive cyclones.

Friday, November 13, 2020

Hurricane Iota (2020)

Storm Active: November 13-18

On November 8, a tropical wave entered the Caribbean sea. It was producing disorganized thunderstorms throughout the Leeward Islands and Puerto Rico. It moved southwestward over the following days into a more favorable upper-level environment and the disturbance began to organize. On November 13, it was organized enough to be designated Tropical Depression Thirty-One. The depression's circulation was large, with an evident spin on satellite imagery and a convective band extending south and west. Later in the afternoon, the storm was upgraded to Tropical Storm Iota, the record-breaking thirtieth named storm of the 2020 season, and the first ever use of the name "Iota".

When it was first named, Iota was rather disorganized; the mid-level center was well southeast of the low-level center, while the latte was intermittently exposed. This was possibly due to lingering westerly shear, but this was quickly declining. Nevertheless, the storm took about a day to get fully stacked and its winds only increased a bit in the meantime. Though a ridge was gently steering Iota, it didn't move a ton on November 14. In fact, it moved south-southwestward for a time and the vigorous bands south of the center were low enough in latitude to impact northwestern Colombia. That evening, an inner core developed and rapid intensification began.

Overnight, Iota became a hurricane. It underwent an interesting structural shift the morning of the 15th, when an eyewall replacement cycle seemed to take place, even before an eyewall had completely formed. This left the storm with weaker winds and convection temporarily but a higher radius of maximum winds. Meanwhile, the central pressure continued to drop steadily, indicating that the changes were internal and that Iota was still strengthening. The cyclone also assumed a steadier course just north of west that it would maintain through landfall.

An eye appeared intermittently on satellite imagery during that day and the hurricane attained category 2 that evening. Then, an enormous burst of intensification ensued overnight, only the latest of a string of such episodes in the extraordinary 2020 season. During this period, Iota's pressure dropped 26 mb in a single six-hour period, along with a 10 mb drop in a single hour as recorded by reconaissance aircraft. In just 12 hours, the storm went from a 105 mph, 960 mb category 2 hurricane to its peak intensity as a 160 mph, 917 mb category 5 hurricane by the morning of November 16. This made Iota the strongest storm of the 2020 season, surpassing Eta from a few weeks earlier. It also beat the record of the 1932 Cuba hurricane as the latest-forming category 5 on record (the latter had had category 5 intensity November 6-9 of that year) and was only the second ever November category 5 known in the Atlantic. On top of this, 2020 became the first hurricane season ever with two major hurricanes in November and 2016-2020 became the first period of five consecutive years with at least one category 5 recorded per year, following Matthew in 2016, Irma and Maria in 2017, Michael in 2018, and Dorian and Lorenzo in 2019.

Iota maintained category 5 status for some of the day, but underwent another eyewall replacement cycle during the afternoon, which brought it back down to category 4 intensity before landfall. Nevertheless, the cyclone was an extremely intense hurricane when it made landfall in Nicaragua that evening. Tragically, its landfall point was less than 100 miles from Eta's, which had hit only a few weeks earlier. After landfall, Iota quickly weakened as it traversed increasingly mountainous terrain. Some high elevation areas of Nicaragua and Honduras recorded over 20 inches of rain during the storm's passage. It lost hurricane status around midday local time on November 17. The degradation of the circulation was relatively slow for a cyclone over land, though, and the center was still quite evident on satellite imagery as Iota passed inland into southern Honduras that evening.

The storm weakened to a tropical depression early on November 18, crossing into El Salvador, and dissipated a few hours later somewhere near the Pacific coastline. Even though the remnants of Iota moved back over water in the eastern Pacific, conditions there did not favor development.

The above image shows Hurricane Iota at category 5 intensity on November 16. No recorded Atlantic hurricane had ever reached such a strength so late in the season.

The conditions in the western Caribbean were extremely favorable for hurricane developement in October and November 2020. Iota formed in this same area and was the second category 4 landfall in two weeks in Nicaragua.

Monday, November 9, 2020

Tropical Storm Theta (2020)

Storm Active: November 9-15

A frontal boundary stretching across the subtropical Atlantic during the first week of November decayed, leaving a trough of low pressure and unsettled weather in its wake. Around November 7, a non-tropical low formed in association with the system well southeast of Bermuda. The low was generating significant convection in the form of a curved frontal band extending north and east of the center of circulation. Eventually, the low became separated from the band, but was still located within a broad upper-level low. The system also was generated gale force winds, so it was designated Subtropical Storm Theta. This designation of a 29th named storm officially broke 2005's record for the most ever recorded in an Atlantic hurricane season. It was the first ever use of "Theta" as well.

The storm was moving just north of east at a moderate pace. Sea surface temperatures were not especially warm, but a great deal of instability was present in the atmosphere to fuel Theta. Satellite estimates indicated that it strengthened over the following day to near hurricane strength. During the afternoon on November 10, Theta transitioned into fully tropical storm. The cyclone was dealing with wind shear near 50 knots, which would ordinarily overwhelm a tropical cyclone, but things are often different for late-season storms in the subtropics: one factor keeping Theta going was very cold air in the upper atmosphere. This meant that, despite cooler ocean temperatures, the altitude/temperature gradient was quite pronounced and supported deep convection.

After a little weakening overnight, Theta regained a bit of strength on November 11 when shear abated a tad. It was still riding the north edge of a mid-level ridge eastward with a very consistent forward speed. By November 13, ocean temperatures had dropped even further, and wind shear was bringing stable air out of the north. Theta started to weaken, slow down, and turn south of east. Enough convection persisted on the southeastern edge of the cyclone for it to stay tropical through the next day. It also was the first Atlantic tropical storm on record to travel so far east in November, past 20° W. Unfavorable conditions eventually overcame Theta though. It weakened to a tropical depression overnight and to a remnant low the next morning just norhwest of the Canary Islands.

The above image shows Theta as a subtropical storm on November 10.

Theta did not have any land impacts along its journey across the eastern Atlantic.

Saturday, October 31, 2020

Hurricane Eta (2020)

Storm Active: October 31-November 13

Two tropical waves over the central tropical Atlantic passed over the Windward islands in the last week of October, producing stormy conditions there. The second was moving faster than the first, and they merged into a single large disturbance over the eastern Caribbean. On October 30, this disturbance developed concentrated thunderstorm activity and consolidated pretty quickly. The next day, it attained tropical cyclone status as Tropical Depression Twenty-Nine. At the time, it was centered well south of Haiti.

Overnight, the storm strengthened into Tropical Storm Eta. This brought 2020 into a tie with 2005 for the most named storms ever recorded in a single Atlantic hurricane season, with 28. Eta formed much, much earlier than 2005's final named storm, Tropical Storm Zeta, which formed on December 30 of that year and lasted into the first week of 2006. Also, because of 2005's unnamed subtropical storm, this was the first use of the Greek letter "Eta" for any cyclone. The newly named Eta moved nearly due westward under the influence of a ridge to its north. Though late fall was closing down many other parts of the Atlantic basin, the southwestern Caribbean remained close to ideal for tropical cyclone development: high oceanic heat content still prevailed along with ample moisture and low wind shear. Eta took full advantage of these conditions beginning on November 1.

Eta's subsequent intensification event was among the most rapid ever recorded. It became a hurricane early on November 2 and the tropical storm force wind radius greatly expanded. The inner core of the system was still fairly small, supporting rapid intensity changes. Similar to in Hurricane Delta a few weeks previously, a tiny warm spot appeared on infrared imagery surrounded by incredibly cold cloud tops that afternoon. Around the same time, Eta became a category 2 and then, a few hours later, a major hurricane. It was now approaching Nicaragua, and it began to slow down and turn toward the southwest. Unlike Delta before it, it managed to clear out its eye that evening. The ring of cold cloud tops, some colder than -90° C, expanded further. Eta peaked that night as the strongest hurricane yet of the 2020 season, with maximum winds of 150 mph (tying Laura) and a minimum central pressure of 923 mb. At the time, this made Eta the second-most intense November Atlantic hurricane recorded, after only the unnamed Cuba hurricane of 1932. However, Eta would be surpassed in both these records by Hurricane Iota just weeks later.

By that time, some of the western half of the circulation was over Nicaragua. Unfortunately, the storm had slowed to a crawl, causing a prolonged period of flooding rains as Eta meandered just off the coast. An eyewall replacement cycle also took place, leaving the cyclone with slightly lower winds but a larger eyewall; in any case, it still made landfall as a category 4 late in the afteroon on November 3. Once inland, Eta pushed westward and weakened rapidly, especially once it reached mountainous terrain farther inland. Early on November 4, it was downgraded to a tropical storm, and that evening, a tropical depression. What was left of the low-level center crossed the inland border into Honduras.

After another day of traversing central America, the weak depression turned northward back toward the Caribbean. It was unclear whether the low-level center had truly survived, but without significant evidence to the contrary, it was maintained as a tropical depression. During the evening, Eta reemerged over water and convection began to increase again. An upper-level trough began to draw the system northeast at increasing speeds on November 6. Little changed with the storm that day since the circulation was ill-defined and beset by moderate southwesterly shear.

A great deal of upper-level divergence did support deep convection and warm Caribbean waters helped Eta to spin up once again on the 7th. The cyclone regained tropical storm strength while centered near the Cayman Islands, but its structure was quite different from before: it had the comma-shape of a storm with some subtropical characteristics. The center also reformed nearer to the mid-level spin and Eta strengthened as it moved northeast toward central Cuba. Early on November 8, Eta made landfall there as a strong tropical storm. This resulted in minor flooding and storm surge, especially on the east side. Later in the day, the storm reemerged over water in the extreme southwest Atlantic.

An upper-level trough over the northwestern Caribbean had been guiding the cyclone; it closed off into an upper-level low and Eta began to rotate counterclockwise around it, veering first north and then northwest toward south Florida. The storm lost most of its central convection by later in the day as very dry air was entrained from the west. The exception to this was Eta's northern semicircle, which was still very moist, and brought flooding rains with tropical storm conditions into the Florida peninsula. That night, the center made landfall in the Florida keys with maximum sustained winds an estimated 65 mph. Continuing its arc, the storm veered west and soon after southwest across the Gulf of Mexico.

Early in the day on November 9, Eta had little thunderstorm activity to speak of, but a compact core redeveloped as shear diminished and the circulation developed a moisture envelope helping to shield it from the dry air without. That evening, the storm was centered off the coast of far western Cuba, bringing some rainfall there. However, it soon reversed course and moved northward again as the subtropical ridge north of it eroded on November 10. Eta was vigorous, but shear displaced the mid-level center well east of the surface circulation by that evening. After recovering overnight, the storm briefly restrengthened into a category 1 hurricane during the morning of November 11. Dry air overwhelmed the circulation soon after and Eta weakened as it moved northward offshore of the west Florida coast. Nevertheless, there was still a significant storm surge in the Tampa Bay area.

Soon, Eta turned northeast. The storm made its final landfall in northwestern Florida early on November 12 as a weakening tropical storm. It crossed the peninsula quickly and emerged into the Atlantic near the Florida border that afternoon. Eta merged with a front early the next morning off the coast of the Carolinas and became extratropical. Persisting for nearly 13 days, the storm was unusually long-lived for a November tropical cyclone.

The above image shows an infrared image of Eta near its peak intensity on November 3. Notice the remarkably large and intense central dense overcast around the eye.

Eta took an unusual winding track through the Caribbean and Gulf of Mexico due to alternating influences from ridges and troughs.

Saturday, October 24, 2020

Hurricane Zeta (2020)

Storm Active: October 24-29

On October 19, a trough of low pressure developed over the southwestern Caribbean, extending from north of Panama all the way toward the western tip of Cuba. There were some showers associated with the system, but they mostly lay to the east. The disturbance crawled westward for a day or two and then stalled. Meanwhile, upper-level winds were becoming a bit more favorable. Finally, a low pressure center formed early on October 23 west of Jamaica. Gradual consolidation continued, culminating in the designation of Tropical Depression Twenty-Eight the next day.

The depression was nearly stationary. The deepest convection and the mid-level center were southeast of the surface center and actually retreated a bit further south early on October 25. At the same time, the storm strengthened into Tropical Storm Zeta, the 27th named storm of the 2020 Atlantic hurricane season. The only previous season with a 27th storm was 2005, for which it was Epsilon (due to an unnamed storm), forming November 29th of that year. It was the second time "Zeta" was used, again after 2005. Zeta wasn't particularly well organized, but had some extremely strong storms in the southern semicircle. Winds in that region increased some during the day and Zeta strengthened. The outermost of these brought some rain to Honduras. That afternoon, the center of circulation reformed nearer to the mid-level center. All these structural changes notwithstanding, Zeta had scarcely moved since its formation.

By October 26, a ridge building in to the north finally got the storm moving toward the northwest. At first, the center outran the central dense overcast a little bit, but relaxing shear and high oceanic heat content allowed Zeta to come back with a vengeance later that day. It quickly strengthened into a hurricane, reaching an intensity of 80 mph winds and a pressure of 977 mb. That night, it made landfall in the northern Yucatan peninsula, in nearly the same location that Gamma and Delta had earlier in the month.

Zeta weakened over land to a tropical storm before emerging into the Gulf of Mexico during the morning of October 27. The storm had lost much of its central convection, but it redeveloped quickly that day. By the evening, a well-defined eyewall had appeared, beginning another period of intensification. Soon, Zeta was a hurricnae again. A powerful extratropical storm dipping into the Rocky Mountains began to influence the cyclone's track. It turned northward on October 28 as it approached the Gulf coast. Zeta's eye opened up a little that morning as well; rapid intensification brought it up to its peak intensity as a category 3 major hurricane with 115 mph winds and a minimum central pressure of 970 mb. It made landfall with these winds in southeastern Lousiana that afternoon. Note that, operationally, Zeta was classified as a category 2 hurricane, but it was upgraded to a category 3 in post-season analysis.

Remarkably, Zeta was the second major hurricane, third hurricane and fifth tropical cyclone of 2020 to make landfall in Louisiana. It also joined a list of six hurricane landfalls in the continental United States, tying a record set in 1985 for the most recorded in a season. Zeta's upgrade marked the first time three major hurricanes were ever recorded in October, and the latest in the year so far that a major hurricane had made landfall in the continental U.S. By that time, the storm was moving quickly northeast. Therefore, rains were limited, but water rose quickly and the New Orleans area experienced very strong winds and widespread power outages. Hurricane force winds also spread far inland due to Zeta's speed: the center moved crossed into Mississippi and then Alabama before it weakened to a tropical storm. It rocketed across the mid-Atlantic on the 29th and became extratropical that afternoon.

The above photo shows Zeta just before landfall in Lousiana.

Zeta's path through the Caribbean and Gulf of Mexico was extremely similar to that of several other cyclones of the 2020 season.

Monday, October 19, 2020

Hurricane Epsilon (2020)

Storm Active: October 19-25

Early on October 16, a non-tropical low pressure system formed over the central tropical Atlantic, well to the east-southeast of Bermuda. The system moved little over the next few days, but gradually deepened. By October 19, thunderstorm activity had increased near the center. Advisories were initiated on Tropical Depression Twenty-Seven that morning. Initially, the center was exposed due to westerly shear, but conditions soon improved and the system strengthened. It was named Tropical Storm Epsilon a few hours later. Epsilon was the 26th named storm of 2020. 2005 was the only other year to see a 26th named storm (Delta, which formed on November 22 of that year). The name "Epsilon" was also used that year, for a cyclone which was named on November 29th.

Initially, the cyclone moved little, but early on October 20 a ridge built in and began pushing Epsilon northwest. The storm sat in a decidedly mixed environment: water temperatures where warm and there was plentiful atmospheric instability, but wind shear was still affecting the storm, and there were pockets of dry air in the circulation. Nevertheless, it fared well; a central dense overcast developed that morning, and an eye feature appeared intermittently later in the day. Accompanying these structural improvements was a corresponding increase in winds, and Epsilon rapidly intensified into a hurricane that evening.

The storm turned toward the west overnight and continued its intensification trend. The eye cleared out further and cloud tops in the small eyewall cooled. Aircraft reconaissance arriving during the afternoon of October 21 found that Epsilon was remarkably on the verge of major hurricane strength. A few hours later, it reached category 3 intensity, with peak winds of 115 mph and a central pressure of 951 mb. After a wobble, the center then assumed a north-northwest heading. The hurricane was running out of warm waters to traverse as it gained latitude, and, inevitably, began to decay. On October 22 it dropped back to category 1 intensity, though its satellite presentation was still impressive. Epsilon made its closest approach to Bermuda that afternoon, but was almost 200 miles east; gale force winds impacted the island, but little rain.

The hurricane followed a typical trajectory as it moved further into the mid-latitudes, recurving north and east and accelerating as it went. It underwent some normal structural changes too: the windfield broadened, the core became less compact, and the cyclone as a whole became asymmetric. Nevertheless, Epsilon maintained category 1 hurricane status through for the next few days. High surf and rip currents affected the northeast United States and Atlantic Canada. On October 25, the storm reached a forward speed of over 40 mph; by that time, it was north of 45 ° N, and the circulation was elongating. Satellite estimates indicated that Epsilon weakened to a tropical storm. Nevertheless, it maintained enough deep convection to be classified as tropical through the evening, when it finally completed extratropical transition. The next day, the remnants were absorbed by another powerful low.

The above image shows Epsilon near peak intensity over the open Atlantic. The storm exhibited a relatively small core inside a large mesoscale circulation; this structure is common for hurricanes in the subtropics.

Epsilon's path took it past the island of Bermuda well to the east, so it caused only minor land imapcts.

Sunday, October 4, 2020

Hurricane Delta (2020)

Storm Active: October 4-10

Around October 1, a tropical wave entered the Caribbean. Associated thunderstorms were vigorous, but unorganized, and the disturbance continued west-northwestward. Within a few days, a low pressure center developed in association with the wave and it steadily consolidated. Late on October 4, Tropical Depression Twenty-Six was designated a little south of Jamaica. The depression was entering the western Caribbean, where it found nearly ideal conditions for development: the highest oceanic heat content in the Atlantic basin, low shear, and high humidity. It became Tropical Storm Delta, the 25th named storm of 2020, on October 5. This handily beat Gamma of 2005, the 25th storm of that season, which was named on November 15 of that year (a storm identified in post-season analysis in 2005 accounts for the 25th being Gamma, not Delta). It was also the second time the Delta named was used, after 2005.

Vigorous convection wrapped around the cyclone's center soon after it was named and rapid strengthening began; even a few dry slots did not slow it down for long. Delta became a hurricane that very evening. The cyclone was very compact, with a small central dense overcast. Quick deepening continued on October 6: the storm incredibly became a category 4 by midday, reaching its peak intensity of 145 mph winds and a pressure of 956 mb. The core was so compact that no eye was apparent on visible satellite imagery; nevertheless, analysis indicated there was a pinhole eye, only a few miles in diameter. Meanwhile, Delta sped up a bit toward the west-northwest.

The cyclone continued to exhibit curious behavior the next morning: convection was extraordinarily deep near the center, but the eyewall appeared to collapse, weakening the storm. As a result, Delta lost some steam before making landfall in the Yucatan Peninsula as a category 3 hurricane that night. The storm weakened further over land, but emerged back into the Gulf of Mexico by that afternoon. By this time, the hurricane was rounding the edge of a subtropical ridge, and turned more toward the north. The southern Gulf of Mexico was still quite favorable for development and the core reorganized. After bottoming out at category 1, Delta was on the rise again by late on the 7th.

An eye finally cleared out on October 8 as the storm vaulted back up to major hurricane intensity. Delta reached a secondary peak of 120 mph winds and a pressure of 953 mb (its lowest yet) that night, but it soon entered a region of smaller oceanic heat content in the northern Gulf. Simultaneously, a fall frontal system increased southwesterly shear drastically. This fortunately weakened the storm on October 9th. It turned north-northeast and then made landfall in western Lousiana that afternoon as a category 2. Though top winds were down, Delta brought strong winds, storm surge, and flooding rains to the same region that Hurricane Laura had devastated just months earlier. In addition, the storm was the 10th of the season to make landfall in the United States, the most on record.

Rapid weakening ensued once the storm moved inland, toward the northeast. It weakened to a remnant low by the morning of October 10 over western Mississippi. The remnants moved across the eastern U.S., eventually bringing downpours across the mid-Atlantic region a few days later.

The image above shows Delta as it regained major hurricane strength in the Gulf of Mexico on October 8.

Delta was yet another hurricane in the Gulf of Mexico, and a second hurricane landfall for western Lousiana alone.

Friday, October 2, 2020

Hurricane Gamma (2020)

Storm Active: October 2-5

During the last week of September, a tropical wave entered the Caribbean sea and moved westward, bringing precipitation to the Windward Islands as it passed by. A few days later, a broad low-pressure formed in association with the system over the western Caribbean. The nascent circulation received some help from a Central American gyre (CAG), a seasonal monsoonal broad low that tends to persist over the region, especially in May-June and October-November. By October 1, a large-scale swirl was evident, but there wasn't much convection near the center. However, on October 2, the system became well-defined enough to be classified Tropical Depression Twenty-Five well north of the coastline of Honduras.

The depression moved slowly northwest and strengthened into Tropical Storm Gamma that night. Gamma was only the second instance of a 24th named storm in recorded history after Beta of 2005, which was named on October 27 of that year (an unnamed subtropical storm in 2005 was the reason the 24th named storm was Beta, not Gamma). Conditions were very favorable for intensification and the system strengthened quickly as it approached the Yucatan peninsula. The next morning, Gamma was reported as reaching its peak intensity of 70 mph winds and a minimum pressure of 980 mb before making landfall in the northeastern Yucatan around noon local time.

Though the above intensity was the operational peak observed for Gamma, a more careful post-season analysis found that the storm in fact achieved category 1 hurricane status just before landfall, around 16:45 UTC October 3. The revised intensity was 75 mph winds and a pressure of 978 mb.

Land did not weaken the storm much; in particular, flooding rains continued as the center moved slowly north-northwestward. Overnight, Gamma emerged into the Gulf of Mexico. Waters were still warm, but shear increased, halting any reintensification by later on October 4. At the same tame, the trough that had been lifting the cyclone north moved on, leaving Gamma trapped under a weak developing ridge. As a result, the cyclone began meandering. Overnight, shear and dry air removed nearly all thunderstorm activity from the circulation and the storm quickly weakened. It also retreated southwest, back toward land, and weakened to a tropical depression on October 5. Soon, it became a remnant low. Before long, the low was absorbed by the approaching Hurricane Delta.

The above image shows Gamma at peak intensity on October 3, just before making landfall in the Yucatan Peninsula.

Unfavorable conditions prevented Gamma from strengthening further in the Gulf of Mexico; it instead took an unusual path and made a second landfall in the Yucatan.