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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 155 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. On top of this, 2020 became the first hurricane season ever with two major hurricanes in November.

*Note: Iota was operationally classified to have peaked as a category 5 hurricane with maximum winds of 160 mph; however, post-season analysis indicated that the surface wind estimates from aircraft reconnaissance may have been a bit too high, prompting a lowering of estimated peak winds to 155 mph. Though a minor change, this means Iota is no longer considered to have attained category 5 intensity.

Iota maintained top-end category 4 status for some of the day, but underwent another eyewall replacement cycle during the afternoon, which weakened it a bit 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 4 intensity on November 16.


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.