Theta satellite image
Visible satellite image of Subtropical Storm Theta at 10:35 a.m. EST November 10, 2020. (Image credit: Tropical Tidbits)

A record many thought unassailable has fallen: the Atlantic named storm record of 28 storms in a season, set during the hellish hurricane season of 2005. On Monday night, the ridiculously prolific 2020 hurricane season spawned its 29th named storm, Subtropical Storm Theta, beating the old record from 2005. There is still plenty of time for 2020 to add to its record tally, and a tropical wave in the Caribbean is likely to become the 30th named storm of the season, Tropical Storm Iota, by this weekend. The average tally of Atlantic named storms in a season is just 12.

Figure 1
Figure 1. The record Atlantic hurricane season of 2020, compared to 2005 and to average. One of 2005’s storms was recognized only in post-season analysis and was never named, so both Eta and Theta were used in 2020 for the first time. (Image credit: Sam Lillo)

Theta’s formation, in conjunction with Tropical Storm Eta’s continued presence in the Gulf of Mexico, gave the Atlantic two simultaneous named storms on November 10 – a very rare occurrence in November. According to Dr. Phil Klotzbach of Colorado State University, the last time the Atlantic experienced two named storms this late in the year was on November 10, 1932. The latest in the calendar year that the Atlantic hurricane season has had two named storms simultaneously was in December 1887.

Theta little threat to land

At 10 a.m. EST Tuesday, Theta was located about 860 miles southwest of the Azores Islands, headed east at about 15 mph. Theta had top sustained winds of 70 mph, just below hurricane strength. Satellite images showed Theta still had characteristics of a subtropical storm, but it is expected to become fully tropical by Tuesday night.

Theta had seemingly unfavorable conditions for development, with sea surface temperatures (SSTs) near 25 degrees Celsius (77°F), high wind shear of 50 – 60 knots, and a dry atmosphere with a mid-level relative humidity of 45%. However, Theta had highly favorable levels of atmospheric instability, created by very cold temperatures in the upper atmosphere (minus 58 degrees Celsius at 200 mb, about five degrees Celsius colder than what Tropical Storm Eta is currently experiencing). This mix of factors often is found in late-season tropical storms.

Bob Henson tweet

Theta will move in an east to east-northeast motion through the end of the week, taking the storm between the Azores Islands and Canary Islands toward Portugal (see the Tweet from Bob Henson, with last night’s forecast track for Theta). The only land area in Theta’s five-day cone of uncertainty is Portugal’s Madeira Island, which could experience tropical storm conditions on Saturday and Sunday. High wind shear and cold waters are expected to weaken Theta to a post-tropical cyclone with 40 mph winds by Sunday.

Not one Iota more, please!

A tropical wave in the eastern Caribbean (designated 98L by the National Hurricane Center on Tuesday morning), is expected to merge with a larger area of low pressure in the central Caribbean late this week. The top three models for predicting tropical cyclone genesis were giving 98L strong support for development by this weekend, when the wave will likely be in the central or southwestern Caribbean, between Jamaica and Nicaragua. The wave is predicted to move westward to west-northwestward under the steering influence of a ridge of high pressure to its north, resulting in a potential threat to Jamaica, the Cayman Islands, and Central America this weekend. Conditions for development will mostly be favorable this weekend, with the SHIPS model predicting moderate wind shear of 10 – 20 knots and warm SSTs of 29 degrees Celsius (84°F). However, some dry air in the western Caribbean may interfere with development.

In a 1 p.m. EST Tuesday Tropical Weather Outlook, NHC gave 98L two-day and five-day odds of development of 10% and 70%, respectively. The next name on the Atlantic list of storms is Iota, the ninth letter in the Greek alphabet.

Figure 2
Figure 2. GeoColor satellite image of the Atlantic at 10:20 a.m. EST Tuesday, November 10, 2020. (Image credit: RAMMB/CIRA/Colorado State University)

Eta meandering in the Gulf of Mexico

Tropical Storm Eta, which made landfall in the Florida Keys with sustained winds of 65 mph on Sunday night, has parked itself over the southeastern Gulf of Mexico, just northwest of the western tip of Cuba. At 10 a.m. EST Tuesday, the nearly stationary Eta had top winds of 60 mph and a central pressure of 992 mb.

Rainbands from Eta set up over portions of southern and central Florida over the past three days, resulting in torrential rains that caused widespread urban flooding. The heaviest rains, in excess of a foot, fell in Broward County, where Fort Lauderdale is located. Some top 72-hour rainfall amounts reported by the National Weather Service for Florida, as of 7:41 a.m. EST Tuesday, November 10:

16.04 inches, Miramar;
12.27 inches, Plantation;
9.53 inches, Fort Lauderdale;
8.16 inches, Pembroke Pines;
6.87 inches, Miami; and
5.96 inches, Miami Beach.

Eta is expected to bring an additional 1-2 inches of rain to South Florida though Wednesday, and 3-5 inches to western Cuba.

Forecast for Eta

Also see: How climate change is making hurricanes more dangerous

Eta will have mostly favorable conditions for intensification through Thursday, with moderate wind shear of 10-15 knots and SSTs near 28.5 degrees Celsius (83°F). However, Eta is embedded in a dry atmosphere with a mid-level relative humidity of 50%, and this dry air will potentially put the brakes on any rapid intensification.

Figure 3
Figure 3. Track forecasts out to six days for Eta from the 6Z (1 a.m. EST) Tuesday, November 10, run of the GFS ensemble model (GEFS). The black line is the mean of the 31 ensemble members; individual ensemble member forecasts are the thin lines, color-coded by the central pressure they predicted for Eta. The long-term motion of Eta was highly uncertain. (Image credit: Tropical Tidbits)

Steering currents are weak over the Gulf of Mexico, but a trough of low pressure passing to the north of Eta is expected to begin pulling the storm to the north by Tuesday night, with the storm continuing a slow northerly motion at less than 5 mph through Friday. Beyond Friday, there is much disagreement among the models on the future track of Eta, and the default cone of uncertainty depicted on the official forecast is likely too small. The GFS model, which keeps Eta weak, suggests the storm will move mostly westward with the low-level flow. The European model predicts that Eta will be stronger, and move more to the north, steered more by the upper-level winds. In either case, Eta will be encountering cooler waters and higher wind shear late this week and continued incursions of dry air. These unfavorable conditions should cause weakening, and Eta may become a tropical depression by Sunday.

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Posted on November 10, 2020 (2:35pm ET).

Jeff Masters, Ph.D., worked as a hurricane scientist with the NOAA Hurricane Hunters from 1986-1990. After a near-fatal flight into category 5 Hurricane Hugo, he left the Hurricane Hunters to pursue a...

33 replies on “Theta forms in the Atlantic, breaking record as 29th named storm of the 2020 Atlantic hurricane season”

  1. My latest birdseye view chart and post of the Atlantic tropics is up at this link. Explanation on how Eta’s acceleration back towards Florida may have been triggered earlier and more strongly than expected. My prayers continue to be for Honduras and Nicaragua as well due to how badly Eta affected the region, and unfortunately we have another Caribbean tropical wave that could develop and hit the region just after 5 days from now.

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