91L satellite image
GeoColor satellite image of 91L over the Caribbean at 11 a.m. EDT Thursday, October 1, 2020. (Image credit: RAMMB/CIRA/Colorado State University)

A tropical wave over the west-central Caribbean, designated Invest 91L by the National Hurricane Center (NHC) on Thursday morning, is likely to develop into a tropical depression or into Tropical Storm Gamma as early as Saturday. This system is a heavy rain threat to Mexico’s Yucatan Peninsula, Belize, and western Cuba.

Conditions for development of 91L were favorable on Thursday, with sea surface temperatures (SSTs) near 30.5 degrees Celsius (87°F), light wind shear less than 10 knots, and a moist atmosphere with a mid-level relative humidity of 80%. Satellite images showed that 91L’s heavy thunderstorm activity was steadily increasing in intensity, areal coverage, and organization as the system moved west-northwest at about 10 mph. An Air Force hurricane hunter aircraft is scheduled to investigate 91L on Friday afternoon.

Figure 1
Figure 1. Track forecasts out to 10 days for the Gulf of Mexico from the 0Z Thursday, October 1, run of the ensemble forecast of the European model. Most of the thin lines (color-coded by pressure) from the 51 individual members predicted that 91L would make landfall over the Yucatan Peninsula and remain trapped in the southwestern Gulf of Mexico. (Image credit: weathermodels.com)

Forecast for 91L

The ridge of high pressure steering 91L should keep it moving to the west-northwest through Sunday, a track on which all of the major computer models agree. This motion should bring 91L to the coast of Mexico’s Yucatan Peninsula on Saturday, though it is possible the system could slide just north of Cancun and pass through the Yucatan Channel between Cuba and Mexico. It appears unlikely that 91L will be a threat to the U.S. Gulf Coast next week. In a 2 p.m. EDT Thursday Tropical Weather Outlook, NHC gave 91L 2-day and 5-day odds of development of 40% and 70%, respectively.

The track of 91L beyond this weekend will be complicated because it is embedded in a large area of low pressure over Central America called a Central American Gyre (CAG). Further complications may arise as another tropical wave now moving into the Lesser Antilles Islands arrives in the western Caribbean next week. In a 2 p.m. EDT Thursday Tropical Weather Outlook, NHC gave this second tropical wave two-day and five-day odds of development of 0% and 20%, respectively.

Also see: At least three named Atlantic storms likely during October

Conditions currently favoring development of 91L will become more hostile on Sunday, when the system will encounter a large trough of low pressure extending from Mexico into the southeastern U.S., associated with a cold front that moved through the Gulf of Mexico early this week. This trough will bring high wind shear of 20 – 30 knots to 91L Sunday through Tuesday. In addition, plenty of dry air over the Gulf of Mexico will interfere with development, as will land interaction with the Yucatan Peninsula. Because of these obstacles, the models are not bullish that 91L will achieve hurricane strength.

Figure 2
Figure 2. Rainfall outlook (in inches) for the five-day period from 2 a.m. EDT (6Z) Thursday, October 1, 2020, through 2 a.m. EDT Tuesday, October 6, from the 6Z Thursday, October 1, run of the GFS model. The model predicted rainfall amounts in excess of 12 inches (yellow-orange colors) for portions of Mexico from 91L. (Image credit: Tropical Tidbits)

Heavy rains possible in Florida

The trough of low pressure that 91L will encounter this weekend will also act to shunt tropical moisture from the system to the northeast over Florida. This is a favorable setup for what is referred to as a Predecessor Rain Event (PRE), which could bring heavy rain and flooding to parts of central Florida.

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Editor’s note: this post was updated at 2 p.m. EDT October 1 to include the latest forecast information from NHC.

Posted on October 1, 2020 (12:51 p.m.EDT).

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 “Tropical Storm Gamma may form as soon as Saturday”

  1. Broad circulation evident on the visible satellite for 91L (soon to be TD25)…..animated loop….

    1. Skye – I didn’t want to take away from the tribute to Dr. McFadden (even though I’m sure he would love all the weather discussion) so I kept my weather posts to this blog….

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