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. NWS MIAMI....Fri Oct 2 2020
    ...Additional Flooding Possible Into This Weekend...
    This hazardous weather outlook is for South Florida.
    Flooding: flooding of urban and poorly drained areas is possible,
    especially along the east coast where more impactful flooding is
    possible. A Flood Watch is in effect for the Atlantic metro and
    coastal areas.
    Thunderstorms: possible through this evening with greatest chance
    over the east coast metro areas.
    Coastal Flooding: in addition to flooding from heavy rainfall, flooding
    is also possible near high tide cycles along the Atlantic coast.
    Waterspouts: a few waterspouts are possible over the local waters.
    The flooding potential across the region may increase into this
    weekend, especially for the eastern metro and coastal areas due to
    the combination of heavy rainfall and high astronomical tides. There
    is uncertainty in how widespread and impactful this flooding may be,
    so keep up with forecast changes.
    Widespread spotter activation is not anticipated, however individual
    spotters are encouraged to report winds and flooding to the National
    Weather Service forecast office in Miami. Please relay any information
    about observed severe weather to the NWS while following all local,
    state, and CDC guidelines.For more information...visit the National
    Weather Service in Miami website at www.weather.gov/miami.
    For more information...visit the National Weather Service in
    Miami website at www.weather.gov/miami.
  2. Flood Watch
    National Weather Service Melbourne FL
    311 AM EDT Fri Oct 2 2020
    St. Lucie-Martin-
    Including the cities of Port Salerno, Jensen Beach, Port Saint
    Lucie, Fort Pierce, Stuart, and Hobe Sound
    311 AM EDT Fri Oct 2 2020
    The Flood Watch is now in effect for
    * A portion of east central Florida, including the following areas,
      Martin and St. Lucie.
    * Through Saturday afternoon
    * Heavy showers and isolated lightning storms will return to Martin
      and St Lucie counties today, aggravating the flooding that occurred
      on Thursday across the coastal communities of these counties.
      Several locations along these counties reported accumulations of
      rainfall of 9 inches or higher. Additional heavy rain of 1 to 2
      inches per hour is expected today and tonight with accumulations of
      2 to 4 inches with isolated higher amounts possible through tonight.
      Additional flooding will be possible especially in urban, low lying,
      and already saturated areas.
    Do not enter or cross flowing water or water of unknown depth.
    A Flood Watch means there is a potential for flooding based on
    current forecasts. You should monitor later forecasts and be alert
    for possible flood warnings. Those living in areas prone to flooding
    should be prepared to take action should flooding develop.
  3. How did you east coast of Florida folks make out yesterday with over 6 inches of rain or more?…any bad flooding etc?

  4. HWRF, GFS, and GFS Para send the center of 91L into the central gulf and then turn it west towards the western gulf; these models, especially the HWRF separate the Mid level circulation from the Low Level Circulation, due to strong shear that can be seen looking at the mid levels on the models. The Mid Level Circulation accelerates NE towards the Florida Peninsula, and that combined with the moisture and the front bring bands of heavy rain across Florida Sunday and Monday. The center tracks west and encounters more favorable conditions in the Western Gulf, potentially intensifying and threatening areas in the Western Gulf. Quite a bit of rain on the way for the Florida Peninsula, especially central and south. 3-4 inches with higher amounts localized. Parts of the SE Coast of FL have already seen a lot of Flooding from yesterday’s deluge.

  5. 2 more invests coming soon. 92L in the central caribbean and a very impressive TW near 8n 31w to be 93L. The wave in the eastern atlantic is below the westerly shear and should be in an area conducive for development for the next few days

  6. gee up to 6 inhes already fallen wow..be safe east coasters………………………BULLETIN – IMMEDIATE BROADCAST REQUESTED
    Flood Warning
    National Weather Service Melbourne FL
    257 PM EDT Thu Oct 1 2020

    Martin FL-St. Lucie FL-
    257 PM EDT Thu Oct 1 2020

    The National Weather Service in Melbourne has issued a

    * Flood Warning for…
    Northeastern Martin County in east central Florida…
    Eastern St. Lucie County in east central Florida…

    * Until 600 PM EDT.

    * At 257 PM EDT, Doppler radar and automated rain gauges indicated
    heavy rain due to thunderstorms. Flooding is already occurring in
    the warned area. Up to 6 inches of rain have fallen so far today.
    An additional 1 to 3 inches are expected through 6pm.

    Some locations that will experience flooding include…
    Port Saint Lucie, Walton, Fort Pierce, White City, Jensen Beach,
    Indian River Estates, Nettles Island, North River Shores,
    Hutchinson Island South, and Waveland.


    A Flood Warning means that flooding is imminent or occurring. All
    interested parties should take necessary precautions immediately.

    Turn around, don`t drown when encountering flooded roads. Most flood
    deaths occur in vehicles.


    LAT…LON 2748 8043 2749 8031 2750 8026 2748 8025
    2747 8025 2741 8023 2726 8016 2726 8023
    2726 8024 2727 8023 2726 8028 2721 8029
    2721 8033


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