Precipitation forecast
Rainfall outlook (in inches) for the week-long period from 2 am EDT (6Z) Tuesday, September 29, 2020, through 2 am EDT Tuesday, October 6, from the 6Z Tuesday, September 29, run of the GFS model. The model predicted rainfall amounts in excess of 12 inches (yellow-orange colors) for portions of Mexico and Central America. (Image credit: Tropical Tidbits)

The enjoyable lull in Atlantic tropical cyclone activity over the past week may come to an end this weekend, when a large low-pressure system that is expected to develop in the western Caribbean has a 50% chance of spawning a tropical depression, according to the National Hurricane Center.

A large, complex area of low pressure called the Central American Gyre (CAG) will develop in the western Caribbean late this week, generating heavy rain as it moves slowly west-northwest. CAGs tend to develop near the beginning and end of the Atlantic hurricane season, and they can sprawl over hundreds of miles. They are notoriously long-lasting and slow-moving, making them prodigious rain producers. For more background, Phillipe Papin is an expert on CAGs (click here to animate his tweet, shown right), and he maintains an excellent CAG forecast page.

As smaller-scale vortexes spin around the CAG, one or more of them can consolidate into a tropical cyclone and eventually break away from the gyre. That outcome can happen in either the Atlantic or the Pacific. For example, on May 31, the first named storm of the Eastern Pacific hurricane season, Tropical Storm Amanda, developed from a CAG, and it made landfall on the south coast of Guatemala later that day. In October 2018, after a week of gestation, a disturbance on the north end of a CAG became a tropical depression in the western Caribbean. Three days later, that depression had detached from the gyre and barreled into the Florida Panhandle as mighty category 5 Hurricane Michael.

The exact location and timing of any tropical cyclones that may develop from this week’s CAG cannot yet be predicted with much probability of success. If the CAG does manage to spawn a hurricane in the coming week, the most likely location for a storm would be in the waters of the extreme northwest Caribbean, near western Cuba and the northeast Yucatan Peninsula. It is more likely that a tropical depression or weak tropical storm would form farther to the south, as predicted by the 6Z Tuesday run of the GFS Ensemble Forecast System (Figure 1).

Figure 1. Predictions for 6Z (2 a.m. EDT) Saturday, October 3, from the 6Z Tuesday, September 29, run of the recently updated and expanded GFS ensemble forecast system (GEFS), of the locations of surface low- and high-pressure systems. (Image credit: Tropical Tidbits)

In that figure, predicted locations of centers of low pressure are shown as orange numbers in millibars, with the leading “10” or “9” omitted, depending on whether the low’s central pressure were above or below 1000 mb. For example, a 999-mb low pressure center will be displayed as “99”, and a 1000-mb low pressure system will be displayed as “00”. In blue numbers, with the leading “10” left off, are the predicted locations of centers of high pressure. For example, a 1020-mb high pressure system will be coded as “20”. Each of the 31 forecasts from the individual members generated a different location and central pressure for major high- or low-pressure systems. The color-coding is a measure (in standard deviations) of ensemble spread – the difference in pressure between the ensemble mean and the individual member. Six ensemble members predicted a hurricane-strength system with a pressure of 985 mb or lower in the western Caribbean on Saturday. These were color-coded orange, showing a high spread from the ensemble mean, since their central pressure differed greatly from the mean predicted pressure.

Figure 2. Departure of sea surface temperature (SST) from average on September 29. SSTs were approximately 1.0 degrees Celsius (1.8°F) above average in the Caribbean. (Image credit: Tropical Tidbits)

Even busy hurricane seasons have extended lulls

Even the busiest hurricane seasons have lulls. For example, during the record-busy 2005 Atlantic hurricane season, Irene was named on August 7, but the next storm, Jose, didn’t get named until August 22.

Lulls like these are usually caused by sinking air over the tropics, leading to drying, high pressure, and reduced odds of tropical storm formation. Sinking air is caused by an unfavorable state of the Madden Julian Oscillation (MJO), or by the suppressed phase of an atmospheric disturbance called a convectively coupled Kelvin wave (CCKW). The MJO is a pattern of increased thunderstorm activity near the equator that moves around the globe in 30 to 60 days; odds of tropical cyclone formation increase when the MJO is strong and located in the proper location, but typically decrease for ocean basins not in the active portion of the MJO. Similarly, a CCKW is a large but subtle atmospheric impulse, centered on the equator, that rolls eastward at 30-40 mph, with showers and thunderstorms along its forward flank. On either side of this center of action, sinking air, high pressure, and reduced odds of tropical storm formation typically occur.

Passage of the suppressed phase of a CCKW and an unfavorable MJO have been acting to dampen Atlantic tropical cyclone activity since last week, but that situation will change by mid-October. With ocean temperatures still much above average (Figure 2) and a season that has a proven track record for spitting out large numbers of named storms, we likely will see at least three named storms form in October. As discussed above, there is even a chance of one spinning up this weekend in the western Caribbean, despite the relatively unfavorable MJO.

Figure 3. In this velocity potential (VP) anomaly plot* from September 29, the MJO brings positive 200 mb VP anomalies (yellow colors) and reduced odds of tropical cyclone formation to the Caribbean (top). However, by mid-October, this pattern is predicted to shift, with negative 200 mb VP anomalies (blue colors) over the Caribbean. (Image credit: Michael Ventrice).

Also see: August 2020 was the world’s second-warmest August on record, NOAA reports

*The amount of rising or sinking air can be inferred from the 200 mb velocity potential (VP) departure from average (also called the anomaly). Negative 200 mb VP anomalies mean that upper-level winds at the 200 mb level are diverging, causing rising air from below to replace the air diverging away at high altitudes. This rising air helps nurture thunderstorm updrafts, and favors low pressure and increased chances of tropical cyclone formation. Conversely, positive 200 mb VP anomalies imply converging air aloft, where sinking air, high pressure, and dry conditions will be unfavorable for tropical cyclone formation. In this plot, negative 200 mb VP anomalies (divergence) are cool-colored contours (the scale shows the departure from average in standard deviations); positive 200 mb VP anomalies (convergence) are warm-colored contours.

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Posted on September 29, 2020 (2:31pm EDT).

Jeff Masters

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...

76 replies on “At least three named Atlantic storms likely during October”

    1. I have been watching that but dont see any organisation, yet, your eyes better than mine.

  1. well NWS is saying Alot of uncertainty with the tropical situation in the next few one knows what tropical trouble will come and where it goes..stay alert and safe all around the gulf huh

  2. Tropical Weather Outlook
    NWS National Hurricane Center Miami FL
    200 AM EDT Thu Oct 1 2020
    For the North Atlantic...Caribbean Sea and the Gulf of Mexico:
    1. Showers and thunderstorms located over the west-central Caribbean 
    Sea are associated with a tropical wave. A broad area of low 
    pressure is expected to form in a day or so over the northwest 
    Caribbean Sea or the extreme southern Gulf of Mexico in the vicinity 
    of the wave as it moves slowly west-northwestward. Conditions are 
    forecast to be conducive for development thereafter in that region, 
    and a tropical depression could form over the weekend as the 
    system meanders. Interests in Belize, the Yucatan Peninsula, and 
    western Cuba should monitor the progress of this disturbance.
    * Formation chance through 48 hours...low...30 percent.
    * Formation chance through 5 days...high...70 percent.
    2. Another tropical wave located a couple hundred miles east of the 
    Lesser Antilles is producing widespread cloudiness and disorganized 
    shower activity.  This disturbance is forecast to move westward 
    during the next several days where environmental conditions could 
    become a little more conducive for development over the central or 
    western Caribbean Sea by early next week.
    * Formation chance through 48 hours...low...near 0 percent.
    * Formation chance through 5 days...low...20 percent.
    Forecaster Latto
  3. the heat wave arrived in clairemont, san diego two days behind schedule, but it’s here now. still kinda warm at 10:38 PDT.


    Tropical Weather Outlook
    NWS National Hurricane Center Miami FL
    200 PM EDT Wed Sep 30 2020

    For the North Atlantic…Caribbean Sea and the Gulf of Mexico:

    1. A tropical wave located over the west-central Caribbean Sea is
    expected to move westward over the next couple of days, and produce
    a broad area of low pressure over the western Caribbean Sea by
    Thursday night or Friday. Environmental conditions are forecast to
    be conducive for some development thereafter, and a tropical
    depression could form over the weekend while the system moves slowly
    west-northwestward over the northwestern Caribbean Sea.
    * Formation chance through 48 hours…low…10 percent.
    * Formation chance through 5 days…medium…60 percent.

    Forecaster Pasch

  5. Convection in the Caribbean has increased tremendously over the past 12 hours. Environmental conditions are relatively conducive for tropical development. This broad area of moisture is moving west and in my opinion we might have something in the next day or two.

  6. Hydrologic Outlook
    Hydrologic Outlook
    National Weather Service Miami FL
    329 AM EDT Wed Sep 30 2020
    A cold front will stall over South Florida late this week before
    slowly moving northward through South Florida and into Central
    Florida this weekend, as several short waves move across the
    region from the Gulf of Mexico. At this time, a coastal trough
    could set up along the east coast metro areas, as a possible
    tropical low develops in the Northwestern Caribbean Sea. This
    will allow for the PWAT values to increase to 2.1 to 2.4 inches
    over South Florida late this week into this weekend.
    This weather pattern will allow for showers and some thunderstorms
    to develop with periods of heavy rainfall with rainfall amounts in
    excess of 5 inches especially over the eastern areas of South
    Florida. The grounds are still somewhat saturated from the heavy
    rainfall from the daily thunderstorms and any additional heavy
    rainfall could lead to flooding especially over the eastern areas
    of South Florida.
    Interests across the eastern portion of South Florida are
    strongly encouraged to monitor the latest information from the
    National Weather Service in Miami Florida.
  7. While the models send the storm center west through Mexico and the Yucatan, the models unanimously send the heavy moisture across the southern to central florida peninsula; look at the IR simulations on the GFS for example. So we may not get the center in Florida, but we will get some rounds of squalls of heavy rain and some wind.

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    great read YCC!

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