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. well we have about 10 days to wait and see but myself im going over my supplies and prep list, things i might Have to do to protect my house etc..its great i Have to the time before hand to go over all this..hopefully it stays away from Florida..lets see how this plays out..perhaps all the gulf area’s might want to wait and see also huh…be safe

    1. Always good to be prepared this time of year. Here in Dallas, I always have a tornado bag packed in the hallway where we hunker down when the storms get rough. I’ve never had to actually make use of the supplies I keep in it, but it sure helps me sleep better knowing it’s there.

    Tropical Weather Outlook
    NWS National Hurricane Center Miami FL
    200 AM EDT Wed Sep 30 2020
    For the North Atlantic...Caribbean Sea and the Gulf of Mexico:
    1. A tropical wave located over the central Caribbean Sea is expected 
    to move westward to west-northwestward over the next few days and 
    interact with a frontal system, producing 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 Brown
  3. Japan Meteorological Agency
    Tropical Cyclone Advisory #31 – 9:00 AM JST September 29 2020
    Sea Far East of Japan

    At 0:00 AM UTC, Severe Tropical Storm Kujira (985 hPa) located at 39.8N 160.7E has 10 minute sustained winds of 55 knots with gusts of 80 knots. The cyclone is reported as moving northeast at 24 knots.

    Storm Force Winds
    50 nm from the center in southeastern quadrant
    30 nm from the center in northwestern quadrant

    Gale Force Winds
    180 nm from the center in southeastern quadrant
    130 nm from the center in northwestern quadrant

    Dvorak Intensity: T3.5-

    Forecast and Intensity
    12 HRS: 41.3N 166.0E – Extratropical Low in Sea far east of Japan
    24 HRS: 41.0N 172.0E – Extratropical Low in Sea south of Aleutian island

  4. Models can and will change. Nothing can be said for certain when this thing is still over a week out from any possible GOM impacts. They’ll go left then right then left again until it finally gets under 5 day and a relatively accurate forecast will be shown.

  5. Sure – Florida didn’t get a direct hit from Sally but it caused the worse flooding we’ve ever seen – way, way worse than Irma, so once again, you don’t need a direct hit to have damage.

  6. If even one person in Florida had to suffer from the effects of a tropical system just to get you to stop this every-thread downcasting rant of yours, it wouldn’t be worth it. So I hope you are right, Robert. Not for your sake, but for everyone else’s.

  7. I’m sure glad I didn’t get hit by Hurricane Sally here in Florida and that I don’t have to have my roof replace. OH WAIT, I DID get hit by Sally and I DO have to replace my roof. People like you are like a really irritating rash that won’t go away.

    1. Most are thankful in Florida being missed so far in 2020. Those actually there like you White Rabbit. You were always a good Mod back in the day too.

  8. Race Hate Brewed, Small Business Smothered, Supreme Court Farce Kidnapping Of Easily Hoodwinked Christian’s Hearts, Mix With Pandemic, West Burns, World Mourns, War Comes. Mass Death Engulfing, Teetering On The Brink. Here Dancing With The Stars, Bread And Circus Think, It Will Never Happen To Me They Say With A Wink. Let Me Go Back, Let Me Vote, Can’t Wait For The Weakened, To Be Told What I’m Thinking. Thanks For The Update Doc, In A World Deceived, I Hope That You’re Not.

    We need a Category Five of People being able to think for themselves. Soon or it shall be too late. We should unite as a Nation. We have been divided long enough. Yet who is truly going to do that? Who is even trying? No matter what you feel about me, you know I am sincerely trying for all of us. All of us not evil. Love warns. Know who your real friends are.

    1. Why i hug my shadow everyday..come back shadow …come back.

      On the serious side i’m still offering to build a design within power plants to lower the chaotic flows electricity sends via a yet officially undiscovered “intrAEresonance” flow. This flow i state damages humans ability to connect correctly to ones conscience. Just like another man made /mam influenced flow it generates a side effect, maybe those at Yale can discover it. Study how parts of the brain “tightens” the more electrical outputs are near by.

      The device i state is very simple, costs pennies. What it does is via the resonances of electricity being harnessed it takes that resonance onto a geometric design of 2 to 1 and redirects the chaotic flow towards true ground, Earths center then outwardly in forms of yet unseen/recorded”sprite” like outflows.

      My apology to MODS both comments are extreme, but based on reality.

    2. Thank you for another insightful post. Nobody would mistake you for being a little Eichmann in the future if we are all judged for our actions on climate change and other issues. Now is the time to think outside the box while hate homogenizes itself across the US. I am annoyed by everyone’s off topic small talk about sports, and am happy to see posts like this. And enjoy how you express yourself in prose. I think your opinions might be normal say in Humboldt County CA or Oakland, but for this blog maybe uncomfortable. People need to wake up and think what are they going to do about climate change and other issues.

  9. https://s1.gifyu.com/images/20200929eve-EzT.gif

    Cue he beautiful sunset images posted by others be it at the Terra firma level or from the near by heavens of low µM form/designs by nature.

    For those that remember the Dr. Masters’ blogbyte of the South American Gyre, look closely at the LPP (low pressure packets) south of Hispaniola. Does it seem like the eastern side of a LITE Gyre is helping/assisting in forming a rotating path for a developing tLPP?

  10. Thank You Dr. Masters, watching and hoping the lull continues for all of us, especially for those already affected this year.

  11. Thank so much Dr. and excellent recap, again, as to CAG’s and how they can spin off storms in either basin……….Because of the “slow to consolidate issues”, no wonder the models are having a tough time at the moment this far out………..Next week should bring a clearer picture for us.

  12. Thanks for the information. We all seem to agree that CAGs bring a whole new level to forecasting TS and Hurricanes. For now, we wait.

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