Weather models hint at the potential formation of a tropical depression or tropical storm off the west coat of Florida or south of Alabama over the weekend. There's a decent chance that Alabama could be affected by heavy rainfall and gusty winds. (Weatherbell.com)

This story, published Wednesday, is now out of date. For the latest tropical update published Thursday, see: Tropical storm warning issued for hard-hit northwest Bahamas as Humberto may soon form

A tropical system may affect Florida, Alabama and other portions of the Gulf Coast between late this week and early next week, bringing a chance of heavy rainfall and gusty winds. It’s one of three tropical waves forecasters are tracking and has the best chance of developing.

Possible storm brewing for Gulf of Mexico

A clump of shower and thunderstorm activity continues flaring up over the southeastern Bahamas, north of Haiti. It will shuffle northward the next few days, brushing past areas hard-hit by Dorian on Friday. It could douse Grand Bahama and Abaco islands with one to three inches of rain.

From there, the disturbance visits Florida late this week into early Saturday, crossing the peninsula and dropping up to several inches of rain, before potentially reemerging into the gulf.

Passing over the warm gulf waters would allow the fledgling system to consolidate and organize some this weekend. Should this occur, the disturbance won’t have much time over the warm gulf waters, so just how feisty it can get is limited by its short development window. However, it’s becoming increasingly probable that a tropical storm may develop. The National Hurricane Center places these odds at 60 percent. The next name up? Humberto.

The extent to which a disturbance develops is not certain, but the encouraging news is that it’s unlikely to reach hurricane strength given its short lifetime. Instead, it might be one of those sneaky sorts of systems that brings drenching rainfall.

After dousing Florida, Louisiana, Mississippi and Alabama are the most likely locations to be affected by potentially heavy rainfall between Sunday and Tuesday. Where the bull’s eye sets up, a few locations are likely to see up to around a half-foot of rain. At the moment, the European model (shown below) places that bull’s eye over Alabama, but this could shift west or east as this forecast comes into better focus. The American model (not shown) simulates the heaviest rainfall in Mississippi and southeast Louisiana.


European model rainfall forecast Thursday night through Tuesday night. (WeatherBell.com)

The risk to states along the eastern Gulf of Mexico has increased relative to those farther west. Models on Tuesday struggled with their “initialization,” mislocating the low’s center, which lead to a compounding track error over time. As models have begun to more accurately latch onto where the disturbance is located now, they’ve been trending east in their forecasters as to where this system, if it makes it to the gulf, will end up.

Some are welcoming this potential system, hailing it as a well-needed drought-buster. At least a quarter of Alabama is in some sort of drought, a number that has been climbing.


(U.S. Drought Monitor)

Disturbance approaching Lesser Antilles


Invest 94L has a 20 percent chance of developing. It currently sits abotu 600 miles east of the Lesser Antilles. (NOAA/TropicalTidbits.com)

Much farther east in the tropical Atlantic is a “a large but disorganized area of cloudiness and thunderstorms” centered about 650 miles east of the Lesser Antilles. Despite its size, the system lacks the “oomph” necessary to whirl up into a cyclone. Moreover, the environment will become hostile to its development. The National Hurricane Center writes that “this system is forecast to move westward toward unfavorable upper-level winds for tropical cyclone formation.” It has no more than a 10 percent chance of development per the Center.

Disturbance off Africa’s west coast


An additional disturbance emerges off the western part of Africa on Wednesday morning. (NOAA/RAMMB)

This last tropical weather system being monitored is a blob located between the Cape Verde Islands and the west coast of Africa. In the short term, atmospheric conditions don’t favor its development.

The National Hurricane Center estimates it has a 20 percent chance of strengthening in the next five days. However, indications are that this wave may try to amalgamate into more of an organized low pressure zone within about six to 10 days. That, along with more encouraging areas of rising air across the Atlantic basin, may boost its odds of developing thereafter. This is a long way down the road, though. For the time being, it’s best characterized as a “wait and see” disturbance.

Extended discussion: the next surge of hurricane activity

Close to a half-dozen systems will have tried, and failed, to develop since Dorian. But a return to dangerous activity is possible. Using some tropical meteorology, we can estimate a return to an active pattern beginning sometime around the following dates (within a few days):

Gulf of Mexico: Sept. 19

Caribbean: Sept. 21

Tropical Atlantic: Sept. 25

Why? It has to do with something called the Madden-Julian Oscillation, or MJO.

The MJO is a large overturning circulation that propagates east across the globe at about 30 mph. In practicality, it’s just a giant, slow, invisible wave that lifts air to the west and drops it to the east. On the leading edge of these waves is the “suppressed” phase of sinking air. To the west (on the wave’s backside), the air is more prone to rising — favorable for hurricane activity.

You can see that here. Pay attention to the areas of brown and green. Notice what happens in Phases 1, 7 and 8. You see a lot of green across the Atlantic. That indicates rising air:


The Madden-Julian Oscillation spills rising air over the Atlantic during Phases 1, 7, and 8. (NOAA/Climate.gov)

Now we can look at what weather models estimate the MJO to do. Almost all models indicate a return to Phases 1 or 8, the phase that favors hurricane activity over the Atlantic, within the next two to three weeks. The upward-motion would arrive from the west over the Gulf of Mexico first before overspreading the rest of the tropical Atlantic.


Each of those yellow lines represents a forecast within the larger European weather model. Notice the clustering of lines heading toward Phases 1 and 8 of the MJO in the coming two to three weeks. (NOAA/CPC)

In addition, there’s another mechanism — called a Convectively Coupled Kelvin Wave — wrapped up inside the larger MJO envelope that will do the same thing. It too will work toward fostering hurricane production at the end of September into early October.

Even if things seem quiet in the wake of Dorian, they won’t be for long.