GFS model shows storm near coast of the Carolinas Thursday evening (TropicalTidBits.com)

The year’s first tropical or subtropical storm could form about three weeks ahead of the official start of Atlantic hurricane season (which is June 1) later this week. It could affect Florida as soon as Tuesday and the Carolinas as soon as Thursday.

The National Hurricane Center says there is about a 30 percent chance of a tropical storm forming in the next 5 days.  Should a storm form, it would likely be classified a subtropical or hybrid storm – containing some characteristics of both a mid-latitude and tropical weather system.

Computer models remain bullish on spinning up a weak storm system from a stalled remnant frontal boundary that is draped across Cuba and the Bahamas today.  There is no low-level circulation yet, just a lot of convergence which is fueling widespread thunderstorms in the region.


The region of interest over the next several days. This morning’s surface wind is shown with the white streamlines, and total precipitable water (moisture) is shaded, with higher values in lighter blues. (earth.nullschool.net)

Heavy rain is falling over Cuba and parts of the Bahamas, and the whole area of disturbed weather is forecast to gradually ooze northward.

Cuba has been dealing with torrential rain for days now, and it has been both destructive and deadly.  Two people died in Havana and a number of buildings were destroyed or damaged by flood waters.

A national radar mosaic shows the heaviest rainfall focused over central Cuba today.

For the Southeast U.S., this system is something to keep an eye on.  Even if a  full-fledged storm doesn’t develop, disturbed weather could still produce some heavy rainfall from south Florida earlier in the week (Tuesday-Wednesday) up to the Carolinas later in the week (Thursday-Saturday).

If the storm manages to flare up, some gusty winds and coastal flooding/erosion could affect coastal areas of the Carolinas.


Enhanced infrared satellite image over Cuba, Bahamas, and Florida from 9am EDT today. The warmer colors are indicative of thunderstorms and heavy rain. (NASA)

Both the GFS and ECMWF global models show a closed circulation later in the week off of the South Carolina coast, though the timing is different between the two.  Both models track the storm over the warm waters of the Gulf Stream which would aid its development.


Comparison of model forecasts: ECMWF (left, valid on Fri evening) and GFS (right, valid on Thu afternoon). Rainfall is shaded, and surface pressure is shown with the blue line contours. (wunderground.com)

As with any model forecasts, one should look at trends and patterns, not specific locations and features from a single run.  However, consistency among models and between runs boosts confidence in the forecast.

Coastal areas of the Southeast appear most likely to deal with some impacts from this storm system.  Whether any moisture gets drawn farther inland and north into the Mid-Atlantic over the weekend is an open question.

Although the Atlantic hurricane season officially begins on June 1, pre-season activity is certainly possible though not exactly common. Historically, about 0.5 percent of tropical storm activity has occurred prior to June 1.

Just in the last thirty years, ten storms formed during April and May, the most recent being Beryl in 2012.  But if we extend the search back to 1851 (the first year in the Atlantic hurricane database), there are a total of 35 storms.  That works out to an average of once every 4-5 years.


Tracks of all known Atlantic tropical and subtropical cyclones that formed prior to June 1 since 1851. (NOAA)

Notice that many of the pre-season storms that have formed over time, many have formed in the same area as this week’s disturbance.

If this develops into either a subtropical or tropical storm, its name would be Ana. Ana is a name from the original six lists of names -first used in 1979.