Enhanced satellite image showing an area of showers and thunderstorms (circled in orange) that could potentially develop into a tropical or subtropical system some time this week. (NOAA, modified by CWG)

An area of enhanced thunderstorm activity is slowly taking shape in the western Atlantic, and could become an early-season tropical or subtropical storm this week. Regardless of the system’s potential development, it is poised to at least bring wet and stormy weather from Florida north into the Carolinas as it lingers off the Southeast coast over the next few days.

A stalled frontal boundary that has been sitting over Cuba for the past several days is slowly making its way north. The National Hurricane Center is giving this area of thunderstorms a 20 percent chance of developing into at least a tropical or subtropical depression over the next 48 hours, and a 40 percent chance over the next five days.

The center notes that it’s more likely the disturbance will develop subtropical characteristics, though, which are a combination of tropical and non-tropical. Subtropical systems typically develop early and late in the hurricane season, and are frequently the result of a weakening non-tropical storm.


A surface analysis and infrared satellite image from early Tuesday morning, shown on the same scale. (NOAA and NASA)

On Tuesday morning, a trough was showing up in the surface analysis over the western Bahamas. Though there’s currently not a surface low pressure center associated with the area of disturbed weather, the forecast models remain in agreement that a low will develop off the east coast of Florida on Wednesday.

Map of sea surface temperatures averaged over the past seven days. (Johns Hopkins Univ.) Map of sea surface temperatures averaged over the past seven days. (Johns Hopkins Univ.)

A hurricane hunter mission might be sent to investigate the disturbance on Tuesday if it shows signs of development. If it eventually becomes a subtropical or tropical storm, the name would be Ana. The previous time a storm got named prior to May 9 was April 20, 2003 — and coincidentally, that name was Ana, as well.

Thunderstorms began affecting southern Florida overnight, and the coverage will spread northward throughout the day. The ocean temperatures in the Bahamas and northward along the Gulf Stream are plenty warm to sustain a tropical cyclone, but the area of rain and thunderstorms doesn’t appear to be taking advantage of the favorable environment yet, and remains extremely disorganized.

With the exception of European model, all global and regional weather models show development of this disturbance into a closed surface low pressure area by Wednesday evening.

In the figure below, the rightmost two panels are regional models, and their higher resolution naturally allows for a lower pressure to be resolved (all other things being equal). All panels are a forecast for Wednesday evening.


Comparison of five model forecasts (three global and two regional) for Wednesday evening. The shading is surface wind speed (knots) and the line contours are surface pressure (millibars). (tropicaltidbits.com, modified by CWG)

After Wednesday, the models diverge in their forecasts. In general, the U.S. GFS is showing a weak system making “landfall” in South Carolina on Saturday afternoon; the Canadian CMC model shows a weak system wobbling around off the Carolinas from Thursday into Tuesday, and the U.S. NAM is similar to the GFS, but a day quicker in its progress (Friday afternoon into South Carolina).

All coastal areas from Florida up into North Carolina can expect breezy and rainy conditions over the next five days as this area of disturbed weather makes it way north. For specific forecasts for your location, you can always check the National Weather Service website.