There seems to be no end in sight to the deluge over the Southeast. The stagnant wet pattern will persist throughout the remainder of this week and into the holiday weekend. Causing this dreary pattern is a broad upper-level low over the Gulf of Mexico and a strong Bermuda High over the western Atlantic Ocean. Their proximity has resulted in the development of an atmospheric river from the Caribbean Sea into the Southern US, whereby deep tropical moisture is being funneled northward.
Elevating the flood risk for the weekend is a broad surface low centered just east of Central America. The disturbance, dubbed Invest 90L by the National Hurricane Center, will be pulled northward by the atmospheric river toward the Gulf of Mexico. The National Hurricane Center has given the low a 50% chance of tropical development within the next five days. Whether Invest 90L becomes Tropical Storm Alberto or remains a cluster of heavy rain and thunderstorms depends on a variety of factors, the most important of which being its track.
Model guidance is yet to come into agreement about its ultimate path, but there is a strong consensus that this system will emerge into the Gulf of Mexico by Saturday evening. From there, Invest 90L may trek northward toward the northern Gulf Coast between Louisiana and the Florida Panhandle by late Sunday night, or it may travel northeastward into the west coast of Florida Sunday afternoon. With minimal wind shear and with moisture supplied by the atmospheric river, the main limiting factor to the cyclone’s development upon arrival in the Gulf of Mexico is marginal sea surface temperatures. Sea surface temperatures of at least 26.5°C (79.7°F) are required to maintain a tropical cyclone. These warm waters are needed to maintain the warm core of tropical systems.
Below is a graphic depicting satellite measured sea surface temperatures from NOAA’s GOES-East over the Gulf of Mexico. Throughout most of the basin, sea surface temperatures just meet the threshold for tropical development. However, due to persistent rain, sea surface temperatures over the west coast of Florida are too cool for tropical development. Further west, the Loop Current is bringing temperatures warm enough for tropical intensification.
Recent runs of the GFS have taken Invest 90L along the cooler waters of western Florida before making landfall somewhere between Panama City and Naples as a weak, disorganized low pressure system. Conversely, recent runs of the Euro have steered Invest 90L through the Loop Current into the eastern Louisiana coast as a tropical depression or weak tropical storm, which would be named Alberto. How far west the cyclone travels depends on the westward extent of the Bermuda High and the position of an upper-level low arriving on the U.S. West Coast this weekend. Based on the further westward extent of the Bermuda High and the cyclone track depicted by the GFS Ensemble mean compared to the GFS operational model and its similarity to the ECMWF, a track northward towards the coasts of Louisiana, Mississippi, Alabama and western Florida Panhandle is favored at this time. This would draw more precipitation toward Mississippi and Alabama and less towards South Florida.
Regardless of the track of Invest 90L, heavy downpours are expected across the Southeast through Memorial Day Weekend. Upon landfall Sunday, the cyclone will travel northeastward, bringing heavy rain chances to Georgia and the Carolinas. The heaviest rainfall of at least five inches is expected to fall over southern Alabama, the western Florida Panhandle, and parts of the western Florida coast.
The cyclone will be slow to depart the Southeast as the upper-level low over the Gulf of Mexico remains idle. Dry weather will not return until either some steering mechanism aloft can push the stubborn upper-level low away or if the Bermuda High shifts westward. We will know more about the path of what could become Tropical Storm Alberto and when dry weather will finally return to the Southeast as the week progresses.