After a relative lull in widespread downpours for the Gulf Coast states Wednesday, tropical moisture will continue to deluge the Southeast into Memorial Day Weekend and early next week. The persistent rain is the result of an atmospheric river formed by an idle upper-level low over the Gulf of Mexico adjacent to a strong Bermuda High over the western Atlantic Ocean. According to the Southeast Regional Climate Center, many areas in the region have already experienced their top five wettest Mays with still more rain to come.
Escalating the flood risk is the looming threat of impacts from a tropical disturbance located just off the coast of Belize as of Wednesday morning. Over the next five days, the disturbance will be pulled northward by the atmospheric river toward the Gulf of Mexico. Development of what is now being dubbed Invest 90L into a tropical or subtropical depression or even Tropical Storm Alberto is becoming more likely. The National Hurricane Center recently increased the chance of intensification to 80% over the central or eastern Gulf of Mexico across this time span. In order to become a Tropical Storm, a tropical cyclone must exhibit maximum sustained winds of at least 39 mph. If cyclonic circulation is apparent but winds do not exceed this threshold, the storm is an unnamed tropical depression.
Formation of a tropical or subtropical cyclone in the Gulf of Mexico during the month of May is an extraordinarily rare phenomenon. The only observed cyclone that formed in the Gulf of Mexico during May was Subtropical Depression One in 1976. A subtropical storm exhibits characteristics of both warm-core tropical cyclones and cold-core extra-tropical cyclones, the former of which is fueled by condensation and the latter by temperature gradients.
Whether Invest 90L intensifies to become Tropical Storm Alberto is still uncertain. Little development is expected until Saturday due to strong upper level winds in the Caribbean. Once it enters the Gulf of Mexico, environmental conditions will be more favorable for tropical development. However, the primary limiting factor will be sea surface temperatures. Surface temperatures of of 26.5°C or 79.7°F are required to maintain a tropical cyclone’s warm core. This threshold is exceeded throughout most of the Gulf except over the coast of western Florida, where sea surface temperatures are cooler due to heavy rainfall.
The ultimate strength and locations of the, heaviest rainfall are dependent on the track of Invest 90L upon arrival in the Gulf of Mexico, a detail which is still uncertain. The ECMWF and its 52-member ensemble have been consistent with Invest 90L slowly trekking northward across the central Gulf of Mexico and making landfall near New Orleans as a tropical depression or weak tropical storm Monday evening. Alternatively, the 21-member GFS ensemble mean has been consistent with Invest 90L straddling the cooler waters of Florida’s west coast and making landfall as a subtropical depression Monday afternoon. The parent GFS model has been steering the storm across South Florida. This track is being treated as an outlier, but it is still within the realm of possibility.
The primary physical parameter controlling the track of Invest 90L is the westward extent of the Bermuda High. The GFS and its ensembles have modeled a weaker Bermuda high than the ECMWF, and have therefore depicted a further east track. Given that the GFS has slightly underrepresented the strength of the Bermuda High, a track that would have Invest 90L make landfall somewhere between New Orleans and Pensacola, FL is favored at this time. That being said, tracks further west along the Louisiana Coast or further east along the Florida Panhandle should not be discounted. Crucially, the cyclone track does not necessarily represent the center of the heavy rainfall. Most of the rain associated with this cyclone will develop east of its center of low pressure.
Irrespective of the future intensity and track of Invest 90L, with already saturated soils across most of the Southeast, heavy rainfall is the most significant threat associated with this storm. Rain will be steadier and more widespread than the downpours experienced over the last several days, with the heaviest rain falling between Sunday and early Wednesday. After the storm makes landfall somewhere along the Gulf Coast late Monday or early Tuesday, the cyclone’s center will remain drifting over the Southeast, delivering several more days of tropical downpours until the Bermuda High expands westward or it is picked up by the northern jet stream. Florida, Alabama, Mississippi, Georgia, and South Carolina will undergo the brunt of the heaviest rainfall over the next seven days, with more than 5 inches of rain expected to fall over southern Florida, the western Florida panhandle, southern Alabama, southeastern Mississippi and central Georgia.
We will know more about the track of Invest 90L and when the rain will finally clear from the Southeast as the disturbance travels closer to the Gulf Coast.