There is growing confidence that a cluster of showers and thunderstorms located just southeast of the Yucatan Peninsula will intensify in the Gulf of Mexico over the holiday weekend into a tropical cyclone. At its 2pm EDT Tropical Weather Outlook, the National Hurricane Center upgraded the probability of Invest 90L becoming a tropical depression to 90% by Saturday evening. As the disturbance intensifies and advances northward, it will increase the flooding potential to the already inundated Southeast, and introduce the risk for strong rip tides along the entire Gulf Coast.
Its proximity to land has thus far this week hindered substantial development of the disturbance, currently denoted as Invest 90L. By Friday afternoon, intensification of an upper-level low over the Gulf of Mexico will draw the surface low northeast of the peninsula and into the open waters of the Caribbean Sea, where it will begin to intensify and track northward.
Upon entering the eastern Gulf of Mexico Saturday morning, Invest 90L will have developed a defined circulation and a warm core fueled by condensation, the two main characteristics of tropical cyclones. With minimal wind shear, an abundance of moisture, and sea surface temperatures just exceeding the 26.5°C threshold for tropical cyclone intensification throughout most of the Gulf, Invest 90L will continue to strengthen as it travels northward. If the tropical depression’s maximum sustained wind speed exceeds 39 mph, it will become Tropical Storm Alberto, the first named storm of the 2018 Hurricane Season (which does not officially begin until June 1).
The soon-to-be tropical depression will trek north or northwestward across the Gulf of Mexico between Saturday and Monday. It will be drawn towards the Gulf Coast by the constructive flows of the upper-level low over the Gulf of Mexico as well as the Bermuda High over the western Atlantic Ocean. As it travels across the Gulf, heavy rain will develop around its center of circulation, with the heaviest rain confined to its northern and western flanks. Throughout the day Sunday, widespread showers and thunderstorms will also develop to the north and east of the cyclone in Florida, Georgia, and the southern Gulf Coast due to the mutual amplification of the upper-level low over the Gulf of Mexico and the Bermuda High. This is the same pattern that resulted in the recent period of heavy rain in the Southeast. With the presence of a tropical cyclone in the Gulf of Mexico, moisture transport to the region will be amplified.The cyclone will make landfall as either a tropical depression or Tropical Storm Alberto somewhere between eastern Louisiana and the western Florida panhandle late Monday or early Tuesday before stalling over the southeastern U.S. Flooding is a much bigger concern than winds, which will be confined to just outside of the storm’s center, which will be located over water throughout most of its life cycle. The most persistent downpours and therefore the greatest flood risk will initially occur near the cyclone’s center, gradually shifting to its northern and eastern flanks as the cyclone loses its tropical characteristics early next week. Widespread but intermittent showers and thunderstorms will continue further north and east away from the cyclone’s center until the storm is carried northeast by an expanding ridge in the southwestern U.S.
Where precisely the tropical cyclone will make landfall and whether it will be a tropical depression or tropical storm is still uncertain. The future positions of an upper-level low off the U.S. West Coast and the Bermuda High both will contribute to the future track. If the trough over the Western U.S. weakens and broadens too soon, the upper-level low over the Gulf of Mexico steering the tropical cyclone will shift further east. Alternatively, if the Bermuda High retreats further east than the model consensus, the cyclone may also track further east. These details will be better understood Friday, when the upper-level low over the Pacific reaches the west coast of the U.S. More data will be available from land than sea, so the synoptic dynamics will be modeled more accurately.
Despite uncertainty related to data assimilation, the GFS is finally conforming to a landfall on the northern Gulf Coast. Most guidance points to this track as well, including its own ensemble, the ECMWF and HWRF (the HWRF is the specialized Weather Research and Forecasting model specially optimized for the prediction of tropical cyclones). Therefore, we are now decently confident that the entire coast between Mississippi and and the Florida pan handle will experience significant rainfall.
At this time, a cyclone track resulting in landfall between far eastern Louisiana and the western Florida Panhandle is favored, with landfall likely right around Mobile, Alabama. The heaviest and most persistent rain and therefore the greatest flood risk will be within close proximity to the center of circulation. This bullseye should not diminish the significant flood risk posed by widespread showers and thunderstorms that will continue to develop to the north and east of the cyclone’s center, regardless of its track. Over the course of the next seven days, rainfall of ten inches or more may deluge the Mobile and Pensacola coasts, with up to eight inches falling across the western Florida Panhandle, South Florida, central and southwestern Georgia, southern Alabama, and southeastern Mississippi.
During lulls in the precipitation, it is not recommended to visit the beach with the intent of bathing. Dangerous rip tides will develop across the shores of the Gulf of Mexico due to the slow propagating speed of the tropical cyclone.
There is still a lot of uncertainty associated with Invest 90L. Regardless of the track, be prepared for several days of heavy rain and possible flooding in the Southeast. We will continue to provide updates as we know more about what is now Invest 90L’s eventual landfall and its future impacts.