Friday morning, the National Hurricane Center declared Invest 90L as Subtropical Storm Alberto as it slowly tracks to the northeast from the eastern coast of the Yucatán Peninsula. A subtropical storm contains some tropical characteristics but is not fully tropical because it is attached to an upper-level trough, making it a cold core storm. Alberto is expected to transition into a tropical storm this weekend, however, as it tracks over the warmer waters of the Gulf of Mexico. Alberto is currently moving through significant wind shear, displacing its convection to its eastern side. Despite this, due to the persistent convection and presence of a surface low, the National Hurricane Center decided that the disturbance has become a subtropical cyclone.
As the weekend progresses and Alberto tracks farther north, wind shear is expected to weaken as sea surface temperatures warm up and become conducive for further intensification. A pocket of low shear will likely set up over the northern Gulf of Mexico. Weak wind shear provides a favorable environment for tropical cyclones because the thunderstorm is not being blown away from its center, thus allowing for further organization and intensification. Due to a favorable environment prior to landfall on the northern Gulf Coast, the National Hurricane Center is expecting Alberto to have maximum sustained winds of 65 mph at landfall on Monday between New Orleans and Pensacola.
Due to the combination of weak wind shear and warm sea surface temperatures, there is the low chance Alberto strengthens into a category one hurricane. Some of the model guidance is suggesting that this happens, but the current forecast calls for a strong tropical storm to make landfall at this time. It would be unprecedented if a hurricane formed in May over the Gulf of Mexico, so the formation into a hurricane would be a historic one.
There has never been a hurricane that has affected the Gulf of Mexico during the month of May in recorded history. There is the low chance Invest #90L strengthens into one. The strongest storm in the Gulf of Mexico in May was Alice in 1953. Winds reached 60 knots (70 mph). pic.twitter.com/HxpJ4DEDy7
— Jackson Dill (@Jackson_Dill) May 25, 2018
Flooding rains will be the main risk with Alberto, with a widespread three to five inches of rainfall forecast all across the Southeast. This will not solely be a Gulf Coast event and its effects will spread outside of the forecast cone of uncertainty, so locations as far north as the Carolinas and southern Appalachian Mountains may experience some flooding and heavy rain as well. There are several other threats that Alberto brings, including tornadoes, storm surge, strong winds, and rip currents. Below, we will break down and detail each of these threats.
Heavy Rain and Flooding:
Flooding will be the main threat from Alberto. On Saturday, scattered showers and thunderstorms will develop across the Southeast ahead of Alberto. The only locations that will directly experience impacts from the subtropical storm is South Florida, where rounds of heavy rain can be expected.
On Sunday the storm will make its closest approach to the western Florida coast before making a turn to the northwest. This storm will not make landfall on the Florida Peninsula. Instead it will remain over one hundred miles offshore, but it will still bring more rounds of heavy rain and thunderstorms to all of the state. Those storms will also spread into portions of the Southeast by the end of the day, including the northern Gulf Coast east of New Orleans and into much of Georgia and parts of the Carolinas.
Monday, which is Memorial Day, will be the big day as Alberto makes landfall as a strong tropical storm or low-end hurricane. Periods of heavy rain and thunderstorms will continue to pivot into the Carolinas, Florida, and the northern Gulf Coast. Farther inland toward the southern Appalachian Mountains and the Tennessee Valley, just a few thunderstorms will be possible. Near the center of low pressure where Alberto makes landfall is where the heaviest of rainfall will be, thus also the best chance for flooding.
Then on Tuesday, the center of low pressure will move inland as Alberto transitions into a post-tropical cyclone. The heaviest rain will be focused on parts of Mississippi and Alabama, while occasional showers and storms affect the remainder of the Southeast.
For the remainder of the week, we’ll be watching the remnants of Alberto slowly track farther inland before getting picked up by the jet stream, carrying it over to the Northeast or Mid-Atlantic by next weekend. There may be the concern for some flooding in these regions, but it’s is too early to know at this time.
In terms of rainfall, we’re expecting the heaviest of rain near the Gulf Coast, where totals may exceed ten inches. Many other locations will measure three to five inches. If several inches fall in a short period of time, that will likely cause severe flooding.
Tornadoes are always a concern with tropical cyclones, especially in the northeastern quadrant of the storm. As Alberto moves to the north, that will put much of the Southeast in that northeastern quadrant, posing the risk for a few — but brief — tornadoes. The threat will begin on Saturday across South Florida, then will move to the north along with the storm.
Below is a general view at where tornadoes will be possible into early-next week:
Another threat with Alberto is the storm surge. There will be several days of an onshore flow across parts of the northern Gulf Coast. This persistent push of water, beginning on Sunday and continuing through Tuesday, will allow for days of minor to moderate coastal flooding due to the several feet of storm surge expected. There are no current specific forecasts of the rise in water levels, but prepare for some coastal flooding, especially between New Orleans and the Florida Panhandle. Even portions of the western Florida coast may deal with minor coastal flooding as the storm parallels the coast.
Strong, potentially damaging winds will remain confined to a small part of the Southeast. It remains too early to predict where these strongest of winds may be, which have the chance at exceeding hurricane strength (74+ mph). That will be based on the exact landfall location, which is still somewhat uncertain. We know landfall will occur somewhere on Mississippi, Alabama, or the Florida Panhandle on Monday or Monday night. Due to the friction of all the trees inland, the best risk for any damaging winds (60+ mph) will be at the immediate coast. As the storm weakens, winds will do so as well. It will still be gusty across much of the Southeast early next week, however, generally ranging at 20-40 mph.
The last threat that Alberto will bring is rough surf and rip currents, Beginning Friday, the risk for rip currents will exist across much of the Southeast coast. That risk will persist for most through at least early next week due to the cyclone over the eastern Gulf of Mexico. The best tip is to avoid going into the water due to the dangers rip currents pose. There will also be rough surf with waves possibly exceeding 10 feet along portions of the Gulf Coast. Just offshore, waves may top 40 feet near the center of the low pressure.
Stay with WeatherOptics as we continue to track Alberto.