Subtropical Storm Alberto is churning north while strengthening over the Gulf of Mexico today. It’s set on making landfall along the U.S. coast in the hours around sunrise on Memorial Day. With a forward speed of 15 mph and sustained winds of 50 mph, its location near the Florida peninsula is already allowing it to throw bad weather at the state and region.
Before we get into the details, don’t let the name fool you — a subtropical storm comes with all of the same hazards as a tropical storm. These hazards include storm surge, heavy rainfall, tornadoes, plus strong and gusty winds.
While Alberto continues to intensify over the warm waters of the Gulf of Mexico today, a portion of the southeast United States will experience prolonged impact from the storm. If and when it transitions to a tropical storm before landfall, don’t expect anything dramatic to change in the appearance or impacts. The difference between the two types of storm is a technical one.
The top concern with Alberto may be the extremely heavy rainfall forecast across the entire Southeastern United States from today through Wednesday, especially just east of the landfall location and over South Florida.
South Florida could see an additional four to six inches of rain after an already wet May, with isolated spots receiving up to 10 inches. Up in the Florida Panhandle, higher totals are expected, with isolated tallies of about a foot of rain. Flash-flood watches are in effect for those two zones, as well as up in the Carolinas.
Other risks will also come into play today and Memorial Day, such as storm surge and tornadoes.
A two- to four-foot storm surge is possible along the coast, to the right of the storm’s track, where the strongest winds blowing toward the shore are. As always, the timing with the normal astronomical tides plays a significant role — flooding will be worse if the highest surge is coincident with the highest tide (the combination of the storm surge and the astronomical tide is called the “storm tide”).
Tornadoes are also fairly common in a landfalling storm’s front-right quadrant — this happens to be the Florida Peninsula here. These are usually short-lived and rated low on the enhanced Fujita scale (EF-0 or EF-1), but they move quickly and have short warning times.
Going back to 1851, there have been just eight prior preseason — the season officially begins June 1 — classified storms that have made landfall on the continental United States (counting tropical storms and hurricanes even before they were given official names). Alberto will be the ninth.
In the wildly remote event that Alberto manages to strengthen to a hurricane at landfall, it would be the second preseason landfalling hurricane in the continental United States on record (the first and only was 110 years ago, when a hurricane clipped North Carolina’s Outer Banks on May 29).
Seven of these eight storms occurred in May. The exception to the May rule was a February storm that hit South Florida, but it’s debatable whether it was truly tropical or subtropical in origin.
Where conditions have not yet gone downhill in the main impact zones, they will shortly. In addition to the coastal and near-coastal risks, folks north of the main impact zones should continue to keep apprised of Alberto in the days ahead.