Alberto had picked up strength as it headed north through the Gulf of Mexico, bringing with it the warning of life-threatening inundation, said the Miami-based hurricane centre. The storm was moving north at about 14 miles per hour, he added, "so not real fast".
A subtropical storm like Alberto has a less defined and cooler center than a tropical storm, and its strongest winds are found farther from its center.
A tropical storm watch is in effect for parts of MS and Alabama, meaning the conditions for a full storm are possible in the next 48 hours.
The forecast track shows Alberto is set to move over the northern Gulf of Mexico on Sunday night and approach the northern Gulf Coast on Monday.
"Heavy rains and gusty winds continue to spread northward over Florida", it tweeted.
Parts of the area are under mandatory evacuation orders. Steady weakening is forecast after landfall, and Alberto will likely become a subtropical depression tonight or early Tuesday and degenerate into a remnant low by Tuesday afternoon.
View Slideshow A satellite image shows Alberto as it nears landfall on the Florida Panhandle and the Gulf Coast on Monday. NOAA STAR
Governors in Florida, Mississippi and Alabama on Saturday declared states of emergency.
The storm comes after a series of deadly hurricanes in the USA and Caribbean previous year walloped places including Texas, Florida and Puerto Rico - causing billions of dollars in damage.
That's why forecasters rapidly upped the storm's chance for development last week and concern grew for a totally washed out holiday weekend. Hurricane season in the Atlantic basin officially begins on June 1. Subtropical storms can develop into tropical storms, which in turn can strengthen into hurricanes. Once Alberto is inland, and deprived of the warm waters that fuel tropical weather systems, the storm was expected to steadily weaken.
Subtropical Storm Alberto was disrupting plans for Memorial Day barbecues and beach outings in Alabama, Florida and MS, as the storm continued churning north through the Gulf of Mexico on Sunday. According to a report by CNN, the storm has already caused dreary weather for Memorial Day weekend all along the Gulf coast.
Many may see some tropical storm force wind gusts along the coast but not necessarily sustained winds.
"We're going to unplug everything that we can unplug, we're going to lift it up the best that we can and we're just going to let mother nature take its course", he said.
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