The National Hurricane Center warned on Wednesday of "catastrophic" and "life-threatening" flooding along parts of the north-central Gulf Coast. Rainfall could last up to two days.
Sally made landfall as a Category 2 storm with winds up to 105 mph and a creeping, slow pace that makes lingering rainfall a major threat.
The painfully storm was moving painfully slow at about 2 mph, pounding parts of Florida and the Alabama coast with heavy downpours and winds Tuesday, as many residents reported power outages and sought to protect their homes and businesses.
"Hurricane Sally is nothing to take for granted. We're looking at record flooding, perhaps breaking historic levels, and with rising water comes a greater risk for loss of life and loss of property," warned Alabama Governor Kay Ivey on Twitter Tuesday.
Ivey urged residents to either prepare for possible evacuations or seek safe shelter, as the turbulent storm made headway.
Forecasters warned that areas from the western Florida Panhandle to southeastern Mississippi could see up to 30 inches of rain. The National Hurricane Center predicted water heights of six to nine feet from Ocean Springs, Mississippi, to Dauphin Island, Alabama, if peak storm surge coincides with high tide.
Florida Governor Ron DeSantis tweeted Tuesday that he had declared a state of emergency in 13 northwest Florida counties, as Sally approached. "Floridians in these counties should prepare for strong winds and severe flooding," he cautioned.
On Monday, President Donald Trump issued emergency declarations for parts of the states of Alabama, Mississippi and Louisiana, ordering federal assistance due to the emergency conditions of Hurricane Sally.
The storm, which will begin to pick up some speed, is expected to move inland across southeastern Alabama later tonight and into Thursday.
Sally is also forecast to bring heavy downpours to parts of Mississippi, Alabama, Georgia and the Carolinas later in the week.
"A hurricane moving at 2 mph is stalled for all intents and purposes," Brian McNoldy, a hurricane researcher at the University of Miami, told The Associated Press. "If they aren’t moving along and they just kind of sit there, you're going to get a ridiculous amount of rain."
Forecasters warned that tornadoes were also possible Wednesday across the Florida Panhandle and southern Alabama.
Earlier in the week, Louisiana and Mississippi were bracing for Sally, but as the hurricane slightly changed course, forecasters predicted they should largely be spared from the storm.
Sally had already deluged some areas Tuesday night with more than a foot of rain, leaving more than 80,000 customers without power in Alabama and Florida, as the storm slogged its way toward landfall.
This year's hurricane season — which won't end for another 2 ½ months — has already been one of the busiest on record. Forecasters have almost run through the alphabet of names.
Early Wednesday morning, another storm, Teddy, was rapidly upgraded to a hurricane, with sustained winds of 90 mph. The storm is still in the middle of the Atlantic, hundreds of miles from land but is forecast to become a catastrophic Category 4, possibly reaching Bermuda this weekend.