Between 3 and 6in (75-150mm) of rain could fall across the Gulf Coast, from south-east Louisiana to north-west Florida.
A few places could see up to 10in (250mm) with possible flash flooding, especially in urban areas, and the risk of isolated river flooding.
Dangerous winds are anticipated and a storm surge could inundate low-lying coastal areas.
The US National Weather Service has issued Hurricane and Storm Surge Watches along the Gulf Coast.
Florida Governor Ron DeSantis has declared a state of emergency across 25 counties over concerns about potential impacts.
As the storm moves further inland and loses contact with its warm water fuel source it will weaken but it is still expected to bring some potentially disruptive wet weather for large swathes of the south-east USA as far north as the Tennessee Valley and the Carolinas.