Nicholas is stalling over the Gulf Coast, dropping dangerous amounts of rain over areas still recovering from previous storms.
Flash flood watches are in effect through Friday in Louisiana, Mississippi, Alabama and Florida.
Ten to 11 inches of rain has already inundated Alabama and Mississippi with more rain on the way.
The heaviest rain will be from Mississippi to Alabama to Florida over the next 24 hours.
New Orleans will continue to see showers and a few thunderstorms with another 1 to 2 inches of rain possible.
Slow-moving Nicholas is an especially dangerous threat for Louisiana, which is still recovering from deadly Hurricane Ida and other devastating storms in 2021 and 2020.
As of Tuesday, about 87,000 customers in Louisiana were still without power from Hurricane Ida, Louisiana Gov. John Bel Edwards said.
Over 1,000 Louisiana residents remain at shelters in the wake of Ida, he said Tuesday.
The governor requested an emergency federal declaration, which was granted by President Joe Biden.
Before heading to Louisiana, Nicholas first struck the Houston area with over 6 inches of rain, shuttering schools.
In the Houston area, 460,000 customers were without power at the height of the storm early Tuesday, according to CenterPoint Energy. About 300,000 customers saw their power return by Tuesday evening.