At least two people were killed in the downpour, which dumped between 1 foot and 18 inches of rain on Palm Beach County over the course of a few hours.
"An incredible rainfall rate," said Robert Molleda, a meteorologist with the National Weather Service. "There is no way we could forecast that much rain in that short a time."
Rainfall reached 22.21 inches around Interstate 95 and Gateway Blvd., forcing the closure of Interstate 95. School was canceled for many districts and traffic was a mess.
The fatalities included a woman who drove her car into a flooded intersection and 90-year-old Harry Krulewitz, who walked into waist-deep water in his neighborhood and then slid into a canal, the South Florida Sun Sentinel said.
Forecasters said the downpour was the result of thundershowers bumping into a cold front and stalling over Palm Beach County. The rain overwhelmed the area's drainage system.
"We've got a really good drainage system, but it can only handle so much," said Tommy Strowd, operations director for the South Florida Water Management District.
Strowd said the county was actually fortunate the storm struck during the dry season because the water table was low and the water would drain off more quickly. Saturday's weather was expected to be mainly dry.