Emergency crews launched airboats into submerged streets Wednesday to rescue central Florida residents trapped by rising floodwaters from a stalled Tropical Storm Fay, which soaked the state for a third consecutive day.

More Coverage From CNN

Hundreds of homes were flooded in areas of Brevard and St. Lucie counties, some by up to 5 feet of standing water. In three towns, rising floodwaters backed up sewage systems. It wasn't immediately clear how many residents were stranded, but county officials reported making dozens of rescues.

The Florida National Guard mobilized about a dozen guardsmen and some high-water vehicles to assist with damage assessment and help with evacuations, said Jon Myatt, spokesman for the Florida Department of Military Affairs.

The storm could dump 30 inches of rain in some areas of Florida and the National Hurricane Center said up to 22 inches had already fallen near Melbourne, just south of Cape Canaveral on the state's central Atlantic coast.

Forecasters originally expected Fay to energize over the ocean and possibly become a hurricane before landing in Florida for the third time later this week. The erratic storm first struck Monday in the Florida Keys island chain, then veered out to sea before traversing east across the state, briefly strengthening, before losing steam and stalling.

The storm barely moved for most of Wednesday, dumping inches and inches of rain over central Florida.