That's according to the National Hurricane Center in Miami, which said Ernesto regained maximum sustained winds of 40 mph near its center. It strengthened after moving off Florida into the Atlantic.
Meanwhile, hundreds of National Guard troops were on alert and officials in the Carolinas, earlier Thursday, warned residents to be ready for anything as Ernesto crawled north over Florida.
The storm's fluctuating strength was less a concern to emergency officials than the prospect of downpours over an area hit by floods during the season's first named storm in June.
In North Carolina, Ernesto was due to follow a separate storm system that had a severe thunderstorm watch posted across much of the state Wednesday.
"We know we're going to get a lot of rain, we know this is going to be a water event," Gov. Mike Easley said.
Ernesto, at 11:30 p.m., regained maximum sustained winds of 40 mph near its center, the National Hurricane Center said. The storm was expected to turn to the north-northeast Thursday.
A tropical storm warning remained in effect from Sebastian, about halfway up Florida's east coast, to the Wilmington area of southeastern North Carolina.
No evacuations had been ordered in North Carolina or South Carolina, though both states' governors urged residents to keep abreast of forecasts.
Easley activated 150 National Guard troops and the State Emergency Response Team to prepare for possible flooding and power outages and also put two Black Hawk helicopters and 38 high-clearance vehicles on standby. He also said the state has two warehouses stocked with emergency supplies, and shelters are prepared to open to house and care for evacuated residents.
"We know we're going to get a lot of rain and this will be a water event. Whether we get hurricane-force winds remains to be seen," Easley said.
Progress Energy put all of its power line-repair crews on standby, but the utility wasn't bringing in any crews from outside the state because wind damage wasn't expected to be extensive.
Forecasters said a storm surge of 1 feet to 3 feet was possible, with the highest surge coming Thursday night or Friday morning around the time of high tide.
In North Carolina, forecasters said as much as 9 inches of rain was possible as a result of Ernesto and the other storm system. The storm's track looked like it would affect areas from Winston-Salem to the coast.
North Carolina has struggled this year with an on-again, off-again drought. But although the storms from Ernesto could fill depleted reservoirs, back-to-back heavy downpours raised the prospect of runoff and flooding.
"If we could get the rain where it was spread out over 12 to 24 hours, where it was a good, soaking rain, (flooding) would not be an issue," National Weather Service forecaster Phil Badgett said.
Easley reminded residents of floods that hit Raleigh and surrounding counties when the remnants of tropical storm Alberto moved through earlier this summer.
"Don't get complacent," Easley warned. "Everybody out there needs to listen to their local officials."
That storm was blamed for the drowning of a 13-year-old boy playing near a culvert in Louisburg. In Raleigh, it caused numerous traffic accidents and vehicle strandings in high water, and forced the closure of a major shopping mall, the evacuation of a small hotel and boat rescues at an apartment complex.
"We've seen what happens when you get 7 inches of rain. We saw that in Raleigh in June," Easley said.
The storm is expected to exit North Carolina by Friday evening.
Coastal North Carolina officials also were pleased with the forecast. Beachfront businesses feared the storm would scare off Labor Day weekend tourists.
"I wouldn't be changing my (vacation) plans right now," said Randy Thompson, emergency services director in Brunswick County. "It appears the storm is going to be pushing out, and it's shaping up like a great weekend at the beach."