Hurricane Hermine, the first to hit Florida in a decade, has killed one person and caused severe damage along the state's northern Gulf Coast.
As the system pushed into Georgia, it took out power lines in both states, affecting hundreds of thousands.
Gusts of 80mph (130km/h) caused storm surges that flooded part of the Florida coast.
There are concerns over stagnant water in the state, which has been battling with the mosquito-born Zika virus.
In the town of Cedar Key, waters rose more than 9.5ft (2.9 metres), among the highest surges ever seen, according to the National Weather Service.
After making landfall early on Friday, Hermine swept through Florida and Georgia.
Having weakened to a tropical storm, it entered South Carolina, where local officials have reported flooded roads, fallen trees and power outages.
But the National Hurricane Center predicted it would regain hurricane strength after emerging into the Atlantic Ocean, and a tropical storm warning was issued for parts of New Jersey, Connecticut and New York City.
The US Coast Guard also warned boaters and swimmers along the New York and New Jersey coasts to use caution in what is expected to be rough surf and hazardous rip currents this weekend.
Zika fears
There are also fears that the stagnant water caused by the storm surge in Florida could provide a breeding ground for mosquitoes.
There have been 47 cases of Zika in people believed to have contracted the virus through local mosquitoes, according to the Florida Department of Health.
"It is incredibly important that everyone does their part to combat the Zika virus by dumping standing water, no matter how small," Florida Governor Rick Scott told a news conference on Friday.
Police in Taylor County, Florida, that has a population of more than 20,000, said the storm had inflicted "severe damage''
In the state capital Tallahassee, at least 70,000 homes were without power at one point, affecting 60% of people in the region.
Hurricanes in the US
- While Florida is prone to storms and storm surges, it has not seen a hurricane in close to 4,000 days
- The last hurricane to strike Florida was Wilma in October 2005, causing five deaths and an estimated $23bn (£17bn) of damage
- Hurricane Katrina hit Louisiana in the same year, killing nearly 2,000 people and displaced one million
- In fact, Hermine is the first hurricane to make landfall in the US since Arthur in July 2014 - no hurricanes touched down in 2010, 2013 or 2015
In Georgia, Governor Nathan Deal also declared a state of emergency, for 56 counties, but the state was spared the havoc it had expected.
"We're having a bit of a sigh of relief," said Jim Butterworth, director of the Georgia Emergency Management Agency.
But 107,000 customers were reported to be without power across the state as crews worked to repair damage.
Hurricane scales
Category one: sustained winds of 74-95mph (119-153 km/h); some damage and power cuts
Category two: winds of 96-110mph (154-177 km/h); extensive damage
Category three: winds of 111-129mph (178-208 km/h); well-built homes suffer major damage
Category four: winds of 130-156mph (209-251 km/h); severe damage to well-built homes, most trees snapped or uprooted
Category five: winds of 157 mph (252 km/h) or higher; high percentage of homes destroyed, area uninhabitable for weeks or months
Source: NOAA
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