Get breaking news alerts and special reports. The news and stories that matter, delivered weekday mornings.

A storm system in the Florida Straits strengthened to a tropical depression Sunday, threatening to bring heavy rain and floods to parts of the Sunshine State and complicate its efforts to combat the mosquito-borne Zika virus, forecasters said.

The tropical depression was southwest of the Florida Keys and was moving west into the Gulf of Mexico late Sunday evening — but is expected to swing northeast and travel over Florida by Thursday and Friday, the National Hurricane Center said.

Gov. Rick Scott previously warned residents of South Florida to have an emergency plan ready in case flooding occurs. Between 1 and 4 inches of rain could fall over the southern half of Florida through Wednesday, and some parts could get 6 inches, the hurricane center said.

Any heavy rains could also pose a setback for efforts to control mosquitoes that spread the Zika virus, which has been detected in areas of the state and which can cause severe birth defects — including microcephaly, a condition where the head is unusually small.

"The big rains are going to probably make us start all over," Miami Beach Public Works Infrastructure Director Roy Coley said. The city, as well as a neighborhood north of Miami, has seen outbreaks of homegrown Zika.

"It’s not great news for us, but the heavy rains will flush away the larvicide and it will reestablish any standing water," he said. "That will make our work more challenging, but we are up to the task," he said.

Let our news meet your inbox. The news and stories that matters, delivered weekday mornings. This site is protected by recaptcha

Related: Should I Worry About Zika? Your Questions Answered

There have been 587 Zika cases identified in Florida, but the vast majority are believed to be related to travel to countries where Zika transmissions are more widespread, like in Latin America and the Caribbean.

But more than 40 cases were transmitted locally in Florida, state health authorities have said. Seventy-two of the infections involve pregnant women.

Florida Gov. Rick Scott on Saturday warned that heavy rains and flooding are possible, and urged those in the Florida Keys and southern part of the state to have an emergency plan. He also pleaded with the public to eliminate standing water, in which mosquitoes breed.

"Any amount of standing water can serve as a breeding ground for mosquitoes and everyone should dump the water around their homes and businesses following this storm," Scott said.

Another tropical depression formed in the Atlantic Ocean off the coast of North Carolina Sunday, which could prompt a tropical storm warning for the Outer Banks, the hurricane center said.

Related: FDA Suggests Testing All Donated Blood for Zika

The system had winds of 35 mph and was expected to swing by the North Carolina coast by Tuesday morning.

Meanwhile, yet another storm churning in the Atlantic Ocean gained strength Sunday to become the first major hurricane of the season, the hurricane center said.

Hurricane Gaston was hundreds of miles away from Bermuda Sunday afternoon and there are no warnings for land.

The Category 3 hurricane, which had winds of 120 mph late Sunday, was about 575 miles east of Bermuda and was stationary at 11 p.m., according to the hurricane center.

Gaston is expected to turn north by Monday and then northeast, away from Bermuda, the hurricane center said. Some "slow weakening" was expected Monday and Tuesday.