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MIAMI (CBSMiami/AP) — A flood watch remains in effect for South Florida through Sunday as subtropical storm Alberto, the first named storm of the 2018 hurricane season, moves northward over the eastern Gulf of Mexico.

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The governors of Florida, Alabama and Mississippi all declared states of emergency ahead of the storm Saturday.

Heavy downpours were expected to begin lashing parts of Florida, Louisiana, Mississippi and Alabama on Sunday. The Florida Keys could see as much as 10 inches of rain, the National Hurricane Center said.

A Tropical Storm Warning has been issued for the west coast of the Florida peninsula from Bonita Beach to the Anclote River, the Dry Tortugas, and the Aucilla River to the Mississippi/Alabama border..

A Tropical Storm Watch is in effect for Mississippi/Alabama border to the Mouth of the Pearl River.

There is also a Storm Surge Watch in effect for Crystal River to the Mouth of the Pearl River.

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At 11 p.m., the National Hurricane Center in Miami said Alberto was centered about 120 miles west southwest of the Dry Tortugas and moving north northeast near 13 mph. Its top sustained winds were 40 mph. Gradual strengthening is forecast until the system reaches the northern Gulf Coast on Monday.

The hurricane center said up to 12 inches of rain was possible across the Florida Keys and southern and southwestern Florida. Residents in the storm’s expected path were advised to monitor the storm’s progress.

“Flooding potential will increase across this region early next week as Alberto is forecast to slow down after it moves inland,” the hurricane center said.

The National Weather Service said a flash flood watch would be in effect from Saturday evening through Tuesday evening for southeastern Mississippi, southwestern Alabama, and the western Florida Panhandle. A storm surge watch was also issued for parts of Florida, Alabama, Mississippi and Louisiana.

A subtropical storm has a less defined and cooler center than a tropical storm, and its strongest winds are found farther from its center. Subtropical storms can develop into tropical storms, which in turn can strengthen into hurricanes. Alberto comes ahead of schedule: the six-month hurricane season doesn’t begin until June 1.

Parts of Florida, Georgia, Alabama, Mississippi and Louisiana have already seen heavy rain this week, and further deluges could leave those areas vulnerable to flash flooding and river flooding. Some beachfront and riverfront communities are already handing out sandbags.

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(© Copyright 2018 CBS Broadcasting Inc. All Rights Reserved. The Associated Press contributed to this report.)