While the U.S. West Coast sweated out a heat wave, forecasters warned of severe thunderstorms along the East Coast on Sunday and issued a tropical storm warning for Florida's Gulf Coast.

Weekend plans could be rained out for millions of people from New York through North Carolina, as thunderstorms were expected to blast winds up to 55 miles per hour later on Sunday. Hail and tornadoes were also possible. 

National forecasters also issued a tropical storm warning for Florida's Gulf Coast after a tropical depression gusting winds around 35 miles per hour formed about 125 miles northwest of Cozumel, Mexico.

The warning affected the coastline from Englewood to Indian Pass in the Florida Panhandle. The system was expected to strengthen into a tropical storm before approaching Florida's coast on Monday afternoon and evening, the National Hurricane Center in Miami said.

On its current track, the center would land around the Big Bend region of the Panhandle, the agency said.

The storm could dump as much as 8 inches of rain on the state, with the potential for 1 to 3 feet of flooding if storm surge occurs at high tide, the hurricane center said. Sandbags were being made available in the Tampa area.

Florida heightened the response level of its State Emergency Operations Center on Sunday to provide emergency planning and support, but had not fully activated all emergency functions.

"As we continue to closely monitor this tropical depression, Floridians should remain vigilant and have an emergency plan for their families and businesses in place today," Gov. Rick Scott said in a statement.

The storm, which forecasters will name Colin, is part of a brisk start to the Atlantic hurricane season running through Nov. 30. Over Memorial Day weekend, the Carolinas were lashed by heavy rain and winds from Tropical Storm Bonnie.

Meanwhile, the western United States sweltered under a heat wave that was expected to bring record high temperatures.

The National Weather Service predicted temperatures well above 100 degrees Fahrenheit for parts of southern California, Arizona and Nevada, with unseasonable heat also afflicting inland areas of the Pacific Northwest.

In Texas, where torrential rains led to flooding last week that killed at least 16 people, the waters were expected to recede as the weather dries out, said Mark Null, hydrologist-in-charge of the National Weather Service's West Gulf River Forecast Center. 

"It's going to be a slow drain," he said in a telephone interview.