Tornado warnings that were in effect for three counties in Missouri and one in northern Texas on Thursday have expired, but the National Weather Service says it's too soon for residents in the Midwest to let down their guard.

Tornado watches, which are less urgent than warnings, remain in effect for parts of Tennessee, Mississippi and Alabama, authorities said Friday. There were a number of flash flood warnings in effect for central Missouri, stretching eastward to southern Indiana. 

Four people were injured Thursday night when a suspected tornado destroyed a farmhouse and a mobile home near Merit, about 40 miles northeast of Dallas. Hunt County Sheriff Randy Meeks said the injuries weren't believed to be life-threatening. 

That storm first hit Farmersville, a town 35 miles northeast of Dallas, toppling utility poles and trees and damaging roofs.

Earlier, severe afternoon thunderstorms pummeled Denton with grapefruit-sized hail. The National Weather Service reports hail of 3½-inch diameters and larger fell north of downtown Denton Thursday afternoon. City spokeswoman Lindsay Baker said damage such as broken windows and skylights was widespread.

The Associated Press and FoxNews.com's Karl de Vries contributed to this report.

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