MORE STORMS FORECAST

Rescue workers comb through debris May 16 after tornados swept through the town of Granbury, Texas late May 15, 2013. [Photo/Agencies]

Preliminary reports showed that the National Weather Service issued a tornado warning for Granbury 26 minutes before the twister struck, according to Mark Wiley, emergency response meteorologist at the agency's Forth Worth office.

This is an unusually long lead time as the average warning time is 10 to 12 minutes, he said.

Wiley said the rating of the deadly tornado would not be determined until later on Thursday, but "it was a strong tornado just based on the damage."

More severe storms could be coming to Texas, parts of Arkansas and northern Louisiana later on Thursday, said Mead. But he said the possible tornadoes would be in northeast Texas, not in the area hit on Wednesday night.

Until Wednesday, the tornado season had been unusually mild so far in 2013 after two years of intense activity. Only three people have died in tornadoes so far in 2013, according to weather service statistics.

The deadliest tornado year in decades was 2011 when 553 people were killed, including 161 from a massive tornado that struck Joplin, Missouri in May that year.

In March 2012, at least 39 people were killed in a chain of tornadoes from the Midwest to the Gulf of Mexico. A total of 70 people were killed in tornadoes last year.

The tornado season in the United States typically starts in the Gulf Coast states in the late winter, and then moves north with the warming weather, peaking around May and trailing off by July.