‘‘Straight-line winds are probably the highest threat,’’ Kines said by telephone. “That’s not to say there won’t be tornadoes, because there probably will be; nevertheless, it’s something to be very concerned about.”

‘Severe’ Criteria

Severe thunderstorms aren’t just the kind of showers that show up to rain out a baseball game. The National Weather Service said for a storm to reach the classification of severe, it has to have hail at least an inch (2.5 centimeters) in diameter and winds gusting to at least 58 miles (93 kilometers) per hour.

“Lightning, no matter how frequently it is striking, is not a criterion for determining whether a storm is severe,” according to the weather service.

For anyone who has watched a tree get uprooted in a severe thunderstorm, it probably isn’t hard to imagine the difference between a summer boomer and something that is going to ruin the house. For everyone else, the numbers tell the tale.

In the U.S., insured losses from severe thunderstorms reached $12.3 billion in 2014, according to the Insurance Information Institute in New York. From 2005 to 2014, 1,606 people were killed by the storms, which caused losses of $180 billion, second only to hurricanes.

Slow Start

This year has been a slow one for the systems.

Only 41 tornadoes were reported in the U.S. from January to March, according to the Storm Prediction Center. That’s fewer than the three-year average for any one of those months.

Last year got off to a relatively slow start as well but managed to make up for lost ground in April, May and June.

April is the one of the peak months for strong and violent tornadoes, primarily across the South, said Greg Carbin, a warning coordination meteorologist at the center. May and June then become active across the Great Plains and Midwest.

“We are on the really steep slope of increasing severe weather activity right now,” Carbin said by telephone.

The current round of severe weather is a classic pattern of cool air dropping in from Canada meeting warm air from the Gulf with dew points reaching into the 60s, said Kines. Meanwhile, a storm system is moving in from the Pacific.

Masses Clash

“The clashes of the air masses to begin with, then a pretty good system coming from the West Coast, that’s going to set the stage for severe weather into Thursday,” Kines said. “Even on Friday, it’s not out of the question that as this system moves to the East Coast, there could be severe weather from Philly to D.C. to Richmond on down to Charlotte.”

The Storm Prediction Center gives the mid-Atlantic from about Philadelphia to Georgia a 15% chance of severe storms, although Carbin said storms are possible from New York to New Orleans.

“It will be a pretty extensive line of storms,” Carbin said.

It’s that time of year again, and another sure sign spring is here.

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