(Bloomberg) -- It’s again the time of year when a mix of coolair from Canada, warm moist air from the Gulf of Mexico and a touchof energy off the Pacific can all wrap together to unleash stormsacross the U.S. Midwest and South.

From Tuesday into the weekend, the potential for severethunderstorms is on the map for the Midwest through the Atlanticcoast, according to the U.S. Storm Prediction Center in Norman,Okla. Hail and high winds have been reported in six states fromMissouri to Ohio Wednesday, as of 2:30 p.m. New York time.

“Thursday is probably the big day, primarily because it’s goingto include the Chicago, St. Louis and Springfield, Mo., corridor,”said Tom Kines, a meteorologist with AccuWeather Inc. in StateCollege, Pa.

About 72.3 million people are in the path of potentially severestorms as the bull’s-eye for the worst weather shifts fromOklahoma, Kansas and Missouri on Wednesday into Illinois and partsof Iowa and Indiana on Thursday, the U.S. Storm Prediction Centersaid Tuesday. The entire region has a 30% chance of severethunderstorms and the host of troubles they can bring, includinglarge hail, straight-line winds and, of course, tornadoes.

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

‘Severe’ Criteria

Severe thunderstorms aren’t just the kind of showers that showup to rain out a baseball game. The National Weather Service saidfor a storm to reach the classification of severe, it has to havehail at least an inch (2.5 centimeters) in diameter and windsgusting to at least 58 miles (93 kilometers) per hour.

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

For anyone who has watched a tree get uprooted in a severethunderstorm, it probably isn’t hard to imagine the differencebetween a summer boomer and something that is going to ruin thehouse. 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 InformationInstitute in New York. From 2005 to 2014, 1,606 people were killedby the storms, which caused losses of $180 billion, second only tohurricanes.

Slow Start

This year has been a slow one for the systems.

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

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

April is the one of the peak months for strong and violenttornadoes, primarily across the South, said Greg Carbin, a warningcoordination meteorologist at the center. May and June then becomeactive across the Great Plains and Midwest.

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

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

Masses Clash

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

The Storm Prediction Center gives the mid-Atlantic from aboutPhiladelphia to Georgia a 15% chance of severe storms, althoughCarbin 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 ishere.

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