FARGO -- Last week's tornadoes left some areas of Minnesota and North Dakota looking like war zones, sobering signs of the current battle between hot and cold air over the central and northern plains.
"For this time of year, for us, we become the battle zone," said Mark Ewens, data acquisition program manager for the National Weather Service in Grand Forks.
The area's peak tornado season normally lasts 45 to 60 days, from about Memorial Day to mid-July, Ewens said.
"We're right in the thick of it right now," he said.
The upcoming weekend could bring more severe weather. Last Thursday, at least 18 tornadoes swept through Minnesota, killing three people.
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Moist and unstable air remains over the area, and another big storm rolling out of the central Pacific is forecast to move inland and set up the potential for severe weather Friday and Saturday, Ewens said. Thunderstorms and high temperatures in the mid- to upper 80s are expected.
In explaining why this time of year is ripe for tornado activity, Ewens likens the atmosphere to a bathtub, with the air being the liquid -- it sloshes, it moves, and it has a memory like wave action.
"The storms of the winter and spring basically are still going on," he said. "They're decreasing, obviously, but there's still enough energy because of the abundance of hot air now in the lower latitudes and the relatively colder air in the midand northern latitudes."
According to the calendar, summer started Monday. But there's still a relative abundance of cold air in the northern latitudes that may mix with storms coming out of the Pacific Ocean, Ewens said.
"We become the zone where the greatest amount of energy is available," he said.
"This year just so happens to be that the atmospheric signals are driving the weather systems equally through the central and northern plains," he added. "So we've had several 'perfect storms' -- that term kind of gets overused, but Thursday's was a classic example -- of all of the elements coming together necessary for tornadoes to form."
Tornadoes happen most often in June because it tends to be the month when strong upper-atmospheric winds blow in a different direction than lower-level winds, said WDAY-TV Chief Meteorologist John Wheeler.
"And I think that was the key ingredient to why we had so many strong tornadoes on Thursday -- wind shear," he said. "Winds were blowing a completely different direction at the ground than aloft."
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Severe weather patterns typically don't taper off until mid-July, Ewens said.
"If you went just from a climatological standpoint, we could easily be in a storm pattern for the next several weeks," he said.
However, there are indications that the weather is going to dry out and heat up as July arrives, he said.
While the frequency for tornadoes peaks in June, the threat doesn't disappear after the Fourth of July, Wheeler said, noting the severe weather season usually lasts from midto late April to late August or early September.
"Severe weather up here in the summertime is a little bit like cold snaps in the winter," he said. "It's more likely to be really, really cold in January, but some winters the coldest weather we have is right after Thanksgiving or first day of March.
Mike Nowatzki is a reporter at The Forum of Fargo-Moorhead, which is owned by Forum Communications Co.
