Why the rotten weather? Blame jet stream

MINNEAPOLIS (AP) -- The jet stream was flowing unusually far south for this time of year when it hit a front of hot, moist air rubbing against a cool-air mass and sparked the violent storms in Minnesota this week.

The Upper Midwest is stuck in this pattern for the time being.

"I would be surprised if the weather became as extreme next week, but there could be more that is at least marginally severe," said Christopher Davis, a research scientist with the National Center for Atmospheric Research who is in St. Louis this summer studying Midwestern storms.

Acceptance that it was Minnesota's turn is one way to deal with the sweat, the sleepless nights and the power failures that turned out the lights for about 200,000 Xcel Energy customers in the Twin Cities. As of Thursday morning, about 22,000 Twin Cities homes were still without power, although the company said 400 line crew workers were working 16-hour shifts to restore power.

"We had been coasting through a very tranquil severe-weather season," said Craig Edwards, chief meteorologist for the National Weather Service in Chanhassen. "We were about due. It's that time of the year."

In short, it's the time of year when the Midwest is the battleground where tropical air masses collide with cooler, drier air.

"We have had a semi-stationary front extending in Minnesota from the northeast to the southwest, a boundary between warm and moist air to the east of it and cooler dry air to the west," said Greg Spoden, assistant state climatologist. "Storms often form along this zone of conflict."

The cool air pushes the warmer stuff upward. On Tuesday, the muggy surface air got an added shove from temperatures that rose to 88 degrees in central Minnesota, lending speed to the updraft and power to the storm that was forming. Once aloft, the air cools, causing the moisture to condense and form rotating clouds.

The upshot wasn't just your garden-variety thunderstorm. It was what weather experts call a supercell, Davis said. Supercells are one focus of his team's research this summer in a study funded by the National Science Foundation.

At the center of a supercell is a rotating and rising column of air called a mesacyclone. At the bottom, tornadoes tend to form. This dangerous combination hovered over south-central Minnesota when a tornado roared through Buffalo Lake Tuesday evening.

Later, the storm cluster produced a broad wall-cloud formation in the Twin Cities area. It tends to produce damaging straight-line winds near its center and can spawn tornadoes, Davis said.

The semi-stationary front that set up conditions for this week's weather is gradually moving east. But another contributing element may hover over Minnesota for some time.

The jet stream has parked farther south than usual for this time of year. This highway of air, which influences much of our weather, separates cold polar air from warmer air to the south, and it usually shifts north toward Canada in the summer.

Now, however, it has dipped deep into the United States, sweeping north over Minnesota, then returning to Canada. This week, its high-speed winds paralleled the storm front closer to the ground, Spoden said.

"It creates a hole in the upper atmosphere," Spoden said. "Mother Nature doesn't like a hole, and she tries to fill it."

Nature's mechanism is to lift air from the lower atmosphere. So it became another force sucking muggy surface air to the level where it forms clouds -- and trouble for people on the ground.

Supercells could form without help from the jet stream, but the stream adds to the weather's overall instability, said Edwards of the National Weather Service.

"The jet stream gets the atmosphere rotating," he said.

That might help explain why the tornado-producing storm cluster seemed to stay over Minnesota on Tuesday rather than race through the region, as tornadoes often do, Edwards said. Rather than run along predictable lines, the storm seemed to walk in circles, he said.

"We are dealing with the dynamics of the atmosphere that are always changing, and one thunderstorm over Anoka is going to change the atmosphere over Chisago County," he said. "We had multiple thunderstorms that were creating their own environment, fighting for energy, buoyance and rotation."