When strong winds, combined with rising water and humidity, combine to create a large band of very moist air that’s sweeping through the upper Midwest, you get weather like this.
Atmospheric rivers occur when air moving over a relatively dry area sinks slowly, and a high pressure center to the east pulls a similar river of moist air southwest. Although it’s not unusual for an atmospheric river to form in the Pacific Ocean, this one is heading towards the middle of the United States from the Gulf of Alaska, colliding with this fally area of air.
This means that a large area of high pressure is sitting over the middle and upper Midwest. The air is trying to push out, but without anyplace to go, it winds up trapped within the atmospheric river. And all of the moisture in the atmosphere is being pulled up through this band of very moist air, which forms what’s called a surge of low pressure.
This surge of low pressure, which is currently centered to the west of Madison, is causing widespread rain and damaging winds to hammer the Northern Plains and Upper Midwest.
In Wisconsin, areas of northern Lower Michigan, and as far south as northern Minnesota have already received in excess of 6 inches of rain in less than a week. According to the National Weather Service, some areas of northern Wisconsin have already received more than 15 inches of rain since the last week of June.
Southeast of the river, flash flooding has already hit and is still moving in parts of Nebraska, Iowa, South Dakota, Minnesota, and Wisconsin. Parts of Iowa’s State Highway 23 have been temporarily closed as water levels in the Des Moines River have risen too high to drive on. As the river rises to its crest, it’ll likely shut down that road again.
Stay with FOX6 News and FOX6Now.com for updates as the weather turns even more soggy.