‘Double whammy’ of low wind, high humidity made for Dane County’s state record rainstorm

August 23, 2018

The storm that dumped more than a foot of water on parts of Dane County Monday has yet to be certified as a state record, but by almost any measure it was a one-in-a-thousand weather event.

The National Weather Service is still evaluating whether the 14.7 inches of rain captured in a Cross Plains rain gauge can be officially verified. If so, it would shatter Wisconsin’s all-time 24-hour rainfall record of 11.72 inches, which occurred near Mellen in 1946.

Even the highest official reading — 11.03 inches measured 1 mile southwest of Middleton — falls squarely in the range of a 1,000-year rain event.

In a typical summer, Madison gets just under 13 inches of rain.

“A summer’s worth of rain in one day,” said Stephen Vavrus, a scientist at UW-Madison’s Center for Climatic Research. “We haven’t had anything like this in memory around here. Or even beyond.”

The storm was the result of a low-pressure system colliding with very humid air, said Denny VanCleve, a meteorologist with the National Weather Service.

With no strong winds to push it off to the east, the storm parked to the west of Madison with a steady supply of moisture.

“There will usually be an area that kind of stays under the gun longest,” VanCleve said. “We knew there was a potential for floods. We didn’t know it was going to be western Dane County.”

Weather Service meteorologist Sarah Marquardt said that, in the metro area, the rain simply fell faster than the ground and storm sewers could absorb it, while communities to the west were hit by additional river flooding from water funneling downstream.

“It was a combination of a lot of things,” she said. “Different things happening in different places.”

Ken Potter sat up Monday night watching the storm track on his iPad. A retired professor of civil and environmental engineering at UW-Madison, Potter has studied the potential impact of an extreme rain event on the Yahara lakes.

He said he was stunned by the rainfall totals, which surpassed the three-day 14-inch deluge that washed out Lake Delton in 2008.

“It could have been a lot worse,” Potter said. “If you move the storm a little bit to the east — this is exactly the kind of storm I feared.”

Vavrus said climate models suggest that westerly flows are weakening as the atmosphere warms, while warmer air can hold more moisture.

“That double whammy of slow movement and high moisture — that certainly raises concerns for more of these events,” he said.

Potter, who headed a committee that last year called on Dane County to adopt stricter stormwater regulations for new developments, said current urban design standards essentially double the amount of water that ends up in ditches, streams and eventually the lakes.

“What scares me is if this is a weather pattern that’s going to be more prevalent,” Potter said. “Then we’ve got a real issue on our hands.”

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