August 21, 2018

How Thunderstorms Near Madison, Wisconsin, Produced a Foot of Rain in a Few Hours

A significant flash flood event unfolded just west of Madison, Wisconsin, on Monday evening, after thunderstorms dropped more than a foot of rain in just a couple of hours. The resulting flash floods inundated streets and neighborhoods, washing away cars and killing at least one person. The record-breaking rainfall unfolded after all the right ingredients came together over an extremely small area.

The National Weather Service's precipitation analysis for Monday, August 20th, showed a bullseye of heavy rain over a tiny part of west-central Dane County, Wisconsin, just west of the state capital of Madison. There was a sharp gradient between a rainy afternoon and a flooding emergency. The greatest totals were measured in Middleton, where some gauges recorded more than a foot of rain in just a few hours.

Monday's rainfall event was unprecedented in this area. The most rain ever recorded in 24 hours in Wisconsin was 11.72" in Mellen, Wisconsin, back in 1946. The heavy rain near Madison will become the heaviest rain ever recorded in 24 hours in Wisconsin if meteorologists verify reports of more than a foot of rainfall.

There aren't many natural or man-made waterways in the world that can handle that much rain all at once. The effect of the heavy rain and flooding is readily apparent on webcams operated by the USGS. The above video, taken by the USGS at Pheasant Branch in Middleton, Wisconsin, shows how quickly the water rose with the rain on Monday. The time lapse video starts at 7:00 AM on Thursday and runs through noon on Tuesday.

The thunderstorms that triggered the flash flooding near Madison were associated with a low-pressure system over the Mississippi Valley. The lines of storms produced damaging winds and even a couple of tornadoes from southern Wisconsin through Louisiana.

Dane County's deluge started when individual cells began training over the Madison area in the early afternoon hours on Monday. A particularly heavy thunderstorm reached Madison's western suburbs just as a line of thunderstorms approached from the south; the interplay between the existing heavy rain, the approaching line of storms, low-level winds converging near Madison, and a small mesoscale convective vortex all allowed the thunderstorms to sit and pivot over Dane County for several hours, prolonging the heavy rain which led to the flash flood emergency

The storms over south-central Wisconsin formed in just the right environment to wring out every drop of tropical moisture from the atmosphere. The low-pressure system responsible for all the active weather dragged deep tropical moisture north into Wisconsin on Monday.
Source: Pivotal Weather
You can measure the amount of moisture in the atmosphere by looking at precipitable water (PWAT), which measures how much rain would fall if all the moisture in a column of the atmosphere were condensed and fell as rain. PWAT values for southern Wisconsin on Monday evening were greater than 1.50", which is indicative of a moist, tropical atmosphere. These high PWAT values allowed the thunderstorms to tap into a deep reservoir of moisture and produce copious amounts of rain in a short period of time.

It's hard to predict these kind of localized events ahead of time. NWS Milwaukee did issue a flash flood watch for southern Wisconsin ahead of Monday's heavy rain. Meteorologists knew ahead of time that the setup would allow for the potential for flash flooding, but there was no way to know in advance just how bad the rain would be. This is why it's so important to listen for warnings even when you're not expecting the very worst from the storms that day. The atmosphere is complicated and it doesn't take much for regular thunderstorms to quickly turn into a big issue.

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I graduated from the University of South Alabama in 2014 with a degree in political science and a minor in meteorology. I contribute to The Weather Network as a digital writer, and I've written for Forbes, the Washington Post's Capital Weather Gang, Popular Science, Mental Floss, and Gawker's The Vane. My latest book, The Skies Above, is now available. My first book, The Extreme Weather Survival Manual, arrived in October 2015.