With widespread and heavy rain in the forecast Tuesday, the National Weather Service is warning of possible flash flooding across southern Wisconsin, but Dane County officials say the risk of major flooding along the Yahara chain of lakes is far lower than a year ago.
Madison received 6.79 inches of rain — more than twice the normal amount of precipitation — through the first 29 days of September, according to the National Weather Service. That puts it on track to be the 8th wettest September on record.
Forecasters expect another 2 to 4 inches of rain could fall Tuesday and Wednesday, which could trigger flash flooding along creeks and small rivers as well as city streets.
“Soils are pretty saturated,” said Marc Kavinsky, a meteorologist with the National Weather Service in Milwaukee.
A flash flood watch is in effect from 7 a.m. Tuesday through 7 a.m. Wednesday in central and southern Wisconsin, including Columbia, Dane, Green, Green Lake, Iowa, Lafayette, Marquette, Rock, and Sauk counties.
Roadways could become flooded and potentially hazardous for travel, the weather service said.
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While levels on lakes Monona and Waubesa have risen in recent weeks, the Yahara River is flowing fast enough to bring them down about half an inch per day, said John Reimer, deputy director of Land and Water Resources for Dane County.
Lakes Mendota and Monona are about 18 inches lower than they were at their peak during the flooding of September 2018, providing space to take on stormwater runoff.
Excessive rainfall this week — more than three inches, especially over a short period of time — could lead to minor flooding on lakes Mendota and Waubesa, possibly requiring sandbags to protect flood-prone areas.
But Reimer said the greater threat may be from below. He warned that already high groundwater levels combined with heavy rains could result in localized flash flooding and water in basements.
Last year Madison recorded 5.46 inches of rain in September — about 2.3 inches more than normal — on the heels of record downpours in August that led to widespread flash flooding as well as near-record lake levels.
Reimer said the addition of three aquatic plant harvesters — part of the county’s $23.8 million investment in flood protection and mitigation — have helped keep water moving through the lakes, though the main reason for lower water levels is the lower amount of rain in August.