Muggy Monday, then storms tonight

Maybe it's just me, but doesn't a root beer float or a cold glass of lemonade sound refreshing on a day like this? Temperatures in the 80s this afternoon are near normal for late July, but the very sticky dew points in the upper 60s and low 70s over most of Minnesota are beyond our usual mugginess.

An unsettled weather pattern will take some of that moisture and turn it into showers and storms. A huge cluster of thunderstorms has been pouring on Iowa this afternoon while, to our west, storms have been much slower to develop across the Dakotas.

july 28 - ltng2
A very large system of thunderstorms has been crossing Iowa. Vaisala

Some storms could become severe this evening from the Dakotas into western Minnesota and then continue overnight. Strong thunderstorms are likely to cross Minnesota from the west tonight and reach the metro area late tonight.

July 28 - severe outlook today
Severe weather today into tonight is most likely across the Dakotas. Storm Prediction Center

Warm, moist air is the fuel for thunderstorms. Expect atmospheric moisture levels, as measured by the dew point, to increase toward morning. The cause is not only an advancing warm front but also a nocturnal low-level jet stream expected to form overnight.

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These "low-level jets" are common in the central United States during the warm season if the atmospheric flow above the surface boundary layer is set up from the south or southwest.

In simplistic physics, the atmosphere cools more quickly in the evening over the high plains east of the Rocky Mountains than it does over lower elevations closer to the Mississippi River. That thermal gradient creates a broad southerly flow.

When the usual nighttime boundary layer inversion develops, warm, moist air from the Gulf of Mexico about 1,000 feet above the surface becomes disconnected from the stable air lying beneath the inversion. That muggy air then accelerates northward toward the Upper Midwest, where it can fuel thunderstorms during the night and into the morning. The low-level jet collapses when daytime heating mixes the slowly moving boundary layer up into the faster jet.

July 28 - 925 mb 12Z fcst
Low level jet stream winds are forecast to exceed 40 miles per hour (35 knots) from Texas to Minnesota late tonight. NOAA/College of DuPage

There are a few questions about the chances of precipitation later on Tuesday. The main issues have to do with whether clouds from overnight storms will clear out so that the sun can warm the unstable atmosphere, and also where any surface boundaries might be, as they can be prime locations to initiate convection.

The most likely scenario seems to be that a cold front advancing from the west will trigger strong and possibly severe thunderstorms Tuesday afternoon and evening, mainly southeast of the metro area. The southeast corner of the state, such as around Rochester, Wabasha and Winona, and on toward La Crosse, Wis., might be in the target.

July 28 - Day 2 severe wx risk
A slight risk for severe storms after 7 a.m. Tuesday has been posted for mainly southeastern Minnesota. Storm Prediction Center

By Wednesday we will be enjoying a slightly cooler, less muggy flow from the northwest.

July 28 - Wednesday fcst map
Wednesday will be breezy and more comfortable. Weather Prediction Center