A stalled frontal boundary will spark widespread thunderstorms Thursday afternoon and evening in parts of the Midwest. The hot, oppressively humid environment will provide an abundance of fuel for torrential downpours and scattered severe weather. Some of the storms Thursday evening could be capable of producing damaging wind gusts and large hail. Unfortunately, the frontal boundary is expected to stay put, yielding little relief from the humidity behind the storms.
An upper-level ridge of high pressure will shift southeastward Thursday towards the Ohio River Valley. This will place the northern Plains and the upper Great Lakes on the periphery of the high’s circulation, along the “ring of fire” for thunderstorms. A potent mid-level trough will edge into the upper Midwest in place of the departing ridge, leaving the region to lie in the right entrance region of a strong jet streak. The divergence of air near this region will drive and accelerate ascent generated by the stalled frontal boundary.
The severe storms will develop over a narrow swath between the upper Michigan peninsula and northeastern Nebraska. Cities like Eau Claire, Minneapolis, Sioux Falls, and Pierre stand in the line of fire from damaging winds and large hail. This corridor resides just south of the stalled frontal boundary, underneath the right entrance region of the jet streak and along the outer periphery of the departing upper-high. All of these upper-level ingredients will act together with the callously soupy air mass and extreme instability to generate widespread thunderstorms and scattered severe thunderstorms in this region.
Storms will first develop over the central Plains and southern Minnesota late this afternoon, joining to form clusters and spreading to western Wisconsin by early this evening. A few linear systems will develop by this time. These will be the most potent storms of the evening, maintaining the best chance of producing damaging wind gusts. Outflow from the storms will eventually trigger storm development in the western upper Michigan Peninsula later in the evening.
Tornadoes are not expected with Thursday’s thunderstorms. The sparing factor limiting the extent and intensity of the severe weather will be the lack of wind shear over most of the threat region. Wind shear is necessary to separate ascending air in the updraft from descending air in the downdraft. If these are not separated, storms cannot organize or intensify because ascending air will be overwhelmed by descending air. Wind shear is also necessary to generate the rotating updrafts of tornadoes.
The cold front will slide slightly southeast by Friday afternoon. Cloud cover and remnant morning storms will keep the vicinity near the front cooler, but the high humidity levels will only modestly decrease, from dew points in the mid to upper 70s to the low to mid 70s. The very warm, humid conditions will continue into the weekend as a ridge of high pressure over the Southwest US expands eastward and a ridge over the Mississippi River Valley shifts southward.