Valid 2026-10-28 12:00 UTC – 2026-10-29 12:00 UTC

THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS FOR PARTS OF THE UPPER MISSISSIPPI VALLEY

Isolated severe thunderstorms are possible over the Upper Mississippi Valley on Wednesday.

Synopsis

Broad mid-level troughing will encompass the upper air pattern over much of the CONUS on Wednesday, with a pronounced embedded impulse poised to traverse the northern Plains and impinge on the Upper MS Valley by late afternoon into early evening. The approach of this mid-level impulse will support rapid surface troughing over the Upper MS Valley, with some low-level moisture return expected. While buoyancy will be relatively weak, strong tropospheric flow accompanying the passing mid-level and surface troughs, along with adequate deep-layer ascent, will promote thunderstorm development over the Upper MS Valley by evening. There is some potential (albeit conditional) that a couple of these thunderstorms may become severe.

Upper MS Valley

As the surface trough intensifies through the day, a narrow ribbon of 60+ F surface dewpoints will advect northward ahead of the trough, beneath 7+ C/km mid-level lapse rates, contributing to 500-1000 J/kg MLCAPE. Furthermore, the strengthening of the surface trough will also encourage the development of a 40+ kt south-southwesterly low-level jet just before sunset, when buoyancy should be relatively maximized. Considerable veering and strengthening flow fields with height will support enlarged and curved low-level hodographs, especially closer to a warm front along the MN/WI border.

The vertical shear profile and expected MLCAPE magnitudes would certainly support supercells capable of producing all severe hazards. However, potential cloud cover may inhibit robust boundary-layer heating, and some MLCINH may linger just above the boundary layer as well. Therefore, any surface-based severe potential that can materialize will be highly dependent on stronger surface heating and subsequent CIN erosion. Still, at least some elevated buoyancy may be realized in the absence of stronger surface heating, so thunderstorms forced along the lee trough/cold front may still be accompanied by a hail threat.

Squitieri · 10/27/2026