Valid 091630Z – 101200Z
…THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS OVER PORTIONS OF
NORTHERN IOWA…SOUTHERN MINNESOTA…AND WESTERN WISCONSIN…
Severe thunderstorms are possible this afternoon and evening over
parts of the Upper Midwest and upper Mississippi Valley regions.
A series of shortwave troughs are rotating across the
central/northern Plains today. Forcing for ascent is evident in
water vapor imagery ahead of these features, moving into eastern
SD/NE and much of MN/IA. Meanwhile at the surface, a warm front
extends from southeast SD across central IA. This boundary will
lift slowly northward through the afternoon. Forecast soundings
suggest that continued slow heating along/south of the warm front
will sufficiently weaken the capping inversion, leading to scattered
thunderstorms by late afternoon over western IA. These storms will
track into the frontal zone, where backed low-level winds and
enhanced vorticity may aid in the development of low-level
mesocyclones and isolated tornadoes. Forecast hodographs show
effective helicity values of 300-400 m2/s2 and sufficient deep layer
shear to maintain supercell characteristics. Otherwise, hail and
gusty winds will be possible in the strongest cells. This activity
should persist into the early evening – moving across parts of
southern MN and into western WI.
Widespread cloud cover is present this morning over eastern SD.
This should limit the risk of strong convection through much of the
day. However, the combination of a midlevel dry slot and the
approach of the surface low may result in a few thunderstorms
developing late this afternoon over northeast SD. The strongest of
these cells could produce gusty winds.