Valid 092000Z – 101200Z
…THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF
NORTHERN/CENTRAL IOWA…SOUTHERN MINNESOTA…AND WESTERN
WISCONSIN…
…SUMMARY…
Severe thunderstorms will remain possible this afternoon and evening
over parts of the Upper Midwest and upper Mississippi Valley
regions. Isolated tornadoes, large hail, and damaging winds could
occur.
…20Z Update…
A subtle shortwave trough near the NE/IA border will likely
encourage additional storm development later this afternoon across
northern/central IA. A surface warm front will continue lifting
slowly northward over this region and into southern MN through this
evening. Isolated tornadoes along with some risk for large hail and
damaging winds will be possible with any storms that form in the
warm sector and then cross the front. Generally small changes have
been made to the southern extent of the Marginal and Slight Risk
areas in central IA to account for observational and short-term
model trends. See Mesoscale Discussion 1941 for more information on
the near-term severe threat across this region.
The Marginal Risk has been expanded a little westward to include
more of northeastern/eastern SD. Isolated storms may form along a
surface trough and to the east of a weak low centered over central
SD, posing some threat for hail, gusty winds, and perhaps a tornado
or two.
No changes have been made to the Marginal Risk across parts of
VA/NC. Isolated hail and strong/gusty winds will remain possible
through the early evening with any storms that can develop and move
southeastward.
..Gleason.. 09/09/2019
.PREV DISCUSSION… /ISSUED 1113 AM CDT Mon Sep 09 2019/
…IA/MN/WI…
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.
…Eastern SD…
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.
$$