Mesoscale severe weather outlook – Includes portion of the Chicago area – Watch unlikely

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Mesoscale Discussion 1440
NWS Storm Prediction Center Norman OK
0518 PM CDT Thu Aug 03 2017

Areas affected…Southern WI…extreme eastern IA…and
northern/central IL

Concerning…Severe potential…Watch unlikely

Valid 032218Z – 040045Z

Probability of Watch Issuance…20 percent

SUMMARY…Isolated strong to severe storms will be possible into the
early evening, with hail and locally strong wind gusts being the
primary threats from southern Wisconsin and extreme eastern Iowa
into parts of northern and central Illinois. A tornado threat
cannot be ruled out, mainly in southern Wisconsin. However, the
limited potential for severe storm coverage is expected to preclude
the need for a watch.

DISCUSSION…Late afternoon surface analysis showed an area of low
pressure centered in southwest WI with a cold front trailing to the
south-southwest through extreme eastern IA to northwest to western
MO, while a warm front extended east through southern WI into
central Lower MI. Objective analyses through the late afternoon
indicated the environment in the wake of earlier convection and east
of the cold front across southern WI and northern/central IL has
destabilized some, despite weak midlevel lapse rates, with MLCAPE of
1000-1500 J/kg.

Stronger forcing for ascent (attendant to a progressive upper MS
Valley shortwave trough) appears to be spreading across the cold
front and warm sector, at this time, per water vapor imagery with a
band of 40-50 kt westerly 500-mb winds per area VAD data across
WI/northern IL. Although storms have developed along the cold
front, weak midlevel lapse rates and veered low-level winds appear
to be limiting the coverage thus far of stronger/sustained storms.
Although effective bulk shear of 30-40 kt suggests organized storms
will be possible as the cold front and shortwave trough advance east
this evening, veered low-level winds should continue to limit
convergence and greater coverage of storms along the cold front.

The tornado threat appears to be greatest with any storms that
interact with the warm front, where low-level shear is the greatest.
Stronger surface vorticity and surface-3 km MLCAPE of 100-125 J/kg,
objectively analyzed in vicinity of the lake breeze in northeast IL
to southeast WI, suggest an isolated tornado threat cannot be ruled
out. Despite the tornado potential, the coverage of storms is
expected to remain limited through the evening.