Strong thunderstorms expected over western Illinois this afternoon/evening

The National Storm Prediction Center has issued the Mesoscale Outlook discussion below indicating a good chance  of strong thunderstorms – possibility a few severe storms over western Illinois Outlined area on highlighted map) this Wednesday afternoon/evening. Scattered showers and thunderstorms will be likely in the Chicago area during this same time-frame.

Mesoscale Discussion 1508
NWS Storm Prediction Center Norman OK
0235 PM CDT Wed Aug 16 2017

Areas affected…Eastern Iowa…Western Illinois…Eastern Missouri

Concerning…Severe potential…Watch unlikely

Valid 161935Z – 162200Z

Probability of Watch Issuance…20 percent

SUMMARY…A potential for wind damage and hail will be possible late
this afternoon from eastern Iowa southward across western Illinois
into far eastern Missouri. The severe threat should remain isolated
and weather watch issuance appears unlikely.

DISCUSSION…The latest surface analysis shows a 1006 mb low over
western Iowa with a very moist airmass located eastward and
southward from the low across much of Iowa into northeast Missouri
into northwest Illinois. Surface dewpoints are generally in the
lower to mid 70s F and temperatures have risen into the lower 80s F
across much of the area. This is contributing to between 1000 and
1500 J/kg of MLCAPE according to the RAP. In addition, water vapor
imagery shows a pronounced dry slot moving into southwestern Iowa
and northwestern Missouri. Ahead of this feature, strong large-scale
ascent and moderate deep-layer shear could be enough for an isolated
severe threat with wind damage and hail possible.

Further to the south across eastern Missouri, the RAP is showing
stronger instability with MLCAPE values in the 1500 to 2000 J/kg
range. In addition, the St. Louis, MO WSR-88D VWP shows 0-6 km shear
at around 30 kt with some directional shear in the low-levels. This
will be enough for an isolated severe threat. Wind damage will be
possible with the faster moving line segments. An isolated rotating
storm with large hail will also be possible.