Severe thunderstorm watch likely to be issued for the Chicago area early this afternoon

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The Storm Prediction Center in Norman, Oklahoma has indicated that it is almost certain  (80 percent) to issue a severe

thunderstorm watch for much if not all of the Chicago Metro area this warm, windy and increasingly more humid afternoon.

The storm are expected to exit the area to the east by early this evening, making it likely that the Chicago Cubs home opener

will be played this  evening.

Mesoscale Discussion 0475 NWS Storm Prediction Center Norman OK 1157 AM CDT Mon Apr 10 2017 Areas affected…Portions of northern and central IL…southern WI…northern IN…southwest Lower MI…and southern Lake MI Concerning…Severe potential…Severe Thunderstorm Watch likely Valid 101657Z – 101930Z Probability of Watch Issuance…80 percent SUMMARY…The area is being monitored for increasing severe- thunderstorm potential this afternoon. While the timing of more robust severe-thunderstorm development is somewhat uncertain, Severe Thunderstorm Watch issuance will be likely within the next few hours. DISCUSSION…Recent water vapor imagery highlights a compact midlevel vorticity maximum advancing across the middle MO Valley region, with attendant speed maxima extending into the southern Great Lakes region. Recent development of elevated convection has been noted across eastern IA, in association with the left-exit region of one of the speed maxima overlying a north-northeast/ south-southwest-oriented frontal zone. As the forcing for ascent continues to overtake the western extent of partially modified Gulf moisture in a broad warm sector, convection will likely increase across parts of central and northern IL and southern WI during the next couple of hours, and subsequently spread eastward and east-northeastward into the late afternoon and early evening hours. With only filtered sunshine over parts of the open warm sector, and surface dewpoints generally in the lower/middle 50s, diurnal gains in buoyancy will remain muted. Nonetheless, VAD wind profiles across the area are sampling 50-60 kt of midlevel flow offering appreciable deep shear with a component oriented orthogonal to the convection-initiating boundary. As such, supercell structures will be possible, with storm-scale upward accelerations bolstered by steep midlevel lapse rates around 8.0-9.0 C/km overspreading the area (based on 12Z observed soundings). As a result, large hail will be possible — particularly during incipient stages of convective development. Subsequently, increasingly numerous cell interactions should foster convective clustering and increasing damaging-wind potential as this activity progresses to the east and east-northeast. Surface observations and short-range RAP forecast hodographs suggest that locally backed surface winds will exist within an area bounded by Peoria, Rockford, Chicago, and Valparaiso. This may support sufficiently long/curved low-level hodographs for brief/transient low-level mesocyclones, and perhaps a brief tornado. However, the relatively scant low-level moisture and only modest gains in diabatic surface-layer heating will greatly minimize low-level buoyancy, which will tend to mitigate tornado potential.

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