Possibility of severe thunderstorms downstate overnight
The National Storm Prediction Center has issued a Mesoscale Outlook detailing reasons why severe thunderstorms could develop overnight across southern and central portions of Illinois, including the southernmost portion of the Chicago area well south of Interstate-80 (escalloped area shown on the headlined map).
The technical discussion follows below…
Mesoscale Discussion 1305 NWS Storm Prediction Center Norman OK 1040 PM CDT Thu Aug 16 2018 Areas affected...Parts of east central Missouri and southern/central Illinois Concerning...Severe potential...Watch unlikely Valid 170340Z - 170545Z Probability of Watch Issuance...20 percent SUMMARY...Isolated to widely scattered strong to severe storm development is possible through Midnight to 2-3 AM CDT. While the severe weather threat seems limited enough that a watch is not currently anticipated, trends will continue to be monitored. DISCUSSION...A mid-level closed low now over central Iowa is forecast to gradually turn eastward into the middle Mississippi Valley through 06-09Z. As it does, it appears that associated forcing for ascent will continue to support scattered vigorous thunderstorm activity, which is already ongoing in an arcing band across northern through west central Illinois. Ahead of this band, the boundary layer remains relatively warm with surface temperatures in the upper 70s to around 80f in a narrow corridor, from around the St. Louis area northeastward through central Illinois. Coupled with dew points in the lower/mid 70s, this appears to be supporting moderate to large CAPE on the order of 2000-3000 J/kg. Although shear is still somewhat modest, the eastward progression of a belt of 30-40 kt flow in the 700-500 mb layer may contribute to at least some strengthening before the mid-level forcing and associated convection progresses through the lingering instability axis overnight. This could be accompanied by at least a short-lived period with some increase in potential for storms capable of producing severe hail and perhaps locally strong surface gusts. ..Kerr/Thompson.. 08/17/2018