Severe thunderstorms still packing a punch as the reach the Chicago Metro area

MESOSCALE DISCUSSION 1386

NWS STORM PREDICTION CENTER NORMAN OK

0954 PM CDT TUE AUG 28 2018

AREAS AFFECTED...PORTIONS OF NORTHERN/CENTRAL IL...NORTHERN IN...AND

SOUTHWESTERN LOWER MI



CONCERNING...SEVERE THUNDERSTORM WATCH 357...359...

VALID 290254Z - 290400Z

THE SEVERE WEATHER THREAT FOR SEVERE THUNDERSTORM WATCH 357, 359

CONTINUES.

SUMMARY...AN ISOLATED STRONG TO LOCALLY DAMAGING WIND THREAT



CONTINUES ACROSS WW 357/359. ADDITIONAL DOWNSTREAM WATCH ISSUANCE

INTO NORTHERN IN AND SOUTHERN LOWER MI WILL PROBABLY NOT BE NEEDED.

DISCUSSION...TWO SEPARATE CLUSTERS OF THUNDERSTORMS ARE ONGOING

ACROSS NORTHERN/CENTRAL IL AS OF 0250Z. ONE OF THESE CLUSTERS WILL



IMPACT THE CHICAGO METRO AREA OVER THE NEXT HOUR OR SO, WITH MAINLY

A STRONG TO LOCALLY DAMAGING WIND THREAT. A MEASURED 59 MPH WIND

GUST OCCURRED AT THE DUPAGE AIRPORT IN IL AT 0237Z. AN INCREASE IN

RADAR REFLECTIVITY ALOFT HAS RECENTLY BEEN NOTED WITH A CELL

EMBEDDED WITHIN THE SOUTHERN PORTION OF THE LINE. WITH AROUND 35-45

KT OF EFFECTIVE BULK SHEAR PRESENT, SOME UPDRAFT ROTATION MAY OCCUR

WITH THIS EMBEDDED THUNDERSTORM, WITH PERHAPS AN ISOLATED HAIL

THREAT.

FARTHER SOUTH, ANOTHER CLUSTER IS MOVING EASTWARD AT 40-45 KT ACROSS

WEST-CENTRAL INTO CENTRAL IL. RECENT RADAR VELOCITY SIGNATURE FROM

KILX SHOWS STRONG (50-60+ KT) INBOUND VELOCITIES AROUND 5000 FT AGL.

THESE WINDS MAY NOT BE FULLY REACHING THE SURFACE GIVEN DIURNAL

COOLING AND AT LEAST SOME DECOUPLING OF THE BOUNDARY LAYER. STILL, A

MODERATELY UNSTABLE AIRMASS RESIDES DOWNSTREAM OF THE ONGOING

CONVECTION, AND STRONG TO SEVERE WINDS SHOULD REMAIN POSSIBLE FOR

THE NEXT HOUR OR TWO. THE LONGER-TERM EVOLUTION OF BOTH OF THESE

CLUSTERS REMAINS SOMEWHAT UNCERTAIN, BUT THERE WILL PROBABLY BE A

TENDENCY FOR THEM TO WEAKEN WITH EASTWARD EXTENT AS CONVECTIVE

INHIBITION CONTINUES TO INCREASE, AND THE COMPOSITE OUTFLOW FROM

FROM BOTH CLUSTERS SAGS SOUTHWARD INTO A WEAKER SHEAR ENVIRONMENT

WITH TIME. CURRENT RADAR AND MESOSCALE TRENDS SUGGEST THE OVERALL

SEVERE THREAT WILL PROBABLY REMAIN TOO ISOLATED TO JUSTIFY

ADDITIONAL SEVERE THUNDERSTORM WATCH ISSUANCE IN NORTHERN IN AND

SOUTHERN LOWER MI.