Overnight threat for additional heavy rainfall and flash flooding remains high

MESOSCALE PRECIPITATION DISCUSSION 0528
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
851 PM EDT FRI JUL 21 2017

AREAS AFFECTED…FAR EASTERN IA…SOUTHERN WI…NORTHERN
IL…NORTHERN IN…SOUTHWEST LOWER MI…NORTHWEST OH

CONCERNING…HEAVY RAINFALL…FLASH FLOODING LIKELY

VALID 220045Z – 220600Z

SUMMARY…FLASH FLOODING WILL BE LIKELY IN ASSOCIATION WITH STRONG
AND ORGANIZED THUNDERSTORMS THROUGH THE REMAINDER OF THE EVENING
HOURS.

DISCUSSION…SEVERAL CLUSTERS OF INTENSE COLD-TOPPED CONVECTION
ARE SEEN CONTINUING TO BECOME BETTER ORGANIZED OVER PARTS OF FAR
EASTERN IA…SOUTHERN WI AND NORTHERN IL AS A WAVE OF LOW PRESSURE
ADVANCES EAST ACROSS NORTHWEST IL ALONG WITH A SUPPORTING
SHORTWAVE IMPULSE. THE CONVECTION IS FORMING WITHIN A RATHER
HIGHLY SHEARED ENVIRONMENT WITH OVER 40 KTS OF EFFECTIVE BULK
SHEAR AND ALSO ALONG A STRONG INSTABILITY GRADIENT INVOLVING THE
FRONT. JUST SOUTH OF THE FRONT…THE AIRMASS IS VERY UNSTABLE WITH
OVER 3000 J/KG OF MLCAPE…AND IT IS VERY MOIST WITH PWATS NEAR
2.25 INCHES AND WITH SURFACE DEWPOINTS IN THE MID TO UPPER 70S.

GIVEN THE LEVEL OF SHEAR AND VERY IMPRESSIVE THERMODYNAMICS…THE
ENVIRONMENT OVER THE NEXT SEVERAL HOURS WILL BE HIGHLY CONDUCIVE
FOR A STRONG MCS TO EVOLVE AS CONVECTIVE CLUSTERS MERGE AND
CONSOLIDATE INTO A LARGER CONVECTIVE COMPLEX. THIS WILL BE AIDED
BY SHORTWAVE DYNAMICS AND WARM ADVECTION AHEAD OF THIS SURFACE
WAVE WHICH IS TRAVERSING A WAVY QUASI-STATIONARY FRONT. A
SOUTHWEST LOW-LEVEL JET IS FORECAST BY THE LATEST RAP GUIDANCE TO
INCREASE TO NEAR 50 KTS BY 06Z AHEAD OF THE SURFACE WAVE..AND THIS
SHOULD STRONGLY ENHANCE THE LARGER SCALE FORCING AND THERMODYNAMIC
SUPPORT FOR THE CONVECTIVE COMPLEX.

EXPECT RAINFALL RATES OF UP TO 3 INCHES/HR GIVEN THE DEEP COLUMN
OF MOISTURE AND STRONG INSTABILITY ALONG THE FRONT. RAINFALL
AMOUNTS THROUGH 06Z OF AS MUCH 4 TO 6 INCHES WILL BE POSSIBLE AND
ESPECIALLY WHERE CELL-MERGERS AND SOME EPISODIC TRAINING OF
CONVECTIVE CLUSTERS OCCUR. THESE AMOUNTS ARE LIKELY TO RESULT IN
FLASH FLOODING…WITH SOME OF IT SIGNIFICANT GIVEN RECENT HEAVY
RAINFALL AND THE URBAN CORRIDORS AREAS…INCLUDING THE CHICAGO
METROPOLITAN AREA. THIS WILL LIKELY BE A LONG-LIVED MCS..AND
ADDITIONAL MPDS WILL BE ISSUED ACCORDINGLY.