MESOSCALE PRECIPITATION DISCUSSION 0452

NWS WEATHER PREDICTION CENTER COLLEGE PARK MD

700 PM EDT TUE JUL 05 2022



AREAS AFFECTED...SOUTHERN WI, NORTHERN IL, NORTHWEST IN, EXTREME

SOUTHWEST MI



CONCERNING...HEAVY RAINFALL...FLASH FLOODING LIKELY



VALID 052300Z - 060500Z



SUMMARY...THUNDERSTORMS DEVELOPING ALONG A STATIONARY FRONT WILL

INTENSIFY THROUGH THIS EVENING AND TRAIN FROM NORTHWEST TO

SOUTHEAST. RAINFALL RATES WILL AT TIMES EXCEED 2"/HR PRODUCING

3-5" OF RAINFALL. FLASH FLOODING IS LIKELY.



DISCUSSION...THE GOES-E IR IMAGERY THIS AFTN INDICATES EXPLOSIVE

GROWTH OF CONVECTION NOTED BY RAPIDLY COOLING CLOUD TOPS ALONG A

LINE FROM FAR NORTHEAST IA TO NEAR CHICAGO, IL. THESE

THUNDERSTORMS ARE BUILDING IN RESPONSE TO EXTREME INSTABILITY

NOTED BY SPC RAP ANALYZED MLCAPE OF 5000 J/KG AND PWS MEASURED BY

GPS OF MORE THAN 2 INCHES, WELL ABOVE THE 90TH PERCENTILE FOR THE

DATE AND APPROACHING DAILY RECORDS. A STATIONARY FRONT WAS

ANALYZED BY WPC LAYING WEST TO EAST NEAR THE IL/WI BORDER, INTO

WHICH 850MB FLOW WAS CONVERGING TO DRIVE SOME ENHANCED ASCENT AND

RAISE 850MB DEW POINT TEMPERATURES TO +18C, ALSO NEAR DAILY

RECORDS FOR THE REGION. ADDITIONAL BROAD SYNOPTIC ASCENT WAS

OCCURRING WITHIN THE TAIL OF AN UPPER LEVEL JET STREAK DISPLACED

NORTH OF THE REGION. RAINFALL RATES WITHIN THIS BLOSSOMING

CONVECTION WERE ALREADY BEING ESTIMATED AS HIGH AS 1.5"/HR VIA

KARX WSR-88D, AND DESPITE NORMALIZED CAPE VALUES ABOVE 0.3, WARM

CLOUD DEPTHS OF MORE THAN 13,000 FT INDICATE AT LEAST HYBRID

WARM-CLOUD PROCESSES TO ALLOW THESE EFFICIENT RAINFALL RATES TO

INCREASE WITH TIME.



THUNDERSTORMS DEVELOPING ALONG THIS BOUNDARY ARE LIKELY TO

INCREASE BOTH IN COVERAGE AND RAINFALL INTENSITY THROUGH THE

EVENING WITHIN THIS FAVORABLE OVERLAP OF ASCENT AND

THERMODYNAMICS. IT IS POSSIBLE THAT AS SOME OF THE INSTABILITY

GETS EXHAUSTED, RAINFALL RATES COULD INTENSIFY TO EXCEED 2"/HR AS

UPDRAFTS PERSIST LONGER IN THE EXTREMELY MOIST ENVIRONMENT. THIS

IS SUPPORTED BY HREF HOURLY RAIN PROBABILITIES FOR 2"/HR PEAKING 

AFTER 00Z. ADDITIONALLY, A POTENT MCS/MCV MOVING ACROSS NORTHERN

IOWA WILL RACE EASTWARD AND EVENTUALLY CLEAR THE INSTABILITY AND

CONVECTION TO THE SOUTH, BUT A BRIEF UPTICK IN THE LLJ IN ADVANCE

OF THIS FEATURE COULD HELP INTENSIFY MOIST ADVECTION AND THE

SUBSEQUENT RAINFALL RATES.



THESE RAIN RATES ARE LIKELY TO TRAIN FROM NW TO SE ALONG THE FRONT

NOTED BY BOUNDARY PARALLEL 0-6KM MEAN WINDS AND CORFIDI VECTORS.

DESPITE MODEST INFLOW OF JUST 15 KTS, THIS SHOULD BE JUST ENOUGH

TO OFFSET SOME OF THE LOSS OF CAPE THROUGH THE EVENING TO PERSIST

THIS HEAVY RAIN BEYOND NIGHTFALL. WHERE THE HEAVIEST RAINFALL

OCCURS, RAINFALL AMOUNTS COULD REACH 3-5" AS REFLECTED BY HIGH RES

AND HREF PROBABILITIES. SOME OF THIS RAIN COULD FALL ATOP SOILS

THAT ARE ALREADY PRIMED FROM 24-HR RAINFALL THAT HAS BEEN MEASURED

BY MRMS TO BE 1-3" FROM SOUTHERN WI, THROUGH THE CHICAGO METRO

AREA, AND INTO NW INDIANA. ALONG THIS STRIPE, BOTH 1-HR FFG AND

3-HR FFG ARE EXTREMELY COMPROMISED, WHICH IS REFLECTED BY HREF

EXCEEDANCE PROBABILITIES REACHING ABOVE 40% TONIGHT. WHILE THERE

EXISTS SOME UNCERTAINTY INTO THE EXACT EVOLUTION AND TIMING OF

THIS EVENT, IT IS BECOMING MORE LIKELY THAT THE AXIS OF HEAVIEST

RAINFALL WILL FEATURE LITTLE LATITUDINAL MOTION DURING THE NEXT

SEVERAL HOURS UNTIL THE MCS SWEEPS THROUGH. THIS SUGGESTS AN

ENHANCED FLASH FLOOD RISK, AND FLASH FLOODING IS LIKELY WHERE THE

MOST PRONOUNCED TRAINING OCCURS, ESPECIALLY IN ANY URBAN AREAS.