MESOSCALE PRECIPITATION DISCUSSION 0377
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
522 AM EDT FRI JUN 23 2017

AREAS AFFECTED...NORTHEAST IL AND NORTH-CENTRAL IN

CONCERNING...HEAVY RAINFALL...FLASH FLOODING POSSIBLE

VALID 230921Z - 231351Z

SUMMARY...THUNDERSTORMS ARE SHOWING SOME TRAINING AND BACKBUILDING
CHARACTER ACROSS NORTH-CENTRAL IN.  HOURLY RAIN RATES UP TO 2"
WITH LOCAL AMOUNTS TO 4" ARE POSSIBLE.

DISCUSSION...THUNDERSTORMS ARE TRAINING/BACKBUILDING DOWNWIND OF
AN INSTABILITY POOL TO A WEAK DEW POINT GRADIENT ALONG THE IN/IL
BORDER, WHILE OTHER ACTIVITY IS BUILDING TO ITS WEST AND NORTH.  A
JET STREAK ALOFT FROM TROPICAL DEPRESSION CINDY ALONG WITH AN
INCOMING SHORTWAVE MOVING ACROSS WESTERN WI ARE HELPING DIVERGENCE
ALOFT IN THIS REGION.  PRECIPITABLE WATER VALUES ARE 1.75-2". 
INFLOW AT 850 HPA IS CONVERGENT AT 30-35 KTS PER VAD WIND
PROFILES.  EFFECTIVE BULK SHEAR IS ~25 KTS WHICH WHEN COMBINED
WITH NEARLY UNIDIRECTIONAL FLOW WITH HEIGHT IS ORGANIZING
CONVECTION INTO A BAND.  MUCAPE VALUES OF 1000+ J/KG LIE ACROSS IL.

THIS ACTIVITY LIES WELL AHEAD OF A FRONTAL BOUNDARY WHICH SHOULD
INTERACT/POSSIBLY MERGE WITH CONVECTION FROM THAT BOUNDARY AFTER
13Z.  ACTIVITY SHOULD INCREASE IN COVERAGE OVER THE NEXT SEVERAL
HOURS IN THE MEANTIME, WITH RADAR AND SATELLITE TRENDS SUGGESTING
SOME ADDITIONAL NORTHWARD EXPANSION IS POSSIBLE.  SLIGHT VEERING
WITH TIME SHOULD ALLOW FOR SOME SOUTHWARD DRIFT OVER THE NEXT
SEVERAL HOURS.  WHILE THE MESOSCALE GUIDANCE HAS A SIGNAL FOR
RAINFALL NEAR HERE, THE MAGNITUDE OF THE THREAT APPEARS TO BE
HIGHER THAN ADVERTISED.  HOURLY RAIN RATES UP TO 2" AND LOCAL
AMOUNTS UP TO 4" APPEAR POSSIBLE.  THE UNCERTAINTY ACROSS THE
REGION ALONG WITH RELATIVELY HIGH FLASH FLOOD GUIDANCE VALUES KEPT
THE CATEGORY AS POSSIBLE.  

ROTH

ATTN...WFO...ILN...ILX...IND...IWX...LOT...

ATTN...RFC...NCRFC...OHRFC...

LAT...LON   41568536 40548472 39688695 40688846 41338734