MESOSCALE PRECIPITATION DISCUSSION 0250
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
730 PM EDT SAT MAY 20 2017

AREAS AFFECTED...CENTRAL AND NORTHEAST IN

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

VALID 202330Z - 210330Z

SUMMARY...THUNDERSTORMS ARE BEGINNING TO FORM NEAR AND IN ADVANCE
OF A MESOSCALE WAVE.  HOURLY RAIN RATES UP TO 2", WITH LOCAL
AMOUNTS TO 4", COULD LEAD TO ISSUES.

DISCUSSION...A WEAK MESOSCALE WAVE IS APPARENT IN THE THERMAL
FIELD ACROSS EAST-CENTRAL IL, WITH A WEAK WARM FRONT ACROSS
CENTRAL IN.  AN MCV IS APPROACHING THE REGION FROM SOUTHERN IL,
WHICH IS LIKELY AIDING DIVERGENCE ALOFT AS OF LATE.  CONVERGING
INFLOW AT 850 HPA IS 25-40 KTS PER VAD WIND PROFILES, WHICH IS
IMPORTING MLCAPES OF 1000-2000 J/KG INTO THE REGION.  THE FLOW IS
NEARLY UNIDIRECTIONAL OUT OF THE SOUTH-SOUTHWEST, WHICH APPEARS TO
BE FAVORING THE SPLITTING CELLS SEEN ON RADAR ACROSS NORTHWEST
INDIANA AS OF LATE.  PRECIPITABLE WATER VALUES ARE 1.5-1.75". 
BULK SHEAR IN THE 0-6 KM LAYER IS 30-40 KTS PER SPC MESOANALYSES.

TRENDS IN THE RAP/HRRR RUNS HAVE BEEN UPWARDS IN AMOUNTS IN THIS
AREA.  THE 12Z GFS-BASED GLAVEZ-DAVISON INSTABILITY INDEX SHOWS
INCREASING VALUES HERE THROUGH 03Z, WHICH IMPLIES INCREASING
CONVECTIVE COVERAGE.  THE LOW-LEVEL INFLOW IS ALSO FORECAST TO
INCREASE SLIGHTLY MORE OVER THE NEXT FEW HOURS.  THERE IS A
GENERAL SIGNAL FOR LOCAL AMOUNTS IN THE 2-4" RANGE ACROSS PORTIONS
OF CENTRAL AND NORTHERN IN.  FLASH FLOOD GUIDANCE IS RATHER LOW
HERE, AS RAINFALL DURING THE PAST 30 DAYS HAS BEEN ABOUT DOUBLE
THE AVERAGE VALUE.  THE AVAILABLE MOISTURE AND INSTABILITY SUPPORT
HOURLY RAIN RATES UP TO 2" WHERE CELLS TRAIN/MERGE.

ROTH

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

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

LAT...LON   41248535 41028478 40278476 40018496 39908553
            39928653 40228751 41018731 41218692