MESOSCALE PRECIPITATION DISCUSSION 0062
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
119 PM EDT FRI MAY 08 2015
 
AREAS AFFECTED...PORTIONS OF IL AND WESTERN IN 
 
CONCERNING...HEAVY RAINFALL...FLASH FLOODING POSSIBLE 
 
VALID 081718Z - 082318Z
 
SUMMARY...THUNDERSTORMS WITH HEAVY RAINFALL ARE EXPECTED THIS
AFTERNOON AND EVENING AHEAD OF AN ADVANCING COLD FRONT AND ITS
PRECEDING OUTFLOW BOUNDARY, WITH HOURLY RAIN RATES UP TO 1.75"
EXPECTED.  FLASH FLOODING IS CONSIDERED POSSIBLE.

DISCUSSION...A THERMAL/OUTFLOW BOUNDARY WAS SEEN IN THE SURFACE
OBSERVATIONS ACROSS CENTRAL AND NORTHEAST IL, ALIGNED PARALLEL TO
AND AHEAD OF A COLD FRONT WHICH LAY ACROSS WESTERN IL.  AN MCV IS
MOVING INTO THE REGION FROM SOUTHEAST MO.  INFLOW AT 850 HPA IS
CURRENTLY 25 KTS.  PRECIPITABLE WATER VALUES OF 1.35" WERE SEEN IN
THE ILX SOUNDING.  MIXED LAYER CAPE VALUES ARE 1000-2000 J/KG
ACROSS THIS REGION PER THE SPC MESOANALYSIS.  THESE PARAMETERS ARE
NEAR THE LOWER THRESHOLD FOR WET MULTI-CELLS.

POSITIVE FACTORS FOR HEAVY RAINFALL INCLUDE WINDS AT 850 HPA
RAMPING UP TO 30-35 KTS PER RECENT RAP RUNS OVER THE NEXT SEVERAL
HOURS, LEADING TO 25 KTS OF CLOUD BASE INFLOW WHICH EXCEEDS THE
MEAN 850-400 HPA WIND BY 10-15 KTS.  THE FLOW IS NEARLY
UNIDIRECTIONAL WITH HEIGHT NEAR THE OUTFLOW BOUNDARY.  INCREASING
PRECIPITATION EFFICIENCY IS EXPECTED.  THE BIGGEST NEGATIVE FACTOR
IS THE ORIENTATION OF THE INSTABILITY, WHICH LIES AHEAD OF THE
SYSTEM RATHER THAN TO ITS SOUTH/UPSTREAM.  SOME FORWARD
PROPAGATION TO THE EAST IS EXPECTED, BUT CONSIDERING THE LOW THREE
HOURLY FLASH FLOOD GUIDANCE VALUES IN THE REGION WHICH EQUAL THE
EXPECTED HOURLY RAIN RATES (1.75"), FLASH FLOODING IS CONSIDERED
POSSIBLE. 

ROTH

ATTN...WFO...ILX...IND...LOT...LSX...PAH...

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

LAT...LON   41828797 41648724 40378717 39208761 38068894 
            38258980 39328991 40618959 41468872