MESOSCALE PRECIPITATION DISCUSSION 0502
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
354 PM EDT FRI JUL 29 2016
 
AREAS AFFECTED...NORTHERN IL....NORTHERN IN 
 
CONCERNING...HEAVY RAINFALL...FLASH FLOODING LIKELY 
 
VALID 291950Z - 300050Z
 
SUMMARY...CONVECTION FORMING AND TRAINING ALONG A SURFACE BOUNDARY
IN A HIGH MOISTURE AIRMASS SHOULD RESULT IN FLASH FLOODING THROUGH
THE LATE AFTERNOON HOURS. 

DISCUSSION...CONVECTION CONTINUES TO FORM ALONG A SURFACE BOUNDARY
EXTENDING ACROSS NORTHERN IL INTO NORTHERN IN. THE CONVECTION IS
DEVELOPING IN AN AXIS OF 1000-1500 J/KG MLCAPE...AS WELL AS A
GRADIENT OF 1.50 TO 1.75 INCH PRECIPITABLE WATER AIR. IN THIS
AIRMASS...CONVECTION SPREADING OUT ALONG THE SURFACE TROUGH IS
FORMING OUTFLOW BOUNDARIES...ON WHICH NEW STORMS ARE FORMING.
HOURLY RAINFALL RATES OF 2.00+ INCHES HAVE BEEN INDICATED BY THE
KLOT RADAR ACROSS NORTHEAST IL INTO NORTHWEST IN. 

THE TREND FOR THE CONVECTION HAS BEEN TO BECOME QUASI STATIONARY
ALONG THE FRONT... AND THIS COULD CONTINUE AHEAD OF A SHORT WAVE
APPROACHING FROM IA/MO THROUGH THE LATE AFTERNOON HOURS. MODEL
SOUNDINGS SHOWED THE COLUMN BECOMING INCREASINGLY MOIST WITH
TIME...INCREASING THE RAINFALL EFFICIENCY OF THE STORMS. ONE AND
THREE HOUR FLASH FLOOD GUIDANCE VALUES ARE FAIRLY LOW ACROSS
NORTHERN IL (AS LOW AS 1.00 TO 1.50 INCHES IN ONE HOUR)...SO
TRAINING IN THESE AREAS POSES AN ENHANCED FLASH FLOOD RISK. 

THERE IS A FAIRLY GOOD HIGH RESOLUTION MODEL SIGNAL (INCLUDING THE
HRRR/HRRR PARALLEL/12Z WRF ARW/WRF NMMB FOR A SWATH OF 2-4 INCHES
OF RAINFALL ACROSS NORTHERN IL INTO NORTHERN IN. GIVEN WHAT HAS
ALREADY OCCURRED...THESE NUMBERS COULD END UP BEING
LOW...ESPECIALLY WHERE TRAINING AND CELL MERGERS ASSOCIATED WITH
OUTFLOW BOUNDARIES OCCUR.   

MOST OF THE HIGH RESOLUTION GUIDANCE SHOWED THAT THE ACTIVITY
SHOULD BEGIN TO WEAKEN AS INSTABILITY BECOMES EXHAUSTED DURING THE
EARLY EVENING HOURS. BEFORE THAT TIME...FLASH FLOODING IS
CONSIDERED LIKELY. 

HAYES

ATTN...WFO...DVN...IWX...LOT...

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

LAT...LON   42418936 41848586 41568509 41288502 40948593 
            41208883 41849023