MESOSCALE PRECIPITATION DISCUSSION 0498
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1152 PM EDT THU SEP 17 2015
 
AREAS AFFECTED...SRN WI...NRN IL...FAR NW IN 
 
CONCERNING...HEAVY RAINFALL...FLASH FLOODING LIKELY 
 
VALID 180351Z - 180751Z
 
SUMMARY...ESCALATING FLASH FLOOD THREAT OVER SOUTHERN WISCONSIN.
HEAVY RAIN WILL LATER SINK SOUTH TOWARD CHICAGO.

DISCUSSION...AT 0330Z FLASH FLOODING APPEARED IMMINENT IF IT WAS
NOT ALREADY OCCURRING IN SOUTHERN WISCONSIN. THUNDERSTORMS WHICH
HAD PRODUCED RADAR-ESTIMATED 3 INCH TOTALS NEAR JEFFERSON HAD
EXPANDED TOWARD KENOSHA...AND HOURLY RAIN RATES WERE INCREASING
PER DUAL-POL ESTIMATES AS EXPANDED COLD POOL FORCING WAS COMING
INTO PHASE WITH BOTH MID LEVEL SUPPORT AND THE NOCTURNAL LOW LEVEL
WIND MAXIMUM. WHILE CELLS WERE ANCHORED ALONG A NW-SE AXIS IN
SOUTHERN WISCONSIN...OTHER CELLS WERE BLOSSOMING UPSTREAM
BEGINNING IN EASTERN IOWA...AND WILL QUICKLY MOVE NORTHEASTWARD
AND MERGE INTO THE DEVELOPING COMPLEX. PER RAP FORECASTS THIS
SHOULD PERSIST THROUGH ABOUT 06Z BEFORE PASSAGE OF A LOW-AMPLITUDE
700 MB SHORTWAVE ALLOWS THE ACTIVITY TO DRIFT FARTHER SOUTH AND
EAST.

THIS RAINFALL HAD A HIGHLY EFFICIENT LOOK IN DUAL-POL DATA WITH
WIDESPREAD HVY RAIN PER HYDRO CLASSIFICATION...AND SHOULD BE
EFFICIENT GIVEN PW ABOVE 1.7 INCHES AND AN EXPANDING COMMON ANVIL
OR LARGE SCALE SATURATION. THIS SETUP COULD PRODUCE LOCAL TOTALS
OF 4-6 INCHES...FALLING ESPECIALLY QUICKLY ACROSS FAR SE WI AND
MAYBE EXTREME NE IL. THE 12Z NSSL WRF AND WRF-ARW HANDLED THIS
DECENTLY...AND WOULD THEN HAVE THE CONVECTION SINKING
SOUTHWARD...WITH PERHAPS LESS FOCUS AFTER 06Z-07Z BUT WITH FLASH
FLOODING STILL POSSIBLE FOR MORE SUSCEPTIBLE LOCATIONS SUCH AS
CHICAGO.

BURKE

ATTN...WFO...DVN...LOT...MKX...

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

LAT...LON   43568811 43208722 42708669 41828683 41368786 
            41528961 42359021 43398946