MESOSCALE PRECIPITATION DISCUSSION 0268
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1112 PM EDT THU OCT 03 2013
 
AREAS AFFECTED...SOUTHEAST NE...SOUTHWEST IA...FAR NORTHWEST MO 
 
CONCERNING...HEAVY RAINFALL...FLASH FLOODING POSSIBLE 
 
VALID 040311Z - 040901Z
 
SUMMARY...AN EXPANDING AREA OF THUNDERSTORMS WITH RAINFALL RATES
OF 1-2 INCHES AN HOUR OVER SATURATED GROUND COULD LEAD TO REGIONAL
FLASH FLOODING.

DISCUSSION...A STRENGTHENING LOW-LEVEL JET -- ON THE ORDER OF
40-50 KT -- HAS HELPED TO GENERATE A THUNDERSTORM AREA NEAR AND
BEHIND A STATIONARY FRONT ACROSS SOUTHEAST NE.  THE GUIDANCE THAT
HAS CAPTURED THIS DEVELOPMENT BEST HAVE BEEN THE 23-00Z HRRR, 12Z
NAM CONEST, AND 01-02Z RAP RUNS.  PRECIPITABLE WATER VALUES ARE
1.25-1.50 INCHES, WHICH IS IN THE 90TH PERCENTILE FOR EARLY
OCTOBER, SUPPORTIVE OF EFFICIENT RAINFALL RATES.  THE LATEST RADAR
IMAGERY SHOWS RAINFALL RATES OF 1-2 INCHES AN HOUR, OVER AN AREA
WHERE FLASH FLOOD GUIDANCE VALUES ARE FAIRLY LOW -- ON THE ORDER
OF 1.5-2 INCHES IN THREE HOURS.  
CAPE VALUES, PER THE 12Z ARW GUIDANCE, ARE IN THE 2000-2500 J/KG
RANGE, WHICH COMBINED WITH THE AVAILABLE 0-2.5 KM SHEAR WOULD
NORMALLY INDICATE A FORWARD-PROPAGATING ORGANIZED CONVECTIVE
COMPLEX, DRIVEN AHEAD BY A LOW-LEVEL COOL POOL.  THE PROBLEM
OVERNIGHT IS THAT ANY COOL POOL IS MORE LIKELY TO REINFORCE THE
EXISTING FRONTAL ZONE RATHER THAN ACCELERATE OFF TO THE EAST OR
SOUTH, WHICH IS DIFFERENT THAN LAST NIGHT'S CONVECTIVE SCENARIO. 
IT IS EXPECTED THAT A LENGTHENING CONVECTIVE BAND WOULD FORM NEAR
AND BEHIND THE SURFACE FRONT AS CIN VALUES INCREASE OVERNIGHT,
WHICH COULD ALSO ACT TO ADJUST THE ONGOING CONVECTIVE BAND
NORTHWARD.  CORFIDI VECTORS ARE FROM WEST TO EAST AT 5-10 KTS. 
CONSIDERING THE ABOVE, SLOW EAST-NORTHEAST EXPANSION OF THE
CONVECTIVE AREA AT AROUND 10 KTS IS ANTICIPATED.

THE LOW-LEVEL JET IS EXPECTED TO REMAIN QUASI-STATIONARY AND
STRENGTHEN SLIGHTLY MORE THROUGH 06Z, BEFORE BEGINNING TO VEER
THEREAFTER.  DUE TO VEERING LOW-LEVEL INFLOW BY 09Z WHICH SHOULD
INDICATE GREATER EASTWARD MOTION OF THE CONVECTIVE AREA, CHOSE 09Z
AS THE VALID TIME OF THE MPD.

ROTH

ATTN...WFO...DMX...EAX...OAX...GID...

ATTN...RFC...ABRFC...MBRFC...NCRFC...

LAT...LON   40949458 40409604 40289829 41399836 42019630 41769470
            40949458