MESOSCALE PRECIPITATION DISCUSSION 0417
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
117 AM EDT TUE AUG 04 2015
 
AREAS AFFECTED...SOUTHWEST SD & NORTH-CENTRAL NE 
 
CONCERNING...HEAVY RAINFALL...FLASH FLOODING POSSIBLE 
 
VALID 040517Z - 041117Z
 
SUMMARY...THUNDERSTORMS HAVE BEGUN TO DEVELOP ACROSS THE REGION. 
WITH HOURLY RAIN RATES UP TO 1.75" POSSIBLE, FLASH FLOODING IS
POSSIBLE IN THIS REGION.

DISCUSSION...AN UPPER LOW IN SOUTHWEST WY HAS EJECTED OUT A FORMER
CONVECTIVE COMPLEX ACROSS NORTHEAST WY.  THIS HAS SPURRED THE
DEVELOPMENT OF NEW THUNDERSTORM ACTIVITY NEAR THE WY/SD BORDER AND
THE CENTRAL SD/NE BORDER, WHICH IS ELEVATED OVER A SURFACE FRONT
DUE TO CIN IN THE REGION.  INFLOW AT 850 HPA IS CURRENTLY
SOUTHEAST AT 25 KTS PER VAD WIND PROFILES.  PRECIPITABLE WATER
VALUES OF 1.25-1.50" INHABIT THE AREA PER GPS INFORMATION.  MUCAPE
VALUES ACROSS SOUTHWEST SD AND WESTERN NE ARE IN THE 2000-4000
J/KG RANGE.

THE EXPECTATION IS FOR CONVECTION TO TRY TO CONGEAL INTO A BAND
STRETCHING ACROSS SOUTHWEST SD INTO NORTH-CENTRAL NE.  AS 850 HPA
INFLOW BECOMES MORE SOUTH TO SOUTHWESTERLY AT 30 KTS -- DOUBLING
THE MEAN 850-400 WIND -- FORWARD PROPAGATION IS EXPECTED TO DROP
OFF TO ROUGHLY 5 KTS TO THE EAST TO SOUTHEAST.  LESS ORGANIZED
CONVECTION ACTIVITY WOULD MOVE NORTH-NORTHEAST AT 10 KTS. 
BACKBUILDING CONVECTION AND CELL MERGERS ARE QUITE POSSIBLE IN
THIS REGIME, AND APPEAR TO BE THE BIGGEST FLASH FLOOD THREAT.  THE
MESOSCALE GUIDANCE INDICATES LOCALIZED MAXIMA OF 2-3" IN THIS
AREA, WHICH OCCUR OVER A QUICK TIME FRAME.  PRECIPITABLE WATER
VALUES ARE FORECAST BY RAP GUIDANCE TO RISE TOWARDS 1.75".  HOURLY
RAIN RATES UP TO 1.75" ARE POSSIBLE WHERE CELLS BACKBUILD AND/OR
TRAIN.  THIS WOULD EXCEED THE LOW FLASH FLOOD GUIDANCE VALUES IN
THIS AREA.  BY 11Z, INFLOW INTO THE REGION SHUTS OFF RAPIDLY,
DROPPING BACK DOWN INTO THE 15-20 KT RANGE, SO BELIEVE THE FLASH
FLOOD THREAT IN THIS REGION COULD LAST UNTIL THEN.

ROTH

ATTN...WFO...ABR...FSD...LBF...UNR...

ATTN...RFC...MBRFC...

LAT...LON   44980263 44160038 43249881 42529963 42870115 
            43700354 44030465 44750421