MESOSCALE PRECIPITATION DISCUSSION 0287
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
202 AM EDT MON AUG 18 2014
 
AREAS AFFECTED...SOUTHEAST MN, SOUTHWEST WI, NORTHERN IA 
 
CONCERNING...HEAVY RAINFALL...FLASH FLOODING POSSIBLE 
 
VALID 180601Z - 181201Z
 
SUMMARY...THUNDERSTORMS DEVELOPING ACROSS NORTHERN IA AND
SOUTHEAST MN ARE EXPECTED TO ORGANIZE AND POTENTIALLY TRAIN OVER
THE NEXT SEVERAL HOURS AS THEY MOVE EASTWARD.  WITH RAINFALL RATES
UP TO 2" AN HOUR ANTICIPATED, FLASH FLOODING IS CONSIDERED
POSSIBLE.

DISCUSSION...A WEAK FRONTAL WAVE ACROSS SOUTHWEST MN COMBINED WITH
A SHARP UPPER-LEVEL SHORTWAVE MOVING ACROSS CENTRAL AND EASTERN MN
HAS SPURRED 20+ KNOT 850 HPA INFLOW INTO A STATIONARY FRONT AHEAD
OF THE WAVE, WHICH HAS LED TO RENEWED THUNDERSTORM ACTIVITY ACROSS
THE REGION.  MUCAPE VALUES ON THE SOUTHWEST FRINGE OF THE AREA
REMAIN 1000-2000 J/KG.   PRECIPITABLE WATER VALUES OF ~1.75" ARE
SEEN IN THIS REGION, ABOVE THE 90TH PERCENTILE FOR MID AUGUST.

THIS SYSTEM HAS HAD A HISTORY OF HEAVY RAINFALL OVER THE PAST FOUR
NIGHTS, AND RECENT RAP RUNS SUGGEST THAT 850 HPA INFLOW SHOULD AMP
UP TO 30+ KNOTS BY 09Z -- MEETING THE THRESHOLD FOR THE WET
MULTICELL ENVIRONMENT AND BECOMING DOUBLE THE 850-400 HPA MEAN
WIND -- WHICH WOULD BE THE STRONGEST INFLOW SEEN YET IN
ASSOCIATION WITH THIS SMALL-SCALE SYSTEM AND LEAD TO PRECIPITATION
EFFICIENCY.  BACKBUILDING COULD ALSO RESULT FROM THIS INCREASE IN
THE 850 HPA WIND.  DUE TO THIS, THE 03Z HRRR, RECENT RAP RUNS, AND
THE 00Z NAM CONEST SHOWED A MODERATE SIGNAL FOR HEAVY RAINFALL
NEAR THE MN/WI BORDER THROUGH 12Z, WHICH IS WHEN 850 HPA INFLOW IS
EXPECTED TO WEAKEN.  FLASH FLOOD GUIDANCE VALUES ARE MODERATELY
HIGH, BUT WITH HOURLY RAIN RATES OF 2" POSSIBLE, AS WELL AS CELL
TRAINING AS STORM MOTION OUT OF THE GENERATION REGION SHOULD BE
AROUND 10 KNOTS PER GFS FORWARD PROPAGATING CORFIDI VECTORS, A
QUICK 2-4" OF RAINFALL COULD LEAD TO FLASH FLOODING.

ROTH

ATTN...WFO...GRB...ARX...MPX...DMX...FSD...

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

LAT...LON   43049146 43059299 43319434 43989512 44799472 45339353
            45389180 44739028 43439059 43049146