MESOSCALE PRECIPITATION DISCUSSION 0685
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1157 PM EDT WED SEP 21 2016
 
AREAS AFFECTED...SOUTHERN MN, NORTHEAST IA, & SOUTHWEST WI 
 
CONCERNING...HEAVY RAINFALL...FLASH FLOODING LIKELY 
 
VALID 220356Z - 220856Z
 
SUMMARY...TRAINING THUNDERSTORMS ACROSS PORTIONS OF MN, IA, AND WI
ARE EXPECTED TO CONTINUE OVER THE NEXT SEVERAL HOURS.  FLASH
FLOODING IS CONSIDERED LIKELY.

DISCUSSION...A SOUTHWARD SHIFT WITH TIME HAS BEEN NOTED WITH THE
THUNDERSTORM COMPLEX STRETCHING FROM CENTRAL MN SOUTHEAST INTO THE
DRIFTLESS AREA NEAR THE WI/MN/IA BORDER JUNCTION.  OUTFLOW
BOUNDARIES FROM THE MN AND WI CONVECTION ARE REINFORCING A FRONTAL
BOUNDARY ACROSS NORTHEAST IA AND NORTHERN IL.  THE SHORTWAVE
CULPRIT FOR THE CONVECTIVE MASS IS MOVING INTO SOUTHWEST MN AT THE
PRESENT TIME, BUT ANOTHER STRONGER SHORTWAVE IS MOVING TOWARDS THE
REGION FROM WESTERN NE.  MUCAPE TRENDS ACROSS WI ARE SHARPLY
DOWNWARD DUE TO RECENT CONVECTION, AND RADAR TRENDS SHOW AN
EXPANDING AREA OF STRATIFORM RAIN ACROSS CENTRAL WI, WITH
CONVECTION BECOMING INCREASINGLY LIMITED TO SOUTHWEST WI/NORTHEAST
IA.  PRECIPITABLE WATER VALUES ARE 1.5-1.75", OVER THREE SIGMAS
ABOVE THE NORM FOR LATE SEPTEMBER.  INFLOW AT 850 HPA IS BROAD AND
SOMEWHAT CONVERGENT ACROSS THE REGION, IN THE 30-40 KT RANGE PER
VAD WIND PROFILES.  MUCAPES ARE IN TO 2000-3000 J/KG RANGE ACROSS
MUCH OF IA AND SOUTHWEST MN, WHICH IS ALLOWING THE CONVECTION TO
BE NOT TERRIBLY ELEVATED OVER THE FRONT ACROSS SOUTHEAST MN,
SOUTHWEST WI, AND NORTHEAST IA.

THE EXPECTATION IS FOR THUNDERSTORMS TO CONTINUE SAGGING SOUTHWARD
INTO PORTIONS OF NORTHERN IA WITH TIME, PERHAPS INTO NORTHWEST IL.
 THE WESTERN EDGE OF THE CONVECTIVE MASS MAY BE HELD UP BY THE
UPSTREAM SHORTWAVE IN NE OVER THE NEXT FEW HOURS.  MASS FIELDS
FROM THE GFS AND RAP INDICATE COOLING ALOFT WITH TIME, WHICH IS
SEEN IN THE 700 HPA TEMPERATURE PATTERN AS WELL AS THE 1000-850
AND 1000-500 HPA THICKNESS PATTERNS.  WITH THE INSTABILITY
GRADIENT TRYING TO FINALLY ALIGN WITH THE FRONTAL ZONE ACROSS IA
AND IL, CONVECTION SHOULD FORWARD PROPAGATE IN A SEGMENTED MANNER
EAST-SOUTHEAST AT 25 KTS ALONG THE SHARPENING MUCAPE GRADIENT,
MORE ALONG THE ORIENTATION OF THE 1000-500 HPA THICKNESS PATTERN. 
RECENT HRRR, HRRR EXPERIMENTAL, AND WRF NSSL RUNS APPEAR CLOSEST
TO WHAT IS EXPECTED, THOUGH THEY'RE STILL TOO FAR TO THE NORTH. 
THERE IS A STRONG SIGNAL FOR 4-7" AMOUNTS OVER THE NEXT SEVERAL
HOURS.  HOURLY RAIN RATES TO 2.5" ARE POSSIBLE.  FLASH FLOODING IS
CONSIDERED LIKELY DUE TO CELL TRAINING.

ROTH

ATTN...WFO...ARX...DMX...DVN...FSD...MKX...MPX...

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

LAT...LON   45749457 45119290 43589064 42238956 42079133 
            43249393 43889589 44619541 45419509