MESOSCALE PRECIPITATION DISCUSSION 0358
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1238 AM EDT THU JUN 22 2017

AREAS AFFECTED...IA/MN BORDER

CONCERNING...HEAVY RAINFALL...FLASH FLOODING POSSIBLE

VALID 220438Z - 220838Z

SUMMARY...THUNDERSTORMS ARE EXPANDING IN COVERAGE AND SHOWING SOME
BACKBUILDING AND TRAINING CHARACTERISTICS.  HOURLY RAIN RATES TO
2", WITH LOCAL AMOUNTS TO 4", COULD CAUSE ISSUES.

DISCUSSION...A SHORTWAVE ALOFT IS MOVING INTO WESTERN MN WHICH IS
LEADING TO INCREASED DIVERGENCE ALOFT OVER THE TOP OF A FRONTAL
ZONE IN NORTHERN IA.  CONVECTIVE ELEVATION OVER THE SURFACE
BOUNDARY IS MINIMAL HERE AS THE MUCAPE VALUES ARE 2000-3000 J/KG. 
BULK SHEAR IN THE REGION IS ~45 KTS, WHICH WOULD FOSTER THE
DEVELOPMENT OF MESOCYCLONES.  THE INFLOW AT 850 HPA HAS BEEN
RAMPING UP AND IS 40-50 KTS INTO THE REGION PER VAD WIND PROFILES
UPSTREAM, EXCEEDING THE MEAN 850-400 HPA WIND BY 20 KTS WHICH
SHOULD BOOST RAINFALL EFFICIENCY.

ALTHOUGH THERE ARE CONFLICTING SIGNALS REGARDING CONVECTIVE
PROPAGATION (1000-850 HPA THICKNESS DIVERGENCE POINTS
EAST-SOUTHEAST TO SOUTHEAST WHILE FORWARD PROPAGATING CONVECTIVE
VECTORS ARE MORE EASTERLY, THE BROAD AREA OF 1000+ J/KG MUCAPES
SHOULD LEAD TO THE FORWARD PROPAGATING VECTORS BEING MORE FAVORED
IN THIS SITUATION.  FORWARD CELL MOTION OF 20-25 KTS SHOULD
CONTINUE.  INCREASING INFLOW FOR ANOTHER COUPLE HOURS SHOULD
FOSTER MORE BACKBUILDING -- IT IS UNCLEAR IF THE INCOMING COLD
FRONT COULD ACT AS A NEW CONVECTIVE FOCUS IN THE MEANTIME WHICH
WOULD EXACERBATE ISSUES.  THE MESOSCALE GUIDANCE IS DOING A POOR
JOB HERE -- THE 12Z SPC WRF APPEARS TO BE DOING THE BEST SO FAR. 
HOURLY RAIN RATES UP TO 2" WITH LOCAL AMOUNTS TO 4" COULD CAUSE
ISSUES OVER AN AREA WITH MODEST FLASH FLOOD GUIDANCE VALUES.  USED
A FOUR HOUR WINDOW AS A HORIZON AND POSSIBLE WORDING DUE TO THE
POOR MESOSCALE GUIDANCE SIGNAL DESPITE A CONVECTIVE SITUATION
WHICH WOULD OTHERWISE LEAD TO LIKELY WORDING.

ROTH

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

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

LAT...LON   44099284 43419218 42899306 42999529 43389595
            44079498