MESOSCALE PRECIPITATION DISCUSSION 0468
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1231 AM EDT WED JUL 12 2017

AREAS AFFECTED...FAR NORTHEAST IA...EXTREME SOUTHEAST
MN...SOUTHERN WI...NORTHERN IL...SOUTHWEST LOWER MI...NORTHERN IN

CONCERNING...HEAVY RAINFALL...FLASH FLOODING LIKELY

VALID 120430Z - 120930Z

SUMMARY...AN AXIS OF HEAVY SHOWERS AND THUNDERSTORMS WILL TEND TO
EXPAND IN COVERAGE AND ORGANIZATION OVER THE NEXT SEVERAL HOURS.
SLOW-CELL MOTION AND AND BACKBUILDING OF CELLS WILL PROMOTE A
LOCALLY SIGNIFICANT RISK OF FLASH FLOODING.

DISCUSSION...EXPERIMENTAL GOES-16 10.3 MICRON/IR SATELLITE IMAGERY
CONTINUES TO SHOW AN EXPANDING AXIS OF VERY COLD CONVECTIVE TOPS
FROM FAR NORTHEAST IA/EXTREME SOUTHEAST MN AND DOWN ACROSS
SOUTHWEST WI AND NORTHERN IL.

ALL OF THIS IS BEING FACILITATED BY A STRENGTHENING WARM ADVECTION
PATTERN OVER THE REGION WITH ENHANCED ISENTROPIC ASCENT ALONG AND
JUST NORTH OF A WELL-DEFINED WARM FRONT. MEANWHILE...A STRONGLY
UNSTABLE AND VERY MOIST AIRMASS CONTINUES TO POOL ALONG THE
BOUNDARY AND ALSO IN AN ELEVATED FASHION AND THUS PROVIDING STRONG
THERMODYNAMIC SUPPORT. MORE DYNAMICALLY...THERE IS ALSO SOME
EVIDENCE OF A MODEST UPPER LEVEL JET STREAK CROSSING SOUTHERN WI
AND LAKE MI AT THIS TIME WHICH IS YIELDING SOME LARGER SCALE
ASCENT.

THE HIRES MODELS ARE NOT DOING WELL AT ALL WITH THE CURRENT LEVEL
OF CONVECTIVE ORGANIZATION OR GENERAL PLACEMENT...BUT BASED ON THE
LATEST RADAR/SATELLITE TRENDS...AND PLACEMENT OF WHERE A STRONG
INSTABILITY GRADIENT IS CURRENTLY ALIGNED...IT IS EXPECTED THAT
AREAS OF VERY SIGNIFICANT RAINFALL WILL IMPACT AREAS OF FAR
NORTHEAST IA/SOUTHWEST WI AND NORTHERN IL...INCLUDING AREAS IN AND
AROUND THE CHICAGO METROPOLITAN AREA.

GIVEN PWATS OF 1.8 TO 2.0 INCHES...AND NUMEROUS OVERSHOOTING TOPS
SHOWING UP ON THE EXPERIMENTAL GOES-16 IR DATA...EXPECT RAINFALL
RATES TO REACH BETWEEN 2 AND 3 INCHES/HR. THE LOW LEVEL JET IS
FORECAST TO INCREASE TO BETWEEN 40 AND 45 KTS OUT AHEAD OF AN
UPSTREAM SHORTWAVE AND LOW PRESSURE WAVE APPROACHING THE UPPER
MIDWEST...AND SO A GREAT SET-UP WILL EXIST FOR BACKBUILDING AND
TRAINING CONVECTION. GOING THROUGH 09Z...EXPECT STORM TOTALS OF 3
TO 6 INCHES TO BE POSSIBLE LOCALLY ALONG WITH ISOLATED HEAVIER
AMOUNTS. FLASH FLOODING WILL BE LIKELY WITH THESE TOTALS AND
ESPECIALLY AROUND THE URBANIZED CORRIDORS.

IN TIME...THE HEAVY RAINFALL THREAT WILL TEND TO SHIFT EAST AND
WILL ALSO ENCOMPASS ADJACENT AREAS OF SOUTHERN WI AND GETTING DOWN
ACROSS SOUTHWEST LOWER MI AND NORTHERN IN AS ISENTROPIC ASCENT
INCREASES HERE AND THE FLASH FLOOD THREAT WILL INCREASE FOR THESE
AREAS AS WELL. WILL CONTINUE TO MONITOR.

ORRISON

ATTN...WFO...ARX...DVN...GRR...IWX...LOT...MKX...

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

LAT...LON   44109119 44049027 43768878 42748635 42048504
            41278508 41188663 41818905 42099005 42319090
            42679158 43099196 43759204 44049164