Flash Flood Guidance
Issued by NWS

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000
AWUS01 KWNH 202259
FFGMPD
WIZ000-210253-

MESOSCALE PRECIPITATION DISCUSSION 0815...CORRECTED
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
658 PM EDT WED SEP 20 2017

CORRECTED FOR MISSING LINE IN SECOND DISCUSSION PARAGRAPH

AREAS AFFECTED...CENTRAL WI

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

VALID 202253Z - 210253Z

SUMMARY...THUNDERSTORMS ARE TRAINING ACROSS CENTRAL WI, WHICH
COULD CONTINUE FOR ANOTHER 3-4 HOURS.  HOURLY TOTALS TO 2" WITH
LOCAL AMOUNTS TO 4" ARE EXPECTED.

DISCUSSION...THUNDERSTORMS HAVE BEEN TRAINING/BACKBUILDING ACROSS
CENTRAL WI, NEAR AND DOWNWIND OF AN INTERSECTION OF A PRE-FRONTAL
TROUGH AND WARM FRONT.  PRECIPITABLE WATER VALUES OF 1.5-1.75"
EXIST ACROSS THE REGION PER RAP ANALYSES.  ML CAPE VALUES OF
1000-2000 J/KG EXIST HERE.  INFLOW AT 850 HPA IS CONVERGENT OUT OF
THE SOUTHWEST AT 25-35 KTS PER VAD WIND PROFILES, SLIGHTLY
EXCEEDING THE MEAN 850-400 HPA WIND.  EFFECTIVE BULK SHEAR OF ~50
KTS EXISTS IN THIS REGION, WHICH ALONG WITH THE NEARLY
UNIDIRECTIONAL FLOW WITH HEIGHT HAS ESTABLISHED THIS AS AN
ORGANIZED LINEAR BAND.

A COMBINATION OF COLD POOL FORMATION AND/OR AN INCOMING FRONTAL
ZONE WITH ASSOCIATED VEERING FLOW AT 850 HPA SHOULD CAUSE THIS
COMPLEX TO MOVE ALONG TO THE EAST WITH TIME, WITH ACTIVITY ENDING
FROM WEST TO EAST AS THE FRONT FORCES IT ALONG.  MORE THAN ONE
ROUND OF CONVECTION IS POSSIBLE IN SOME AREAS.  MESOCYCLONE
FORMATION IS ALSO POSSIBLE, WHICH COULD AID HEAVY RAIN RATES.  OF
CONCERN IS WHETHER OR NOT THE COLD WEDGE IN CENTRAL WI BECOMES
REINFORCED DUE TO THIS CONVECTION, WHICH COULD MAGNIFY ISSUES
ALONG ITS SOUTHWEST FLANK PRIOR TO FRONTAL PASSAGE.  THE 12Z NMMB
AND 18Z NAM CONEST HAVE HAD SOME CONCEPT OF WHAT IS EVOLVING AT
THIS TIME.  THE 12Z SSEO NEIGHBORHOOD PROBABILITIES SUGGEST THAT
ISSUES SHOULD PERSIST FOR A FEW HOURS.  HOURLY TOTALS TO 2.5" WITH
LOCAL AMOUNTS TO 4" ARE EXPECTED WITHIN THE TRAINING/BACKBUILDING
BAND.

ROTH

...PLEASE SEE WWW.WPC.NCEP.NOAA.GOV FOR GRAPHIC PRODUCT...

ATTN...WFO...ARX...GRB...MKX...

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

LAT...LON   45408848 45218803 44338793 43638906 44119025
            44679020 45238911



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