Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 272342
AFDARX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LA CROSSE WI
642 PM CDT WED AUG 27 2014

.SHORT TERM...(THIS EVENING THROUGH THURSDAY NIGHT)
ISSUED AT 328 PM CDT WED AUG 27 2014

FORECAST FOCUS IS ON PRECIPITATION TIMING AND AMOUNTS...THREAT
FOR HEAVY RAIN WITH LOCALIZED FLOODING.

AS OF MID AFTERNOON...WATER VAPOR IMAGERY OVERLAID WITH RAP 500
MB HEIGHTS SHOWED DECAYING DISTURBANCE ACROSS IOWA...WITH MAIN
CLOSED LOW ROTATING INTO NORTHWESTERN COLORADO. WEAK IMPULSES WERE
RIDING THE PERIPHERY OF THE TROUGH...AND IT IS THESE IMPULSES THAT
WILL BE THE PLAYERS FOR THE NEXT 24 TO 48 HOURS ACROSS THE UPPER
MISSISSIPPI RIVER VALLEY.

AT THE SURFACE...LINGERING SHOWERS AND THUNDERSTORMS HAVE BEEN
CONFINED TO SOUTHERN AND SOUTHWESTERN IOWA...KEEPING FRONTAL
BOUNDARY ACROSS SOUTHERN IOWA / NORTHERN MISSOURI. THE 65F
DEWPOINT LINE IS GENERALLY SOUTH OF I-80.

THE LARGEST CHANGES TO ONGOING FORECAST HAVE BEEN RELATED TO
PRECIPITATION TIMING...AS 27.12Z MODEL CYCLES TREND TOWARD A LATER
ARRIVAL OF THE GREATEST 850MB MOISTURE TRANSPORT OVERNIGHT THROUGH
THE DAY ON THURSDAY. MOST OF THE DETERMINISTIC AND HI-RES MODELS
SHOW A SHIELD OF RAIN SHOWERS WITH EMBEDDED THUNDERSTORMS REACHING
NORTHEAST IOWA / SOUTHEAST NOT UNTIL THE EARLY MORNING HOURS /4 TO
6 AM/ THEN MOVING INTO WESTERN WISCONSIN MID TO LATE MORNING.
PRECIPITATION CHANCES HAVE BEEN ADJUSTED...BUT OVERALL CONFIDENCE
REMAINS VERY HIGH SO KEPT 90 TO 100 CHANCES.

THE NOSE OF THE 850MB MOISTURE TRANSPORT LIFTS THROUGH THE AREA
DURING THE DAY...SO ANY COMPLEX OF SHOWERS/STORMS WILL LIKELY BE
PROGRESSIVE. HOWEVER...THE ATMOSPHERE WILL INCREASINGLY BECOME
MORE FAVORABLE FOR HEAVY RAINFALL RATES...AS PWATS APPROACH 2
INCHES. WARM CLOUD DEPTHS EXCEED 4 KM ACROSS MUCH OF THE AREA. THE
STORM TOP WINDS ARE PROGGED TO BE 25 TO 30 KTS...WHICH SHOULD HELP
KEEP CONVECTION PROGRESSIVE ENOUGH. THOUGH WITH BROAD WARM AIR
ADVECTION AND ISENTROPIC LIFT DURING THE DAY...HAVE KEPT HIGH
PRECIPITATION CHANCES THROUGH THE EVENING HOURS. THE GREATER FOCUS
SEEMS TO BE ON WESTERN INTO NORTH CENTRAL WISCONSIN...THOUGH MOST
AREAS WILL SEE SHOWERS/STORMS THROUGHOUT THE DAY.

.LONG TERM...(FRIDAY THROUGH WEDNESDAY)
ISSUED AT 328 PM CDT WED AUG 27 2014

THE MAIN 500MB SHORTWAVE BEGINS TO SWING THROUGH THE UPPER
MIDWEST FRIDAY EVENING INTO SATURDAY MORNING...WITH THE GREATEST
500-300MB PV ADVECTION ALIGNED FROM CENTRAL AND EASTERN IOWA INTO
SOUTHERN WISCONSIN. THE MOIST AIRMASS WILL LINGER THROUGH THIS
PERIOD AS WELL...AS THE OVERALL ENVIRONMENT SUPPORTS EFFICIENT
RAIN PRODUCES. THE SURFACE FRONT WILL LIKELY STALL FROM IOWA INTO
SOUTHERN WISCONSIN AND COULD BE THE FOCUS FOR SEVERAL EPISODES FOR
SHOWERS/STORMS. DEPENDING ON THE HEAVY RAINFALL OUTCOME IN ROUND
ONE...THIS PERIOD WILL NEED TO BE WATCHED FOR REPEATED ROUNDS.
THOUGH AT THIS POINT...IT LOOKS LIKE DIFFERENT AREAS WILL SEE THE
ROUNDS OF HEAVIER RAINFALL.

IT DOES LOOK LIKE BY SATURDAY MOST OF THE UPPER LEVEL ENERGY
MOVES EAST...AND THE PRECIPITATION CHANCES ARE CONFINED TO
SOUTHERN AND CENTRAL WISCONSIN. FOR THOSE PLANNING ACTIVITIES THIS
UPCOMING HOLIDAY WEEKEND...THE SATURDAY NIGHT TO SUNDAY MORNING
TIMEFRAME HAS THE GREATEST LIKELIHOOD TO SEE DRIER PERIODS.

SUNDAY THROUGH MONDAY WILL FEATURE AN APPROACHING UPPER LEVEL
SHORTWAVE FROM THE NORTHERN PLAINS. 27.12Z MODELS ALREADY SHOW
THIS FEATURE TO BE A BIT VIGOROUS AS IT SWINGS THROUGH. AT THE
SURFACE...THIS WILL DRIVE A COLD FRONT THROUGH THE UPPER MIDWEST.
THERE REMAINS SOME TIMING DIFFERENCES BETWEEN THE GFS/ECMWF/GEM
BUT THE TREND HAS BEEN FOR THE FRONTAL PASSAGE TO BE LATE SUNDAY
NIGHT OR EARLY MONDAY MORNING. SEVERE CHANCES LOOK LIMITED RIGHT
NOW GIVEN TIMING BUT 500 TO 1000 MUCAPE TRANSLATES AHEAD OF THE
FRONT WITH MARGINAL 0-6KM SHEAR VALUES /20 KTS/. SOMETHING TO KEEP
AN EYE WITH THE HOLIDAY WEEKEND...ESPECIALLY IF THE TIMING BECOMES
MORE FAVORABLE DURING PEAK HEATING.

&&

.AVIATION...(FOR THE 00Z TAFS THROUGH 00Z THURSDAY EVENING)
ISSUED AT 642 PM CDT WED AUG 27 2014

27.18Z FORECAST GUIDANCE HAS SLOWED NORTHEAST PROGRESSION OF
SHOWERS AND THUNDERSTORMS THURSDAY MORNING AND THUS DELAYED ONSET
OF -SHRA/TSRA AT BOTH SITES BY A COUPLE HOURS. IF CURRENT TRENDS
CONTINUE...RAIN MAY HOLD OFF UNTIL LATE THURSDAY MORNING. STILL
EXPECT VFR CONDITIONS OVERNIGHT WITH INCREASING MID-LEVEL CLOUD
COVER AND LIGHT WIND WITH A SLIGHT SOUTHEAST DIRECTIONAL PREFERENCE.
CEILINGS/VISIBILITY WILL LOWER WITH SHOWERS AND THUNDERSTORMS...
DIPPING AT TIMES INTO THE MVFR/IFR RANGE BY THURSDAY AFTERNOON...
ESPECIALLY DURING PERIODS OF HEAVY RAINFALL.

&&

.HYDROLOGY...THURSDAY THROUGH FRIDAY NIGHT
ISSUED AT 328 PM CDT WED AUG 27 2014

VERY FAVORABLE SETUP FOR EFFICIENT...HEAVY RAIN PRODUCERS OVER
THE NEXT FEW DAYS. WARM CLOUD DEPTHS CONTINUE TO HOVER NEAR 4000 M
/IF NOT HIGHER/ WHILE PWATS PEAK AROUND 2 INCHES THURSDAY-
THURSDAY NIGHT. HEAVIER RAIN SIGNALS ARE PROGRESSIVE...THROUGH
NORTHERN IOWA INTO SOUTHEAST MINNESOTA AND THEN NORTH CENTRAL
WISCONSIN. FOR ROUND TWO...THE HIGHER THREAT LOOKS TO BE ACROSS
EASTERN IOWA INTO SOUTHERN WISCONSIN.

RUN TOTAL QPF FROM THE EC/GFS/GEM THROUGH SATURDAY RANGE FROM 2
TO 4 INCHES. LOCALLY HIGHER AMOUNTS ARE LIKELY. AT THIS
MOMENT...THINK THERE COULD BE QUICK RISES ON SOME RIVERS...BUT
SHOULD STAY WITHIN BANK. AS ALONG AS STORMS MOVE...AND AREAS DON/T
GET REPEATED HITS...THE FLASH FLOOD THREAT IS REDUCED. ALL
SAID...WITH THE HIGH LIKELIHOOD OF HEAVY RAIN...AND THE POTENTIAL
FOR SIGNIFICANT AMOUNTS...WILL MAINTAIN HYDROLOGIC OUTLOOK TO
HIGHLIGHT THE THREATS. A FLASH FLOOD WATCH MAY BE NECESSARY IF
SIGNALS ARE FOR THE ROUNDS OF HEAVY RAINFALL LOOK TO REPEAT OVER
THE SAME AREA.

&&

.ARX WATCHES/WARNINGS/ADVISORIES...
WI...NONE.
MN...NONE.
IA...NONE.
&&

$$

SHORT TERM...ZT
LONG TERM....ZT
AVIATION...ROGERS
HYDROLOGY....ZT/RIECK


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