Area Forecast Discussion
Issued by NWS La Crosse, WI

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000
FXUS63 KARX 171805
AFDARX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LA CROSSE WI
105 PM CDT WED SEP 17 2014

.SHORT TERM...(TODAY AND TONIGHT)
ISSUED AT 318 AM CDT WED SEP 17 2014

LATEST INFARED SATELLITE IMAGERY INDICATE STRATOCUMULUS DECK ACROSS
SOUTHERN MINNESOTA INTO IOWA. THIS IS IN ASSOCIATION WITH A WEAK
IMPULSE OVER SOUTHWEST MINNESOTA AND NORTHWEST IOWA EVIDENT ON
LATEST WATER VAPOR SATELLITE IMAGERY. LATEST 07Z SURFACE ANALYSIS
SHOWS RIDGE OF HIGH PRESSURE EXTENDING FROM NORTHERN ILLINOIS
INTO NORTHERN WISCONSIN. WITH CLEAR SKIES AND LIGHT WINDS EAST OF
THE MISSISSIPPI RIVER...TEMPERATURES HAVE FALLEN INTO THE MIDDLE
TO UPPER 30S ACROSS CENTRAL WISCONSIN PER LATEST 07Z METARS.

MAIN FORECAST CONCERN TONIGHT IS FOG POTENTIAL IN RIVER VALLEYS
AND CLOUD FORMATION OVER NORTHERN WISCONSIN.

THE LATEST 17.00Z GFS/NAM PUSH WEAK IMPULSE SOUTH OF THE FORECAST
AREA THIS MORNING AND THIS WILL CONTINUE TO ADVECT THE STRATOCUMULUS
DECK OVER THE FORECAST AREA THROUGH 18Z. ANY RESIDUAL MOISTURE
LEFT WITH DEPARTING IMPULSE WILL EVAPORATE DUE DAYTIME HEATING AND
ALLOW SKIES TO BECOME MOSTLY SUNNY AFTER 18Z.

FOCUS TURNS TO FOG POTENTIAL TONIGHT. THE LATEST 17.00Z
GFS/NAM/ECMWF CONTINUE TO PUSH WEAK SURFACE FRONT NORTH TO SOUTH
ACROSS THE FORECAST AREA THIS EVENING. ALL MODELS INDICATE 850MB
MOISTURE ALONG WEAK SURFACE FRONT AND PRODUCE CLOUDS EAST OF THE
MISSISSIPPI RIVER...MAINLY OVER CENTRAL WISCONSIN. THE QUESTION WILL
BE HOW FAR SOUTH AND WEST THE DECK OF CLOUDS WILL BE TO INHIBIT ANY
RIVER VALLEY FOG FORMATION AFTER 06Z THURSDAY. THE 17.00Z GFS/NAM
BUFKIT SOUNDINGS SHOW WINDS LIGHT ENOUGH UNDER INVERSION AFTER 06Z
THURSDAY AND NEAR SURFACE BOUNDARY LAYER MOISTENING. THIS WOULD
SUGGEST FOG IS POSSIBLE IN RIVER VALLEYS AND WILL DEPEND ON IF ANY
CLOUDS MOVE OVER SOUTHWEST AND WESTERN WISCONSIN TO INHIBIT
FORMATION OF FOG. FOR NOW...HAVE LEFT MENTION OF FOG IN RIVER
VALLEYS IN FORECAST.

.LONG TERM...(THURSDAY THROUGH TUESDAY)
ISSUED AT 318 AM CDT WED SEP 17 2014

THE 17.00Z GFS/NAM/ECMWF LINGER THE 900-850MB MOISTURE OVER
CENTRAL WISCONSIN INTO THURSDAY PER BUFKIT SOUNDINGS. IF CLOUDS
FORM AND LINGER OVER THIS AREA. TEMPERATURES MAY NOT REACH THE
CURRENT FORECAST HIGHS OVER CENTRAL WISCONSIN.

MAIN FORECAST CONCERNS THURSDAY NIGHT THROUGH SATURDAY ARE
PRECIPITATION CHANCES. THE 17.00Z GFS/NAM/ECMWF/GEM CONTINUE TO
INDICATE TIMING DIFFERENCES WITH SHORTWAVE TROUGH AND ASSOCIATED
SURFACE FRONT MOVING INTO THE GREAT LAKES REGION. THE 17.00Z ECMWF
IS TRENDING TOWARDS A BROADER AND MORE PROGRESSIVE SOLUTION THAN THE
GFS/GEM. THE 17.00Z GFS/GEM/ECMWF/NAM FOCUS LIFT/MOISTURE
TRANSPORT AND CONVERGENCE OVER THE NORTHERN SECTIONS OF THE
FORECAST AREA MAINLY AFTER 18Z FRIDAY IN ASSOCIATION WITH THE FIRST
IMPULSE. HENCE...HIGHER PRECIPITATION CHANCES OVER THIS AREA.
SHORTWAVE RIDGE REMAINS ENTRENCHED OVER THE SOUTHERN FORECAST AREA
AND ALLOW FOR LESSER TO NO PRECIPITATION CHANCES OVER THE SOUTHERN
FORECAST AREA FRIDAY.

MAIN SHORTWAVE TROUGH DIGS INTO THE GREAT LAKES REGION AND SURFACE
FRONT MOVES ACROSS THE FORECAST AREA FRIDAY NIGHT INTO SATURDAY
MORNING. WITH THE TIMING DIFFERENCES BETWEEN THE 17.00Z
GFS/NAM/ECMWF/GEM...CONFIDENCE IN EXACT TIMING OF THUNDERSTORMS
CONTINUES TO BE LOW. THE 17.00Z GFS/GEM/ECMWF/NAM INDICATE DECENT QG
FORCING/LIFT/MOISTURE TRANSPORT AND CONVERGENCE ASSOCIATED WITH
TROUGH/SURFACE FRONT FRIDAY NIGHT INTO SATURDAY MORNING. MUCH OF
THE AREA SHOULD SEE SHOWERS AND THUNDERSTORMS. MODELS CONTINUE TO
PROG HIGH PRECIPITABLE WATER VALUES...INSTABILITY AND SHEAR WITH
THE SURFACE FRONT/SHORTWAVE TROUGH. A  FEW OF THE THUNDERSTORMS
COULD POTENTIALLY BECOME SEVERE.

SURFACE RIDGE BUILDS INTO THE GREAT LAKES REGION SUNDAY NIGHT INTO
TUESDAY AND PROVIDE DRIER WEATHER CONDITIONS ACROSS THE FORECAST
AREA. COOLER AIR WILL FILTER IN BEHIND THE SURFACE FRONT/SHORTWAVE
TROUGH AND TEMPERATURES SHOULD BE NEAR NORMAL EARLY NEXT WEEK.

&&

.AVIATION...(FOR THE 18Z TAFS THROUGH 18Z THURSDAY AFTERNOON)
ISSUED AT 105 PM CDT WED SEP 17 2014

PLAN ON VFR CONDITIONS PREVAILING THIS AFTERNOON INTO THIS
EVENING. ATTENTION THEN TURNS TO CLEAR SKIES AND LIGHT WINDS LATER
TONIGHT AND THE POTENTIAL FOR SOME RIVER VALLEY FOG VCNTY KLSE.
NORMALLY WOULD GO WITH THE THINKING THAT FOG/STRATUS WOULD BE A
SLAM DUNK GIVEN LIGHT WINDS/CLEAR SKIES...BUT THERE WILL BE A BIT
OF A CONUNDRUM WITH PASSAGE OF A COLD FRONT AND EXPECTED MVFR CLOUD
COVER IN ITS WAKE. MEDIUM LEVEL OF CONFIDENCE THAT THERE WILL BE
SOME INCOMING MVFR CLOUD COVER WITH THE FRONTAL PASSAGE AT KLSE AND
KRST AS WELL PER LATEST MODEL OUTPUT. DUE TO THIS INCREASING
CONFIDENCE IN MVFR CLOUD COVER...HAVE BACKED OFF ON IFR CONDITIONS
THAT WAS PREVIOUSLY FORECASTED AT KLSE AFTER 09Z. THINKING THAT
THE COMBINATION OF THIS MVFR CLOUD COVER //WHICH WOULD ACT TO
MITIGATE RADIATIONAL COOLING/RIVER VALLEY FOG// AND LIGHT
NORTHEASTERLY FLOW AT BLUFFTOP LEVEL WILL KEEP THE VALLEY MIXED A
BIT. THINKING THERE WILL BE SOME LOWER STRATUS IN THE MISSISSIPPI
RIVER CHANNEL...BUT NOT EXTENSIVE ENOUGH FOR PREVAILING
CONDITIONS AT KLSE. KEPT KRST VFR TIL 09Z...THEN BROUGHT IN 4SM
BR SCT025 IN THE 09-14Z TIME FRAME. LOOKS LIKE MVFR CLOUDS WILL
THEN BECOME PREVALENT 14-18Z.

&&

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

$$

SHORT TERM...DTJ
LONG TERM...DTJ
AVIATION...DAS



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