Area Forecast Discussion
Issued by NWS La Crosse, WI

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000
FXUS63 KARX 180844
AFDARX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LA CROSSE WI
344 AM CDT THU SEP 18 2014

.SHORT TERM...(TODAY AND TONIGHT)
ISSUED AT 343 AM CDT THU SEP 18 2014

MAIN FORECAST CONCERNS TODAY ARE CLOUD TRENDS.

LATEST 08Z SURFACE ANALYSIS SHOWS WEAK COLD FRONT EXTENDING FROM
EASTERN WISCONSIN INTO WESTERN MINNESOTA. LATEST FOG PRODUCT
SATELLITE IMAGERY INDICATES LOW LEVEL STRATUS OVER PORTIONS OF
NORTHERN AND EASTERN HALF OF WISCONSIN AND EAST CENTRAL
MINNESOTA. THIS LOW LEVEL STRATUS IS MOVING SOUTH AND WEST BEHIND
THE WEAK SURFACE FRONT.

THE 18.00Z GFS/NAM AND 18.06Z RAP APPEAR INITIALIZE AND HANDLE
MOVEMENT/EXPANSION OF LOW LEVEL STRATUS WELL INTO WESTERN WISCONSIN
EARLY THIS MORNING/TODAY PER THE 925MB RELATIVE HUMIDITY FIELDS. THE
BIG QUESTION WILL BE HOW LONG WILL THE LOW LEVEL STRATUS HANGS
AROUND THE FORECAST AREA TODAY. THE 18.00Z GFS/NAM AND 18.06Z RAP
SUGGEST THE CLOUDS WILL ERODE DUE TO DAYTIME HEATING AND LIFT BACK
NORTH INTO NORTHERN WISCONSIN AROUND 18Z TODAY. IF LOW LEVEL STRATUS
ERODES LATER IN THE AFTERNOON...THIS WILL HAVE IMPACTS ON FORECAST
HIGH TEMPERATURES FOR TODAY...ESPECIALLY OVER EASTERN PART OF THE
FORECAST AREA.

.LONG TERM...(FRIDAY THROUGH WEDNESDAY)
ISSUED AT 343 AM CDT THU SEP 18 2014

MAIN FORECAST CONCERN ARE PRECIPITATION CHANCES FRIDAY INTO SATURDAY
NIGHT. THE LATEST 18.00Z GFS/NAM/ECMWF SHOW DECENT ISENTROPIC LIFT
AT 305K SURFACE BETWEEN 12-18Z FRIDAY AND ASSOCIATED 925-850MB
MOISTURE TRANSPORT INTO THE WESTERN FORECAST AREA. THE COMBINATION
OF ELEVATED INSTABILITY OF 0-6KM MOST UNSTABLE CAPE OF 300-800
J/KG OVER THE WESTERN FORECAST AREA AND THIS LIFT...WARRANTS A
SMALL PRECIPITATION CHANCE OVER THE WESTERN FORECAST AREA BETWEEN
12-18Z FRIDAY.

AFTER THIS INITIAL WEAK IMPULSE IN ASSOCIATION WITH WARM AIR
ADVECTION...THE 18.0ZZ GFS/NAM/ECMWF INDICATE MUCH OF THE
SOUTHERN FORECAST AREA BECOMES CAPPED AFTER 18Z FRIDAY...AS
SHORTWAVE RIDGE BUILDS INTO THE AREA AND FOCUSES LIFT/MOISTURE
TRANSPORT INTO NORTHERN WISCONSIN. HAVE ADJUSTED PRECIPITATION
CHANCES FURTHER NORTH AFTER 18Z FRIDAY. THE CAP BEGINS TO WEAKEN
AFTER 00Z SATURDAY ACROSS THE FORECAST AREA...IN ADDITION SURFACE
FRONT PUSHES ACROSS FORECAST AREA AND DEEP FORCING/MOISTURE
TRANSPORT/CONVERGENCE WILL ALLOW SHOWERS AND THUNDERSTORMS TO
DEVELOP IN THE 00-06Z SATURDAY TIMEFRAME AND MOVE NORTHWEST TO
SOUTHEAST ACROSS THE FORECAST AREA FRIDAY NIGHT. ANY SEVERE THREAT
WITH THE THUNDERSTORMS LOOKS TO BE CONFINED NORTH OF THE
INTERSTATE 90 CORRIDOR...WHERE THE BETTER INSTABILITY AND SHEAR
WILL BE OVER THE FORECAST AREA. AT THIS TIME...THE SEVERE THREAT
LOOKS TO BE ISOLATED AND THE MAIN THREATS WOULD BE LARGE HAIL AND
DAMAGING WINDS.

SURFACE FRONT/SHORTWAVE TROUGH PUSHES SOUTH AND EAST OF FORECAST
AREA SATURDAY AND TAKING THE BULK OF LIFT AND BETTER INSTABILITY
SOUTH AND EAST OF FORECAST AREA. THE REGION WILL BE UNDER THE
INFLUENCE OF CYCLONIC FLOW SATURDAY...WITH STEEPENING LAPSE RATES
AND A COUPLE HUNDRED ELEVATED CAPE PER BUFKIT SOUNDINGS. SHOWERS
AND THUNDERSTORMS WILL CONTINUE TO BE POSSIBLE ACROSS THE FORECAST
AREA. HAVE LEFT SMALL PRECIPITATION CHANCES IN THE FORECAST.

UPPER LEVEL TROUGH BEGINS TO BREAK DOWN OVER THE GREAT LAKES
REGION AND HEIGHTS RISE OVER THE NORTHERN PLAINS LATE SUNDAY INTO
THE MIDDLE OF NEXT WEEK. THE 18.00Z GFS/ECMWF ARE INDICATING
UPPER LEVEL RIDGE AMPLIFYING OVER THE CENTRAL UNITED STATES BY 00Z
THURSDAY. TEMPERATURES ARE EXPECTED TO BE NEAR NORMAL EARLY TO
MIDDLE OF NEXT WEEK.

&&

.AVIATION...(FOR THE 06Z TAFS THROUGH 06Z THURSDAY NIGHT)
ISSUED AT 1141 PM CDT WED SEP 17 2014

SATELLITE AND SURFACE OBSERVATIONS THIS EVENING INDICATE THE
STRATUS CLOUDS HAVE ADVANCED INTO NORTHERN WISCONSIN BUT ARE
MOVING SOUTH A SLOWER PACE THAN EARLIER EXPECTED. THE 18.00Z NAM
NOW SUGGESTS THESE CLOUDS WILL NOT MAKE INTO THE AREA BEFORE SOME
FOG CAN FORM AND THIS IS CONSISTENT WITH WHAT THE RAP HAS BEEN
SHOWING ALL EVENING. BASED ON THESE TRENDS HAVE GONE FOR A VALLEY
FOG EVENT AT KLSE WITH IFR CONDITIONS DEVELOPING AND PERSISTING
THROUGH THE MIDDLE OF THURSDAY MORNING BEFORE LIFTING. WITH A DEEP
LAYER OF LIGHT WINDS...SHOULD ALSO SEE SOME FOG FORM AT KRST WITH
MVFR/IFR VISIBILITIES EXPECTED. ONCE THE FOG LIFTS THURSDAY
MORNING...VFR CONDITIONS THEN EXPECTED FOR THE REMAINDER OF THE
PERIOD AS HIGH PRESSURE DRIFTS ACROSS THE NORTHERN GREAT
LAKES/SOUTHERN ONTARIO.


&&

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

$$

SHORT TERM...DTJ
LONG TERM...DTJ
AVIATION...04



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