Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 241738
AFDARX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LA CROSSE WI
1138 AM CST WED DEC 24 2014

.UPDATE...
ISSUED AT 1005 AM CST WED DEC 24 2014

WATER VAPOR SHOWS A SHORT WAVE TROUGH MOVING NORTH NORTHEAST
THROUGH CENTRAL ILLINOIS. THIS WAVE WILL HELP TO KEEP SOME WEAK
900 TO 800 MB FRONTOGENESIS ACROSS CENTRAL AND NORTH CENTRAL
WISCONSIN AND PARTS OF SOUTHWEST WISCONSIN. SOUNDINGS SHOW THAT
THERE WILL BE SOME ICE CRYSTAL SEEDING FROM ALOFT...SO EXPECT
MAINLY SNOW. LITTLE...IF ANY...ACCUMULATION IS EXPECTED. DUE TO
THIS...INCREASE THE SNOW CHANCES UP INTO THE 20 TO 30 PERCENT
RANGE.

&&

.SHORT TERM...(TODAY AND TONIGHT)
ISSUED AT 326 AM CST WED DEC 24 2014

THE MAIN FOCUS FOR TODAY IS WITH THE CHANCES FOR SNOW/DRIZZLE.

CURRENTLY...A SHEARED OUT/POSITIVELY TILTED MID LEVEL CLOSED
TROUGH EXTENDS FROM THE UPPER MISSISSIPPI RIVER VALLEY ON DOWN TO
THE WESTERN GULF WITH THE SHEAR AXIS RIGHT OVER WEST CENTRAL
WISCONSIN/EASTERN IOWA. AT THE SURFACE...A LOW/TROUGH IS CENTERED
OVER SOUTHEAST WISCONSIN WITH NORTHWESTERLY FLOW RUNNING ACROSS
THE REGION ON THE BACK SIDE OF THIS SYSTEM. NOT MUCH IN THE WAY OF
COLD AIR BEHIND IT WITH 850MB TEMPERATURES AROUND -5C.

PRECIPITATION-WISE...DRIZZLE HAS DEVELOPED IN A BAND NORTHWEST OF
THE SURFACE LOW AND ALONG THE MID LEVEL SHEAR AXIS ALONG THE
TROUGH. 24.06Z RAP SOUNDINGS FROM ALONG THIS BAND SHOW THAT THERE
IS A LACK OF ICE ALOFT WITH SATURATED LOW LEVELS. THESE SOUNDINGS
SHOW SOME COOLING AROUND 700MB WHICH COULD PUSH THE SATURATED
LAYER DOWN INTO THE -8C RANGE AND PERHAPS CAUSE A CHANGE OVER TO
LIGHT SNOW. WITH FORCING WEAK...EXPECT THAT DRIZZLE WILL BE THE
MAIN PRECIPITATION TYPE WITH IMPACTS MINIMAL THANKS TO SURFACE
TEMPERATURES AT 32 TO 36F. THE 24.06Z HRRR/RAP TAKES THIS MAIN
BAND EASTWARD THROUGH THE MORNING AND DIMINISHING ALONG THE WAY.

AS THE TROUGH/LOW SHIFT OFF TO THE EAST TONIGHT...MID
LEVEL/SURFACE RIDGING WILL PUSH IN FROM THE WEST AND MAY OFFER A
BRIEF PERIOD WHERE SKIES CLEAR OUT LATE TONIGHT. HAVE PLAYED SKY
COVER ON THE PESSIMISTIC SIDE SINCE AS SOON AS THE LOW LEVEL
MOISTURE SHIFTS EAST...SOME HIGH LEVEL CLOUD COVER APPEARS TO MOVE
IN ACCORDING TO THE 24.00Z NAM/GFS. TEMPERATURES WILL NOT DIVE
MUCH TONIGHT WITH LOWS ONLY IN THE MID 20S.

.LONG TERM...(THURSDAY THROUGH TUESDAY)
ISSUED AT 326 AM CST WED DEC 24 2014

THE FOCUS THEN SHIFTS TO PRECIPITATION CHANCES WITH THE NEXT
SYSTEM TO MOVE IN ON THURSDAY INTO FRIDAY NIGHT. RETURN SOUTHERLY
LOW LEVEL FLOW WILL BRING WARMER AIR UP FROM THE SOUTHERN PLAINS.
THERE HAS BEEN A FAIRLY CONSISTENT MESSAGE FROM THE 24.00Z AND
PREVIOUS MODELS RUNS THAT SHOW A 900-800MB FRONTOGENETIC BAND
DEVELOPING THURSDAY AFTERNOON ACROSS CENTRAL MINNESOTA INTO
NORTHERN WISCONSIN. LOCALLY...24.00Z NAM/GFS ARE SHOWING SOME LOW
LEVEL SATURATION WITH THE WARM AIR ADVECTION TAKING PLACE THURSDAY
NIGHT. IF FORCING WAS A BIT STRONGER IN THE LOW LEVELS...THERE
WOULD BE THE POTENTIAL TO GET SOME DRIZZLE TO FORM BUT AT THE
MOMENT IT LOOKS TOO WEAK.

BROAD MID LEVEL TROUGHING WILL MOVE INTO THE REGION FRIDAY/FRIDAY
NIGHT AND BRING THE NEXT BEST CHANCE FOR SOME SNOW AS IT COMES
THROUGH. AS THE DEEPER LIFT COMES IN WITH A MID LEVEL SHORT WAVE
TROUGH TRACKING THROUGH IOWA...SNOW SHOULD DEVELOP IN A SOUTHWEST
TO NORTHEAST BAND. WITH QPF AMOUNTS AROUND A TENTH OF AN INCH OR
LESS...EXPECT THAT ANY ACCUMULATIONS WILL BE MINOR AT AN INCH OR
LESS.

THE PASSING OF THIS TROUGH WILL MARK A PERIOD OF COLDER AIR
ADVANCING INTO THE REGION FROM CANADA. A VERY CONSISTENT SIGNAL IN
THE 24.00Z GFS/ECMWF IS WITH ANOTHER WAVE OF COLDER AIR MOVING
DOWN FROM NORTHERN CANADA FOR THE MIDDLE PART OF NEXT WEEK WITH
850MB TEMPERATURES FORECAST TO DROP TO -20C OR LOWER.

&&

.AVIATION...(FOR THE 18Z TAFS THROUGH 18Z THURSDAY AFTERNOON)
ISSUED AT 1138 AM CST WED DEC 24 2014

AT TAF TIME...A SNOW SHOWER WAS REDUCING VISIBILITIES TO 1 3/4
MILES AT KRST. LOOKING AT THE RADAR...THIS SNOW SHOWER WILL MOVE
QUICKLY SOUTH OF TAF SITE AND NOT EXPECTING ANY ADDITIONAL ONES
SO DID NOT INCLUDE IT IN THE FORECAST. SOUNDINGS CONTINUE TO
SUGGEST THAT KRST CEILINGS WILL REMAIN IFR THROUGH 25.09Z AND
THEN BECOME MVFR. MEANWHILE KLSE WILL CONTINUE TO SEE MVFR
CEILINGS.

AS FAR AS CLOUD COVER ON CHRISTMAS MORNING...THE LAMP GUIDANCE
SUGGESTS THAT CEILINGS WILL BECOME VFR AFTER 25.11Z AT KRST AND
25.13Z AT KLSE. HOWEVER SOUNDINGS CONTINUE TO SHOW LOW LEVEL
MOISTURE TRAPPED BELOW THE SUBSIDENCE INVERSION. FOR NOW...JUST
STAYED PESSIMISTIC.

&&

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

$$

UPDATE...BOYNE
SHORT TERM...HALBACH
LONG TERM...HALBACH
AVIATION...BOYNE


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