FXUS63 KARX 042133
AFDARX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LA CROSSE WI
333 PM CST FRI DEC 4 2009

.SHORT TERM...TONIGHT THROUGH MONDAY NIGHT

MAIN FORECAST CONCERNS DEALT WITH SNOW CHANCES TONIGHT...CLOUDS OVER THE
NEXT SEVERAL DAYS...TEMPERATURES...AND THEN WITH SNOW CHANCES FOR
SUN NIGHT INTO MONDAY MORNING.

WATER VAPOR IMAGERY AT 20Z SHOWED A BROAD UPPER LEVEL LOW OVER
ONTARIO WITH A WEAK SHORTWAVE TROUGH MOVING INTO NORTHERN
MINNESOTA. ALSO SEEN WAS ANOTHER SHORTWAVE TROUGH MOVING INTO THE
PACIFIC NORTHWEST. LOOKING AT RADAR...LARGE AREA OF ECHOES
DETECTED ACROSS MINNESOTA INTO NORTHWEST WISCONSIN AHEAD OF THE
UPPER LEVEL SHORTWAVE TROUGH. VISIBILITIES HAVE GENERALLY BEEN
BETWEEN 2 AND 7 MILES IN THIS SNOW...WITH A FEW ISOLATED
OBSERVATIONS DOWN TO 1 1/2 MILES. 

THE UPPER LEVEL SHORTWAVE TROUGH OVER MINNESOTA WILL ROTATE AROUND
THE BROAD LOW TO THE NORTH...AND ALTHOUGH THERE WILL BE SOME WEAK
QG CONVERGENCE/ISENTROPIC LIFT ACROSS MINNESOTA AND INTO CENTRAL
WISCONSIN THIS EVENING...THE BULK OF THE ENERGY WILL STAY NORTH OF
THE FORECAST AREA. DO THINK THAT THE SNOW SEEN OVER MINNESOTA WILL
PUSH EAST INTO THE EVENING...SO MUCH OF THE NORTHERN PARTS OF THE
FORECAST AREA WILL CONTINUE TO SEE SNOW. BUT WITH THE WEAKENING
SIGNALS IN THE FORCING...DON/T ANTICIPATE ANY STRENGTHENING IN THE
SNOWFALL SO WILL CARRY SNOW IN THE FORECAST...BUT WITH ONLY SMALL
CHANCES OF ANY ACCUMULATIONS.

THE UPPER LEVEL TROUGH OVER THE PACIFIC NORTHWEST WILL DIG SOUTH
ACROSS THE WEST COAST ON SUNDAY. AN IMPULSE OF SHORTWAVE ENERGY
WILL PULL AWAY FROM THE MAIN TROUGH AND WILL PUSH ACROSS THE
CENTRAL PLAINS AND ACROSS THE UPPER MISSISSIPPI RIVER VALLEY
REGION SUNDAY NIGHT. MODELS HAVE BEEN TRANSITIONING TO A MORE
WEAKER WAVE AS WELL AS PUSHING THE PRECIPITATION TO THE SOUTH. AS
THE SHORTWAVE TROUGH MOVES ACROSS THE AREA IT WILL INTERACT WITH A
WEAK FRONTAL BOUNDARY SITUATED FROM THE CENTRAL PLAINS INTO
EASTERN WISCONSIN. THE BULK OF THE QG CONVERGENCE/OMEGA/ISENTROPIC
UPGLIDE WILL STAY ALONG AND SOUTH OF THE FRONT. THIS WOULD SUGGEST
MOST OF THE SNOW WOULD BE IN AREAS ALONG AND SOUTH OF INTERSTATE
90. WITH SOME ISSUES WITH MODEL CONSISTENCY...DIDN/T HAVE THE
CONFIDENCE TO MAKE MANY CHANGES FOR THE SUNDAY NIGHT/MONDAY TIME FRAME.

.LONG TERM...TUESDAY THROUGH FRIDAY

FOCUS ON THIS PERIOD WAS POSSIBLE SIGNIFICANT WINTER STORM FOR
LATE TUESDAY NIGHT THROUGH WEDNESDAY.

THIS STORM WILL BE ASSOCIATED WITH THE UPPER LEVEL SHORTWAVE TROUGH
CURRENTLY PUSHING INTO THE PACIFIC NORTHWEST OVER LONGWAVE RIDGE
OVER THE GULF OF ALASKA. THE 04.00Z MODELS IN FAIRLY GOOD
CONSENSUS WITH THIS WAVE DEEPENING OVER THE WEST THROUGH THE
WEEKEND THEN PUSHING THROUGH THE FLOW ACROSS THE CENTRAL PLAINS
EARLY NEXT WEEK BEFORE EJECTING NORTHEAST TOWARDS THE GREAT LAKES
TUESDAY INTO WEDNESDAY.

A SURFACE LOW IS SEEN AHEAD OF THE UPPER LEVEL SHORTWAVE AS IT
CROSSES THE ROCKY MOUNTAINS...BUT WILL REALLY STRENGTHEN ON THE LEE-SIDE
OF THE ROCKIES OVER EASTERN COLORADO MONDAY NIGHT. AS THE WHOLE WAVE
MOVES NORTHEAST...THE SURFACE LOW WILL STRENGTHEN REACHING AS IT
MOVES OVER THE GREAT LAKES.

THERE ARE MODEL DIFFERENCES THOUGH...WITH THE GFS/GFS ENSEMBLES
BEING A BIT WEAKER WITH THE SHORTWAVE TROUGH/SURFACE LOW. 04.00Z ECMWF/GEM
ALMOST ON TOP OF EACH OTHER WITH SURFACE AND UPPER LEVEL FEATURES
AND LOOKING AT DPROG/DT...HAVE SEEN A CONTINUING TREND OF
STRENGTHENING THE SYSTEM AND MOVING IT TO THE NORTHWEST. THE ECMWF
IS STRONGEST SOLUTION WITH THE LOW DROPPING TO MSLP OF 966MB ON
WEDNESDAY. GFS IS WEAKEST DETERMINISTIC SOLUTION WITH THE LOW AT
982MB AT THIS TIME...WHICH STILL IS QUITE SIGNIFICANT. SO...EVEN
THOUGH THE GFS IS WEAKER...IT TOO CONTINUES TO INDICATE A HIGH CHANCE
FOR PRECIPITATION WITH WRAP AROUND PRECIPITATION ACROSS THE AREA
SO FELT FAIRLY CONFIDENT IN RAISING PRECIPITATION CHANCES FOR
TUESDAY THROUGH WEDNESDAY.

THERE WILL BE WARM AIR ADVECTION AHEAD OF THE SYSTEM...BUT AS
TRACK OF THE LOW WILL STAY TO THE SOUTH OF THE FORECAST AREA...TEMPERATURE
PROFILES WILL STAY BELOW THE FREEZING MARK SO NO PRECIPITATION TYPE
ISSUES SEEM TO COME INTO PLAY AT THIS POINT. TALKING ABOUT
TEMPERATURE PROFILES...GFS BUFKIT SOUNDINGS SHOWING DEEP NEARLY
ISOTHERMAL LAYER IN THE DENDRITIC GROWTH ZONE ON THE WEST SIDE OF
THE SURFACE LOW...WITH DECENT OMEGA SIGNAL. TAKING THIS INTO
ACCOUNT...ALONG WITH LIQUID AMOUNTS APPROACHING 0.8 INCHES FROM
THE ECMWF...PENDING CHANGES IN MODEL OUTCOMES COULD SEE SOME
SIGNIFICANT SNOWFALLS SOMEWHERE IN THE REGION.

NEXT CONCERN IS TEMPERATURES/WINDS/WIND CHILLS/BLOWING SNOW
ASSOCIATED WITH THIS SYSTEM. DID LOWER HIGHS ON WEDNESDAY BEHIND THE
LOW WITH GOOD COLD AIR ADVECTION MOVING IN. 04.00Z ECMWF/GEM
PRODUCING HIGHS WEDNESDAY ONLY IN THE SINGLE DIGITS IN THE WESTERN
PARTS OF THE FORECAST AREA. DID LOWER HIGHS ON WEDNESDAY...BUT
WILL NEED TO KEEP WATCHING THE TREND AND COULD CERTAINLY SEE
NEEDING TO GO COLDER YET. WILL ALSO HAVE TO DEAL WITH STRONG WINDS
ON WEDNESDAY WITH A TIGHT PRESSURE GRADIENT ON THE BACK SIDE OF
THE LOW. IF THIS ALL COMES TO BE...WITH NEW SNOWFALL AND STRONG
WINDS...WIND CHILLS/BLOWING SNOW ALL SEEM POSSIBLE.

&&

.AVIATION...

A CLOUD SHIELD OF MOSTLY MVFR CEILINGS WAS LOCATED ACROSS MUCH OF 
MN/WI THIS AFTERNOON...PER LATEST SATELLITE IMAGERY/SFC OBS. THE 
CLOUDS WERE ASSOCIATED WITH AN UPPER LEVEL DISTURBANCE...WHICH WILL 
ROTATE EAST TONIGHT. CLEAR SLOT SOUTH OF THE THIS MAIN CLOUD MASS 
HAS FILLED IN THIS AFTERNOON.  RH FIELDS AND TIME/HEIGHT X-SECTIONS 
SHOW A DECREASE IN SATURATION AT KRST/KLSE...WHICH WOULD RESULT IN A 
PERIOD OF SCT SKIES OVERNIGHT TONIGHT. ANOTHER SPOKE OF UPPER LEVEL 
ENERGY...COUPLED WITH A SFC FRONT...WILL EASE BACK ACROSS THE AREA 
SAT AFTERNOON...WHICH SHOULD RESULT IN INCREASING CLOUDS...AND 
POSSIBLY BACK TO MVFR CEILINGS BY SAT NIGHT. A FEW FLURRIES WOULD 
ALSO BE POSSIBLE...BUT DON/T EXPECT ANY VSBY RESTRICTIONS AT THIS 
TIME. SO...HAVE CONTINUED TREND TO SCT CLOUD COVER 
OVERNIGHT...ALTHOUGH CONFIDENCE A TAD SHAKY. COULD SEE CLOUDS 
HOLDING ON UNTIL THAT NEXT PIECE OF ENERGY MOVES IN. TRENDS WILL 
HAVE TO BE MONITORED AND UPDATES MADE AS NECESSARY.   

&&

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

$$

SHORT/LONG TERM...BROOKS
AVIATION...RIECK