FXUS63 KARX 060912
AFDARX
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LA CROSSE WI
312 AM CST SUN DEC 6 2009
.SHORT TERM...TODAY THROUGH TUESDAY
FORECAST CONCERNS FOCUSED ON SNOW AMOUNTS TONIGHT AND THEN HANDLING
OF A MAJOR WINTER STORM POISED TO HIT THE AREA BEGINNING ON
TUESDAY AFTERNOON.
WATER VAPOR IMAGERY AT 07Z SHOWING MID LEVEL LOW NEAR JAMES BAY...
WHILE AT THE SURFACE A COLD FRONT EXTENDS SOUTHWARD FROM A WEAK LOW
OVER NORTHEAST IOWA. INFRARED SATELLITE IMAGERY SHOWS SEVERAL BANDS
OF CLOUDS AROUND THE AREA. THE FIRST ONE SITS OVER THE FORECAST AREA
AND IS SLOWLY MOVING EAST-NORTHEAST...WITH CLEAR SKIES ACROSS SOUTH
CENTRAL MINNESOTA AND NORTH CENTRAL IOWA. ANOTHER AREA OF STRATUS
WAS OVER NORTHERN MINNESOTA...SINKING SLOWLY SOUTHWARD.
06Z.00Z NCEP DETERMINISTIC MODELS IN GOOD AGREEMENT THROUGH THE
PERIOD...WITH ONLY MINOR DETAIL DIFFERENCES. HAVE GENERALLY
FOLLOWED THE GFS/NAM BLEND FOR SYSTEM TONIGHT. SHORTWAVE GETS
EJECTED OUT OF THE WESTERN TROUGH TODAY AND SLIDES ACROSS THE UPPER
MISSISSIPPI RIVER VALLEY REGION TONIGHT. DEEP QG CONVERGENCE/290K-
300K ISENTROPIC LIFT SPREAD ACROSS THE AREA THROUGH THE EVENING
HOURS. BROAD FORCING SIGNALS ALONG WITH WEAK FRONTOGENETIC
RESPONSE MAXIMIZED ACROSS NORTHEAST IOWA AND SOUTHWEST WISCONSIN
BETWEEN 06Z-12Z MONDAY. COBB OUTPUT FROM BOTH NAM AND GFS VERY
CONSISTENT WITH PLACING A BAND OF 2-3 INCHES ACROSS THE
AFOREMENTIONED AREA...TAPERING TO AROUND AN INCH FROM ROCHESTER...TO
LA CROSSE TO ADAMS FRIENDSHIP. DID BACK OFF A LITTLE ON THE HIGHS
SNOW PROBABILITIES UNTIL LATE EVENING AND OVERNIGHT. AFTER
COLLABORATION WITH NEIGHBORING OFFICES AND GIVEN AMOUNTS WILL HOLD
OFF ON ANY HEADLINES AT THIS TIME...AS ADVISORY CRITERIA WOULD NOT
BE WIDESPREAD AT THIS POINT. DAY SHIFT CAN MONITOR IF HEADLINES ARE
NEEDED.
SUBSIDENCE QUICKLY SPREADS OVER THE AREA ON MONDAY ON BACKSIDE OF
DEPARTING WAVE. CONTINUED TO HOLD ONTO MID RANGE PROBABILITIES...
MAINLY EAST OF THE MISSISSIPPI RIVER THROUGH THE MORNING HOURS.
QUIET WEATHER IS THEN EXPECTED MONDAY NIGHT AND THE FIRST PART OF
TUESDAY. ATTENTION THEN TURNS TO SIGNIFICANT SYSTEM LIFTING OUT OF
THE ROCKIES LATE TUESDAY. STRONG WARM ADVECTION AHEAD OF IT WILL
LIKELY SPREAD SNOW INTO THE SOUTHERN PORTIONS OF THE AREA LATE
TUESDAY AS ANOMALOUS LOW LEVEL JET TRANSPORTS GULF MOISTURE
NORTHWARD. MAIN IMPACT OF THE STORM WILL BE FELT TUESDAY NIGHT AND
WEDNESDAY...WITH DISCUSSION ON THIS IN THE LONG TERM DISCUSSION
BELOW.
.LONG TERM...TUESDAY NIGHT THROUGH SATURDAY
BIGGEST CONCERN IS CENTERED SQUARELY ON THE TUESDAY NIGHT AND
WEDNESDAY TIME FRAME AS MAJOR WINTER STORM AFFECTS THE AREA.
STILL SOME STRENGTH DIFFERENCES BETWEEN THE 06.00Z GFS AND ECMWF
WITH THE SURFACE LOW...BUT TRACK PRETTY CONSISTENT FROM
MISSOURI..INTO NORTHERN ILLINOIS BY 12Z WEDNESDAY...WHICH MODELS HAVE
BEEN TRENDING MORE NORTHWARD. OPEN WAVE TO BECOME NEGATIVELY TILTED
AS IT PROGRESSES INTO THE AREA TUESDAY NIGHT...WITH 500MB HEIGHTS
SOME 2 TO 3 STANDARD DEVIATIONS BELOW NORMAL. STRONG FETCH OF GULF
MOISTURE AS INDICATED BY 850MB MOISTURE FLUX SOME 3 TO 4 STANDARD
DEVIATIONS ABOVE NORMAL AT 12Z WEDNESDAY. THUS SYSTEM WILL HAVE AMPLE
MOISTURE TO WORK WITH. IN FACT GFS COBB OUTPUT WOULD SUGGEST THE
POTENTIAL FOR 6 TO 10 INCHES ACROSS MUCH OF THE AREA. OTHER BIG
FACTOR IS STRONG PRESSURE GRADIENT RESULTING IN STRONG WINDS FOR
WEDNESDAY. THIS IN TURN WILL RESULT IN POSSIBILITY OF BLOWING AND
DRIFTING OF THE SNOW. THIS ALREADY WELL HANDLED IN THE GRIDS. ALL IN
ALL STILL SOME UNCERTAINTIES...AND DETAILS WILL NOT BECOME CLEAR FOR
SEVERAL MORE DAYS...BUT IF CURRENT TRENDS CONTINUE...HEADLINES WILL
LIKELY BE HOISTED AS EARLY AS THIS AFTERNOON.
&&
.AVIATION...TODAY AND TONIGHT
PRIMARY CONCERN CONCERN THIS PERIOD IS WITH TIMING OF MVFR/IFR CIGS/
VSBYS MOVING IN WITH -SN TONIGHT. FCST AREA REMAINS BETWEEN SYSTEMS
TODAY WITH WEAK SHORTWAVE RIDGING ALOFT OVER THE REGION AND A
GENERALLY DRY LOW LEVEL AIRMASS ACROSS THE TAF SITES. APPEARS CLOUD
TODAY WILL MAINLY BE CIRRUS THIS MORNING...THEN GRADUALLY LOWER INTO
THE 10K FT RANGE BY LATE AFTERNOON AS MOISTURE SPREADS NORTHEAST/
DEEPENS AHEAD OF THE NEXT SHORTWAVE MOVING ACROSS THE CENTRAL
PLAINS. MOISTURE/LIFT INCREASE WITH THE PASSING SHORTWAVE TONIGHT
WITH -SN EXPECTED TO SPREAD INTO THE TAF SITES AROUND 06Z...
CONTINUING THRU ABOUT 12Z OR SHORTLY THEREAFTER. CONSENSUS OF 06.00Z
FCST MODELS HAS TRENDED A BIT SOUTH WITH THE MAIN SNOW BAND
TONIGHT...AND A BIT LIGHTER ON SNOW AMOUNTS. STRONGER LIFT IN THE
850-700MB LAYER...AND GREATER THREAT/LIKELIHOOD OF IFR CIGS/VSBYS...
IS SOUTH OF THE TAF SITES TONIGHT. LIFT ALONG I-90 AXIS MAINLY IN
THE 700-500MB LAYER. GIVEN THIS...APPEARS CIGS/VSBYS IN ANY -SN AT
KRST/KLSE TONIGHT WOULD MAINLY BE IN THE MVFR RANGE WITH PERHAPS A
SHORT PERIOD OF IFR VSBYS. SNOW AMOUNTS AT BOTH KRST-KLSE LOOKING TO
BE AROUND AN INCH.
&&
.ARX WATCHES/WARNINGS/ADVISORIES...
WI...NONE.
MN...NONE.
IA...NONE.
&&
$$
AVIATION..........RRS
SHORT/LONG TERM...RABERDING
