FXUS63 KARX 070925
AFDARX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LA CROSSE WI
325 AM CST MON DEC 7 2009

.SHORT TERM...TODAY THROUGH WEDNESDAY NIGHT

FORECAST CONCERNS THIS CYCLE...IMPENDING MAJOR WINTER STORM/
TUE THROUGH WED...POTENTIAL BLIZZARD CONDITIONS WED.

SFC ANALYSIS AT 06Z HAD A WEAK LOW OVER CENTRAL MO WITH A TROUGH
TO THE NORTHEAST INTO SOUTHEAST WI. BROAD HIGH PRESSURE WAS SPREAD
ACROSS MUCH OF THE CENTRAL/NORTHERN PLAINS...CENTERED OVER ALB. WV
IMAGERY SHOWED A SHORTWAVE MOVING EAST NEAR THE IA/MO BORDER. LIFT
AHEAD/NORTH OF THIS WAVE COUPLED WITH 280-290K ISENTROPIC LIFT AND
WEAK THETA-E ADVECTION RESPONSIBLE FOR SOME -SN OVER THE REGION...
ROUGHLY SOUTH OF A KFSD-KONA-KGRB LINE WHERE PRECIP PRODUCTION
ALOFT ABLE TO SATURATE/OVERCOME THE DRIER AIRMASS IN THE 850-700MB
LAYER. SNOWFLAKES OUTSIDE THE OFFICE A RATHER FINE...DUSTY SNOW.

NO ISSUES NOTED WITH 07.00Z GFS/NAM INITIALIZATIONS...WITH MODELS
PRESENTING REASONABLY SIMILAR SOLUTIONS FOR TONIGHT INTO WED NIGHT.
SOME TRACK/STRENGTH DIFFERENCES AS THE DEEPENING SFC CYCLONE MOVES
ACROSS SOUTHERN LAKE MI WED MORNING...BUT THIS NOT UNEXPECTED IN
THE 60HR TIME-FRAME. MODELS REMAIN WELL SUPPORTED BY UKMET/ECMWF/
CAN-GEM WITH A MAJOR WINTER STORM TO IMPACT THE REGION TUE INTO WED
NIGHT. DPROG/DT OF 500MB HGTS AT 07.00Z SHOWED NAM/GFS/ECMWF RUNS OF
05.00Z AND 06.00Z VERIFIED WELL ACROSS NOAM/EASTERN PAC WITH
SOLUTIONS CONVERGING TOWARD A SOLUTION MOST SIMILAR TO EARLIER ECMWF
RUNS. THRU 36HRS MODELS CONVERGING ON A COMMON SOLUTION WITH GOOD
RUN-TO-RUN CONSISTENCY SHOWN BY GFS/ECMWF. MODELS CONTINUE TO
CONVERGE ON A COMMON SOLUTION IN THE 36-84HR TIME FRAME AS THE
STRONG SHORTWAVE MOVES INTO IL AT 60HRS THEN MOVES QUICKLY INTO
EASTERN CAN BY 84HRS. GFS WITH THE BETTER RUN-TO-RUN CONSISTENCY IN
THIS TIME-FRAME. TREND BY 84HRS IS TOWARD BROADER TROUGHING/FLATTER
FLOW ACROSS THE CENTRAL CONUS. CHECK OF OBS VS. MODEL DATA AT 06Z
SHOWED NAM/GFS WERE REASONABLE WITH THE SFC MASS FIELDS ACROSS THE
CENTRAL CONUS. PER WV IMAGERY AND RADAR DATA...NAM APPEARED BETTER
WITH A STRONGER SHORTWAVE NEAR THE EAST END OF THE IA/MO BORDER. A
BLEND OF NAM/GFS APPEARED BEST WITH THE 00Z-06Z PRECIP ACROSS THE
REGION. WITH GFS/ECMWF OFFERING THE BETTER RUN-TO-RUN CONSISTENCY
OVER THE PAST 48HRS WORTH OF RUNS...FAVORED A BLEND OF THEIR
SOLUTIONS THIS CYCLE...WITH THEIR SFC LOW POSITIONS AT 60HRS /12Z
WED/ CONSISTENT AND IN THE MIDDLE OF THE ENVELOPE OF SOLUTIONS.
INTERESTINGLY...AT 12Z WED 07.03Z SREF MEAN ON THE NORTHERN EDGE OF
THE SOLUTION ENVELOPE WHILE 07.00Z GFS ENSEMBLE MEAN ON SOUTHERN
EDGE OF THE SOLUTION ENVELOPE. CONSIDERING MAGNITUDE OF THE STORM
HEADED FOR THE REGION TUE/WED...FORECAST CONFIDENCE SURPRISINGLY
GOOD THIS CYCLE.

FOR THE SHORT TERM...FIRST SHORTWAVE TROUGH AND LOWER LEVEL WARM
ADVECTION/ISENTROPIC LIFT RESPONSIBLE FOR THE -SN OVER ROUGHLY THE
SOUTHERN 2/3 OF THE FCST AREA WILL EXIT EAST OF THE AREA BY OR
SHORTLY AFTER 12Z. BULK OF ANY LINGERING -SN AFTER 12Z EXPECTED TO
BE FLURRIES WITH MINIMAL ACCUMULATION. DEEPER MOISTURE SHOVES OFF TO
THE EAST AS WELL BY/AFTER 15Z...BUT MODELS QUITE CONSISTENT WITH
SOME SFC-850MB MOISTURE LINGERING OVER THE AREA TODAY/TONIGHT...
UNDER A DEVELOPING INVERSION NEAR 850MB. THUS MOSTLY CLOUDY TO
CLOUDY SKIES EXPECTED TO PERSIST THRU TONIGHT.

CONFIDENCE REMAINS HIGH FOR FCST AREA TO TAKE A NEARLY DIRECT HIT BY
A MAJOR WINTER STORM BEGINNING TUE AND CONTINUING INTO WED NIGHT.
FCST THINKING HAS CHANGED LITTLE FROM 06.00Z AND 06.12Z DATA SETS...
WITH AREA IN FOR A PROLONGED PERIOD OF FORCING/LIFT AND SNOWFALL.
UPPER LEVEL TROUGH WILL DIG THRU THE CENTRAL/SOUTHERN ROCKIES
TODAY/TONIGHT THEN INTO THE TX/OK/KS AREA TUE. BROAD AREA OF 925-
700MB WARM ADVECTION AND 275K-300K ISENTROPIC LIFT AHEAD OF
THIS TROUGH SPREADS ACROSS THE AREA ALREADY ON TUE. DEEP LAYERED
SATURATION ARRIVES QUICKLY WITH 3-4 G/KG OF SPECIFIC HUMIDITY
AVAILABLE IN THE INFLOW AIRMASS. SNOW EXPECTED TO SPREAD RATHER
QUICKLY NORTHEAST ACROSS THE AREA TUE MORNING AND RAISED SNOW
CHANCES IN THE 15Z-18Z TIME-FRAME. ALSO RAISED SNOW CHANCES FOR TUE
AFTERNOON AS MOISTURE/LIFT CONTINUE TO INCREASE OVER THE AREA. DEEP
LAYERED LIFT...THE 925-700MB WARM ADVECTION...275-300K ISENTROPIC
LIFT...400-200MB PV ADVECTION AND 300MB DIVERGENCE BETWEEN COUPLED
JETS ALOFT SPREAD ACROSS THE AREA LATE TUE/TUE NIGHT. MAXIMUM LIFT/
OMEGA INDICATED IN ROUGHLY THE 08.21Z TO 09.09Z TIME-FRAME AS THE
MID LEVEL SHORTWAVE/TROUGH AXIS APPROACHES/PASSES. LOWER LEVEL
THERMO-DYNAMIC FORCING/LIFT ALREADY MOVING EAST OF THE AREA BY 12Z
WED AS THE SFC-700MB LOW PASS EAST OF THE AREA. SOME 900-700MB FN
CONVERGENCE IN THE TIGHT LOWER LEVEL CIRCULATION/WRAP-AROUND/
DEFORMATION BAND REMAINS IN THE DEEPLY SATURATED AIRMASS...TO
CONTINUE SNOWFALL OVER MUCH OF THE AREA THRU AT LEAST WED MORNING.
MODEL SOUNDINGS INDICATE A DEEPER THAN NORMAL PRIME DENDRITIC GROWTH
ZONE /-12C TO 18C TEMP LAYER/...ON THE ORDER OF 100-150MB DEEP...
OVER THE AREA TUE AFTERNOON/EVENING. SNOW-WATER RATIOS COULD BE IN
THE 20 TO 1 RANGE DURING THIS TIME...BUT INCREASING WINDS ARE LIKELY
TO BREAK UP THE BIGGER FLAKES AND KEEP SNOW-WATER RATIOS MORE IN THE
12 TO 15 TO 1 RANGE. BASED TUE-WED SNOWFALL ACCUMULATIONS ON THIS...
VERY SIMILAR TO PREVIOUS FCST CYCLE. ALL MODELS PRODUCING 6 TO 12+
INCHES OF SNOW OVER THE FCST AREA FROM TUE THRU WED. CONSIDERED
RAISING WINTER STORM WARNING ALREADY THIS MORNING GIVEN GOOD
CONFIDENCE ON A CONSISTENT MODEL SIGNAL. HOWEVER STILL PLENTY OF
LEAD TIME TO THE SIGNIFICANT ACCUMULATIONS AND DAY CREW WILL HAVE
07.12Z MODEL SET TO LOOK AT AND IF ALL REMAINS ON TRACK WILL LIKELY
ISSUE WARNINGS THIS AFTERNOON.

ONCE THE SFC-850MB LOW PASSES LATER TUE NIGHT...TIGHT PRESSURE
GRADIENT AND COLD ADVECTION ON ITS BACKSIDE WILL SPREAD ACROSS THE
AREA FOR LATE TUE NIGHT AND WED...WITH 925MB WINDS IN THE 35-40KT
RANGE. WITH WHAT SHOULD BE PLENTY OF FRESH SNOW FOR THESE WINDS TO
BLOW AROUND...BLIZZARD CONDITIONS A GOOD POSSIBILITY FOR THE OPEN
COUNTRY AREAS AND RIDGE-TOP LOCATIONS WED. IF NOT ALREADY SO...
WINTER WEATHER HEADLINES WOULD LIKELY NEED TO BE UPGRADED TO
BLIZZARD WARNINGS FOR PORTIONS OF THE AREA. WILL PROBABLY NOT BE
ABLE TO BE DETAIL SOME OF THIS UNTIL TUE AFTERNOON OR NIGHT...WITH
LATER SHIFTS SORTING IT OUT AS THE STORM EVOLVES AND SNOWFALL/WINDS
ARE BETTER KNOWN. WINDS DIMINISH WED NIGHT AS THE LOW QUICKLY PULLS
AWAY FROM THE REGION...BUT TRAVEL CONDITIONS WILL LIKELY REMAIN POOR
FROM TUE AFTERNOON ALL THE WAY INTO THU.

NUMERICAL TEMP GUIDANCE REASONABLY SIMILAR THRU THE PERIOD AND
GENERALLY USED A BLEND OF THE HIGHS/LOWS FOR TODAY THRU WED NIGHT.

.LONG TERM...THURSDAY THROUGH SUNDAY

NOT A LOT OF TIME SPENT LOOKING AT DAY 4-7 DETAILS THIS CYCLE.
07.00Z MEDIUM RANGE MODELS APPEAR REASONABLY SIMILAR TO EACH OTHER
AND THEIR PREVIOUS RUNS FOR THU-SUN WITH BROAD TROUGHING OVER THE
CENTRAL CONUS THU/FRI AND QUASI-ZONAL FLOW OVER THE CONUS SAT/SUN.
COLD AIRMASS POURS SOUTHEAST ACROSS THE REGION BEHIND THE DEPARTING
LOW FOR THU/FRI...WITH SOME SUB-ZERO LOWS POSSIBLE OVER THE FRESH
SNOW COVER LATE IN THE WEEK. SOME MODERATION OF TEMPS SAT/SUN WITH
THE MORE ZONAL/WESTERLY FLOW AND SOME WEAK/MDT LOW LEVEL WARM
ADVECTION OVER THE REGION. GIVEN THE FLOW PATTERN AND A DRY/COLD LOW
LEVEL AIRMASS OVER THE REGION...THU-SUN LOOKS TO BE A GENERALLY DRY
PERIOD. PREVIOUS DAY 4-7 GRID SET RATHER WELL TRENDED IN THESE
REGARDS WITH NO CHANGES MADE.

&&

.AVIATION...TODAY AND TONIGHT

SURFACE LOW NOW TRACKING ACROSS MISSOURI WILL CONTINUE TO PULL
NORTHEAST AWAY FROM THE AREA TODAY. LARGE SHIELD OF LIGHT SNOW...
WITH VISIBILITIES OF 1 TO 3 MILES NOW MOVING OVER THE AREA. THE
SNOW SHOULD PUSH OUT OF THE TAF SITES BETWEEN 11Z AND 13Z. MAINLY
MVFR CEILINGS THEN EXPECTED THE REMAINDER OF THE DAY...AS ANOTHER
BAND OF STRATUS DROPS IN BEHIND THE DEPARTING LOW. THUS HAVE KEPT
CEILINGS IN BOTH TAF SITES BETWEEN 015-020 FEET THROUGH 06Z
TONIGHT...AND MOST LIKELY ON THROUGH 12Z.

&&

.ARX WATCHES/WARNINGS/ADVISORIES...
WI...WINTER STORM WATCH FROM TUESDAY AFTERNOON THROUGH WEDNESDAY
     EVENING FOR WIZ017-WIZ029-WIZ032-WIZ033-WIZ034-WIZ041-
     WIZ042-WIZ043-WIZ044-WIZ053-WIZ054-WIZ055-WIZ061.

MN...BLIZZARD WATCH FROM TUESDAY AFTERNOON THROUGH WEDNESDAY
     EVENING FOR MNZ086-MNZ087-MNZ094-MNZ095.

     WINTER STORM WATCH FROM TUESDAY AFTERNOON THROUGH WEDNESDAY
     EVENING FOR MNZ079-MNZ088-MNZ096.

IA...BLIZZARD WATCH FROM TUESDAY AFTERNOON THROUGH WEDNESDAY
     EVENING FOR IAZ008-IAZ009-IAZ010-IAZ018-IAZ019-IAZ029.

     WINTER STORM WATCH FROM TUESDAY AFTERNOON THROUGH WEDNESDAY
     EVENING FOR IAZ011-IAZ030.

&&

$$

SHORT/LONG TERM...RRS
AVIATION..........RABERDING