FXUS63 KARX 252103
AFDARX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LA CROSSE WI
301 PM CST FRI DEC 25 2009

.SHORT TERM...TONIGHT THROUGH MONDAY NIGHT

FORECAST CONCERNS THIS PACKAGE...SNOW CHANCES WITH SFC/UPPER LOW
NEAR/OVER THE AREA TONIGHT THRU SAT NIGHT...TEMPERATURES THRU THE
PERIOD.

AT 18Z A 987MB LOW WAS CENTERED NORTHWEST OF KDSM...DRIFTING SLOWLY
NORTHWEST AS ONE STRONG SHORTWAVE ROTATES WESTWARD ACROSS SOUTHERN
MN AND WARMER AIR ALOFT WRAPS AROUND THE WEST SIDE OF THE MID LEVEL
LOW. COLDER LOW LEVEL AIRMASS CONTINUED TO WRAP AROUND THE SOUTH/
SOUTHEAST SIDES OF THE SFC LOW...WITH TEMPERATURES FALLING ACROSS
EASTERN IA AND BEGINNING TO FALL ACROSS THE SOUTH END OF THE FCST
AREA. AHEAD OF THIS...SFC TEMPS IN THE MID 30S TO LOW 40S OVER THE
FCST AREA. PRECIP...MAINLY RAIN...CONTINUED TO WRAP AROUND THE
EAST/NORTH SIDES OF THE SFC/MID LEVEL LOW AND ACROSS THE AREA.
APPEARS ONE STRONGER/MORE WIDESPREAD AREA OF PRECIP...WITH A
SHORTWAVE LIFTING NORTH THRU IL...TO ROTATE ACROSS THE AREA THIS
AFTERNOON...MAINLY AS RAIN OR -RA CHANGING TO -SN AS THE COLDER AIR
ARRIVES.

25.12Z NAM/GFS INITIALIZED WITHOUT LARGE ERRORS. BOTH WERE A BIT TOO
FAR EAST WITH THE SFC LOW IN IA AND GFS APPEARED A BIT BETTER WITH
THE 500MB HGTS/SHORTWAVE DETAILS. SOLUTIONS QUITE SIMILAR FOR
TONIGHT INTO MON NIGHT AND GENERALLY WELL SUPPORTED BY UKMET/CAN-
GEM/ECMWF. DPROG/DT OF 500MB HGTS AT 25.12Z SHOWED NAM/GFS/ECMWF
RUNS OF 24.12Z VERIFIED QUITE WELL ACROSS THE CONUS AS THE 2 MAIN
SHORTWAVES MERGE INTO ONE LARGE UPPER LOW. 23.12Z RUNS DID NOT
VERIFY QUITE SO WELL BUT ECMWF THE BETTER OF THE BUNCH WITH A SLOWER
EVOLUTION OF THE MERGER. THRU 36HRS MODELS HAVE CONVERGED ON A
RATHER TIGHT CONSENSUS AS THE UPPER LOW LIFTS INTO WI...TOWARD A
COMPROMISE OF THEIR EARLIER RUNS. BETTER RUN-TO-RUN CONSISTENCY AT
00Z SUN LOOKS TO GO TO NAM. MODEL SOLUTIONS DIVERGE IN THE 36-60HR
TIME FRAME AS THE LOW/TROUGH MOVES EAST OF THE REGION BUT
CONSENSUS BETTER THEN PREVIOUS RUNS WITH A TREND TOWARD A FASTER
PROGRESSION OF THE LOW BY 60HRS. MODEL CONSENSUS IMPROVES IN THE 60-
84HR PERIOD AS THE TROUGH PROGRESSES INTO EASTERN NOAM. MODELS
CONVERGING TOWARD A BLEND OF THE EARLIER RUNS ON SPEED WITH BETTER
RUN-TO-RUN CONSISTENCY TO ECMWF. CONSENSUS IMPROVING WITH EVOLUTION
OF THE TROUGHING ALONG THE WEST COAST AT 84HRS. FOR RECENT
PERFORMANCE...WARM MODEL OUTLIER /NAM/ APPEARED TO DO BEST WITH THE
NEAR-MISS OF THE AREA BY A MAJOR WINTER STORM. NAM WAS FIRST/WARMEST
TO WRAP THE 850MB WARM AIR WEST/NORTH ACROSS THE ENTIRE FCST AREA
WITH MAIN PRECIP TYPE AS RAIN THU/THU NIGHT/FRI. CHECK OF OBS VS.
MODEL DATA AT 18Z SHOWED NAM/GFS REMAIN A BIT EAST WITH THE SFC LOW
POSITION...UKMET/CAN-GEM BETTER. PER WV IMAGERY...GFS APPEARED
BETTER WITH THE SHORTWAVE DETAILS IN THE UPPER LOW ACROSS THE
REGION...ESPECIALLY WITH A STRONGER SHORTWAVE ROTATING NORTH ACROSS
IL. REASONS TO FAVOR EACH OF THE MODELS THIS CYCLE. HOWEVER...WITH A
TREND TOWARD A COMMON/BLENDED SOLUTION OF THE EARLIER RUNS...FAVORED
THE MODEL/ENSEMBLE CONSENSUS. FCST CONFIDENCE IN THE SHORT-TERM GOOD
THIS CYCLE.

FOR THE SHORT TERM...UPPER LEVEL LOW WILL CONTINUE TO CONSOLIDATE/
DEEPEN OVER MO THRU THIS EVENING...THEN LIFT INTO WI SAT. OCCLUDED
SFC LOW WILL WOBBLE OVER IA TONIGHT AS WARM/COLD AIRMASSES WRAP
AROUND IT...SLOWLY FILL AND END UP IN SOUTHEAST IA SAT MORNING. WITH
OCCLUSION AND SFC-700MB COLD AIR WRAPPED AROUND THE EAST SIDE OF THE
LOW...CENTER OF THE SYSTEM BECOMES FURTHER AND FURTHER CUT-OFF FROM
THE PRIMARY GULF MOISTURE FEED. ANY REMAINING -RA CHANGES QUICKLY TO
SNOW LATE THIS AFTERNOON/EVENING...AND BECOMES QUITE LIGHT ONCE THE
IL SHORTWAVE ROTATES NORTHWEST PAST THE AREA. ANY ACCUMULATIONS THRU
REST OF THIS AFTERNOON/THIS EVENING EXPECTED TO BE AN INCH OR
LESS...WITH WINTER WEATHER ADVISORY CANCELLED AS OF 3 PM/21Z. ALL
MODELS INDICATE A DRY SLOT AND PRECIP MINIMUM TO ROTATE OVER THE
AREA TONIGHT/SAT. SOME MODEL CONSENSUS FOR THIS IN THE AXIS OF
STRONGEST 850-700MB COLD ADVECTION. OPTED TO LEAVE PRECIP CHANCES
FOR TONIGHT INTO SAT NIGHT RATHER GENERAL WITH 80 TO 90 PERCENT
TONIGHT AND 60 PERCENT SAT/SAT NIGHT...AND NOT TRY TO DETAIL WHAT
COULD BE A DIFFICULT TO PLACE PRECIP MINIMUM/DRY AXIS. TRENDED GRIDS
TOWARD OCCASIONAL -SN LATER TONIGHT WITH MINIMAL ACCUMULATIONS.
LIKELY -SN IN GRIDS SAT/SAT NIGHT...BUT AGAIN ANY SNOW AMOUNTS
EITHER PERIOD APPEAR TO BE QUITE LIGHT. SFC LOW WELL EAST OF THE
AREA SUN...BUT MID/UPPER LEVEL LOW TO BE NEAR/OVER THE AREA...AT
LEAST EARLY SUN. MODELS LINGER SOME SFC-700MB MOISTURE ALONG WITH
CYCLONIC FLOW AND WEAK 850-700MB WARM ADVECTION AS THE COLD CORE OF
THE MID/UPPER LEVEL LOW PULLS AWAY. CONTINUED THE -SN CHANCE SUN AND
SCT FLURRIES INTO SUN NIGHT. RISING HGTS AND SHORTWAVE RIDGING ALOFT
BUILD ACROSS THE AREA FOR LATE SUN NIGHT INTO MON NIGHT. MODEL
SOUNDINGS/X-SECTIONS INDICATE DRYING ABOVE 925MB TO SPREAD ACROSS
THE AREA...WITH 850MB TEMPS WARMING TO -4C TO -6C BY 12Z MON. MAIN
PUSH OF ARCTIC AIR IN THE WAKE OF THE DEPARTING SYSTEM EITHER
ACCOMPANIES IT ACROSS THE AREA SAT/SAT NIGHT OR PASSES EAST OF THE
AREA MON/MON NIGHT...WITH TEMPS TO REMAIN NEAR SEASONAL NORMALS THRU
THE SHORT TERM.

NUMERICAL TEMP GUIDANCE FOR TONIGHT THRU SUN QUITE SIMILAR THEN SOME
5 TO 10F DIFFERENT FOR SUN NIGHT. FAVORING THE MODEL/ENSEMBLE
CONSENSUS...FAVORED A BLEND OF GUIDANCE LOWS/HIGHS FOR TONIGHT THRU
SUN NIGHT. FAVORED A BLEND OF THE GFS MEX/ENSEMBLE MOS AND HPC
GUIDANCE FOR HIGHS/LOWS MON/MON NIGHT.

.LONG TERM...TUESDAY THROUGH FRIDAY

25.00Z GFS/ECMWF/UKMET IN REASONABLY GOOD AGREEMENT TUE-WED AND
SHOW BETTER CONSISTENCY WITH THEIR RUNS OF 24HRS AGO...BETTER THAN
THE PAST FEW CYCLES. CAN-GEM A SLOW/STRONG OUTLIER WITH AN UPPER
LOW WEST OF THE CA BAJA EARLY TUE...BUT LOOKS TO CATCH UP WITH THE
OTHERS BY WED. BETWEEN MODEL CONSISTENCY BEGINS TO BREAK DOWN THU.
MODELS BRING A NORTHWEST FLOW TROUGH/SHORTWAVE ACROSS THE AREA
THU...BUT ECMWF QUITE A BIT STRONGER THAN GFS. GFS WITH THE BETTER
RUN-TO-RUN CONSISTENCY WED. BETWEEN MODEL AND RUN-TO-RUN CONSISTENCY
QUITE POOR BY FRI...ECMWF WITH AN UPPER LOW OVER THE REGION...GFS
WITH WEAK SHORTWAVE RIDGING. FCST CONFIDENCE ON THE BETTER SIDE
TUE/WED...THEN LOW BY FRI WITH THE LESSER MODEL/RUN-TO-RUN
CONSISTENCY. QUIET/ DRY TUE WITH SHORTWAVE RIDGING ALOFT/SFC HIGH
PRESSURE ACROSS THE REGION. WITH THE MODEL CONSISTENCY FOR SOME FORM
OF A SHORTWAVE TROUGH TO MOVE TOWARD/INTO THE AREA WED/THU...
CONTINUED THE SMALL -SN CHANCE. WITH 25.00Z ECMWF LOOKING LIKE A
STRONG OUTLIER WITH ITS UPPER LOW OVER THE REGION FRI AND...PER
PMDEPD...NOT WELL SUPPORTED BY ITS ENSEMBLE MEAN...FAVORED THE
FLATTER/MORE PROGRESSIVE GFS/ ENSEMBLE MEANS AND LEFT FRI DRY FOR
NOW. TRUE ARCTIC AIR LOOKS TO REMAIN BOTTLED UP IN CAN TUE THRU
THU...WITH A PUSH SOUTHWARD BEHIND THE THU TROUGH FOR THU NIGHT/FRI.
GENERALLY FAVORED A BLEND OF GFS MEX/ENSEMBLE MOS AND HPC GUIDANCE
FOR HIGHS/LOWS THRU THE PERIOD.

&&

.AVIATION...TONIGHT AND SATURDAY

BANDS OF PCPN CONTINUE TO ROTATE TO THE WEST...PER THE LATEST 88-D
RADAR IMAGERY...WHILE A BAND OF -SHRA ASSOCIATED WITH LOW LEVEL
FRONTOGENETIC FORCING HAS DEVELOPED OVER WESTERN WI. OUTSIDE OF
THESE MAIN PCPN AREAS...SATELLITE IMAGERY...ALONG WITH RUC BUFKIT
SOUNDINGS AND TIME/HEIGHT X-SECTIONS...POINT TO A DECREASE IN THE
DEPTH OF THE CLOUD/SATURATION. THIS WOULD REMOVE SOME OF THE ICE
FROM THE CLOUD...AND BRINGS THE POTENTIAL FOR SOME DRIZZLE. SFC
TEMPS AT KLSE/KRST WOULD KEEP IT AS LIQUID. COLDER AIR FINALLY WRAPS
IN AROUND THE LOW TONIGHT...AND WITH DEEPER MOISTURE ALSO
INDICATED...-SN WOULD BE SUPPORTED. NOT A LOT OF LIFT THOUGH...BUT
ENOUGH ISENTROPIC UPGLIDE ON THE 280-290 K SFCS THAT SOME FLURRIES
OR A SNOW SHOWER WILL BE POSSIBLE. DON/T EXPECT MUCH IF ANY
ACCUMULATION HOWEVER. THE COLDER TEMPS COULD RESULT IN SOME ICY
SPOTS DEVELOPING ON PAVEMENT/RUNWAYS TONIGHT THOUGH.

WINDS SHOULD GRADUALLY SLACKEN THIS EVENING AS THE LOW GETS CLOSER
TO THE TAF SITES AND STARTS TO WEAKEN. EXPECT A MOSTLY EASTERLY
DIRECTION THROUGH SATURDAY MORNING. CEILINGS SHOULD STAY IFR/MVFR
THROUGH MOST OF SATURDAY.

&&

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

&&

$$

SHORT/LONG TERM...RRS
AVIATION..........RIECK