FXUS63 KARX 050912
AFDARX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LA CROSSE WI
312 AM CST SAT DEC 5 2009

.SHORT TERM...TODAY THROUGH MONDAY NIGHT

BIGGEST FORECAST CONCERN IS ON SNOW CHANCES AND AMOUNTS SUNDAY
NIGHT. CLOUDS AND TEMPERATURES UNTIL THEN THE MAIN CHALLENGE.

07Z INFRARED SATELLITE IMAGERY SHOWING BACK EDGE OF THE CLOUDS
EXTENDING FROM NEAR THE TWIN CITIES...TO DECORAH IOWA. SURFACE
REPORTS STILL INDICATING FLURRIES UNDER THE REMAINING CLOUDS.

05.00Z NCEP DETERMINISTIC MODELS AGAIN SHOW SOME DIFFERENCES WITH
THE SYSTEM TO AFFECT THE AREA SUNDAY NIGHT. NAM CONTINUES TO BE THE
MORE ROBUST WITH QPF AND IS ALSO IS THE FURTHEST NORTH WITH THE
SURFACE LOW TRACK. GEM/GFS A LITTLE FURTHER SOUTH. BIGGEST SENSIBLE
WEATHER DIFFERENCE WILL BE IN THE NORTHERN EXTENT OF THE SNOW. FOR
NOW HEDGED MORE TOWARD THE GFS/GEM SOLUTION...BUT NOT TOTALLY
DISCOUNTING THE NAM.

THE BACK EDGE OF THE CLOUDS WILL CONTINUE TO PUSH EASTWARD TODAY...
BRINGING AT LEAST SOME SUNSHINE TO THE AREA. HOWEVER...MODELS
INDICATING SOME HIGH LEVEL MOISTURE OVERSPREADING THE AREA THIS
AFTERNOON...WITH FILTERED SUNSHINE LIKELY BY THEN. MID LEVEL
CIRCULATION OVER SOUTHERN ONTARIO WILL CONTINUE TO PUSH NORTHWARD
TOWARD JAMES BAY BY SUNDAY. WEAK DISTURBANCES WILL ROTATE AROUND
IT...BUT ANY LIGHT SNOW/FLURRIES WILL REMAIN NORTH OF THE AREA TODAY
AND TONIGHT. WITH WEAK WARM ADVECTION EXPECTED TODAY...AND SOME
SUN...WILL SEE SOMEWHAT WARMER TEMPERATURES WITH READINGS NEAR
GUIDANCE VALUES.

ON SUNDAY...SHORTWAVE GETS EJECTED OUT OF THE WESTERN TROUGH AND
TRACKS NORTHEAST TOWARD THE AREA SUNDAY NIGHT. DEEP QG
CONVERGENCE/290K-300K ISENTROPIC LIFT OVERSPREAD THE AREA FROM
THE SOUTHWEST. BOTH THE NAM AND GFS SHOWING SOME WEAK FRONTOGENETICAL
FORCING ACROSS NORTHEAST IOWA AND SOUTHWEST WISCONSIN. WOULD EXPECT
SNOW TO MOVE INTO THE AREA DURING LATE AFTERNOON/EARLY EVENING
HOURS. NORTHERN END OF ACCUMULATING SNOW LOOKS TO BE ALONG OR JUST
NORTH OF THE INTERSTATE 90 CORRIDOR...WITH GFS COBB OUTPUT
INDICATING AROUND A INCH FROM ROCHESTER...THROUGH LA CROSSE AND
MAUSTON...WITH UP TO 3 INCHES FURTHER SOUTH IN NORTHEAST IOWA AND
SOUTHWEST WISCONSIN. WITH THIS IN MIND DID RAISE SNOW PROBABILITIES
INTO THE CATEGORICAL RANGE ACROSS THE SOUTHERN COUNTIES AND THEN
TAPERED THEM TO CHANCE POPS ACROSS CLARK/TAYLOR COUNTIES.

SYSTEM EXITS EARLY MONDAY AS WEAK HIGH PRESSURE BUILDS IN MONDAY AND
MONDAY NIGHT. COLD ADVECTION AND LINGERING CLOUDS LONG WITH THE
FRESH SNOW WILL KEEP TEMPERATURES JUST IN THE 20S FOR HIGHS...WHICH
IS IN REASONABLE AGREEMENT WITH THE GUIDANCE NUMBERS.

.LONG TERM...TUESDAY THROUGH FRIDAY

MAIN FOCUS IN THIS TIME FRAME REMAINS ON POSSIBLE MAJOR WINTER STORM
FOR TUESDAY THROUGH WEDNESDAY. 05.00Z GFS AND ECMWF BOTH TAKING THE
SURFACE FROM THE SOUTHERN PLAINS THROUGH THE OHIO VALLEY...WITH THE
GFS A LITTLE FURTHER NORTH. THIS IS A SOUTHWARD SHIFT TO THE MODELS
FROM THE RUNS 24 HOURS AGO. BOTH DO BRING A LARGE SWATH OF SNOW
ACROSS THE AREA...AND WITH THESE SIGNALS REMAINING CONSISTENT DID
RAISE POPS A LITTLE ACROSS THE AREA. WITH THE CURRENT MODEL TRENDS...
HEAVIEST SNOW/WORST CONDITIONS WOULD BE SOUTH OF THE AREA...BUT
SIGNIFICANT SNOWFALL/STRONG WINDS AND BITTER WIND CHILLS AREA STILL
QUITE POSSIBLE FOR PORTIONS OF THE FORECAST AREA. FOR ANY DETAILS
WILL HAVE TO SEE IF THE MODELS CAN LOCK ONTO A SPECIFIC TRACK. IN
ANY CASE...STORM BEARS WATCHING CLOSELY.

REMAINDER OF THE FORECAST LOOKS ON TRACK AND MADE NO CHANGES.

&&

.AVIATION...

WITH A SWITCH TO A MORE WEST/SOUTHWEST SFC-700MB FLOW...THE 3K TO 5K
FT STRATUS CLOUDS WERE BEING ADVECTED EAST OF THE TAF SITES RATHER
QUICKLY EARLY THIS MORNING. THIS TREND WILL CONTINUE WITH GOOD VFR
CONDITIONS /SKC/SCT250/ EXPECTED THRU THE MORNING. NEXT SHORTWAVE
THRU THE FLOW WILL SPREAD A ROUND OF 850-700MB WARM ADVECTION/
ISENTROPIC LIFT AND UPPER LEVEL DIVERGENCE ACROSS THE AREA THIS
AFTERNOON/EVENING. MID/HIGH CLOUDS OVER THE DAKOTAS WILL SPREAD
ACROSS THE AREA IN THIS LIFT THIS AFTERNOON/EVENING. WITH THE DRIER
LOW LEVEL AIR OVER THE AREA...CIGS THIS AFTERNOON/EVENING EXPECTED
TO REMAIN AT OR ABOVE 10K FT. THE WEAK SFC-850MB LOW IN RESPONSE TO
THE WARM ADVECTION/DIVERGENCE ALOFT WILL MOVE SOUTHEAST OF THE TAF
SITES BY LATE THIS EVENING. NORTHERLY SFC-850MB FLOW BEHIND THIS LOW
LOOKS TO SPREAD AN INCREASE OF SFC-850MB MOISTURE ACROSS THE AREA.
WITH WEAK COLD ADVECTION/SUBSIDENCE INDICATED BEHIND THE WEAK LOW...
NOT CONFIDENT THIS MOISTURE WILL MANIFEST ITSELF AS A FULL CLOUD
DECK. ADDED A SCT 2K-3K FT LAYER LATE TONIGHT INTO SUN MORNING FOR
NOW AND WILL WATCH LATER TRENDS WITH THIS POST-LOW LOW LEVEL
MOISTURE.

&&

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

$$

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