FXUS63 KARX 030926
AFDARX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LA CROSSE WI
326 AM CST THU DEC 3 2009

.SHORT TERM...TODAY THROUGH SATURDAY NIGHT

MAIN FORECAST CONCERN IS ON SNOW CHANCES FOR TODAY AND TONIGHT

09Z WATER VAPOR LOOP AND RUC 500MB HEIGHT ANALYSIS SHOWED RIDGING 
ALONG THE WEST COAST OF THE U.S. INTO WESTERN CANADA...RIDGING ALONG 
THE EAST COAST OF THE U.S....AND TROUGHING FROM HUDSON BAY INTO THE 
CENTRAL U.S.. WITHIN THE TROUGHING...THE MAIN FEATURE OF NOTE 
PERTINENT TO THE FORECAST IS AN UPPER LOW NEAR SIOUX FALLS SD. RADAR 
AND SURFACE OBSERVATIONS SHOW SOME LIGHT SNOW ACCOMPANYING THE UPPER 
LOW OVER EASTERN SOUTH DAKOTA AND SOUTHERN MINNESOTA. INTERESTING TO 
NOTE TOO THAT WHEN COMPARING THE PRECIPITATION TO THE SURFACE 
ANALYSIS...ALL OF THE SNOW LIES BEHIND A SECOND COLD FRONT... 
STRETCHING FROM A 989MB SURFACE LOW NEAR BUFFALO NY WESTWARD INTO 
SOUTHERN SOUTH DAKOTA. 11-3.9 UM SATELLITE IMAGERY ALSO SHOWED 
EXTENSIVE CLOUD COVER BEHIND THE FRONT...COVERING MUCH OF THE 
DAKOTAS... MINNESOTA AND WISCONSIN. AIRMASS OVER THE UPPER 
MISSISSIPPI VALLEY AND NORTHERN PLAINS REGIONS HAS CERTAINLY COOLED 
DOWN OVER THE PAST FEW DAYS...WITH 850MB TEMPS OF -10 TO -15C 
REPORTED ON 00Z SOUNDINGS FROM MPX TO GGW. DESPITE THE COOLING 
ALOFT...THE CLOUDS HAVE HELPED TO KEEP TEMPERATURES UP...WITH 
READINGS ACROSS MOST OF THE FORECAST AREA IN THE UPPER 20S AND LOW 
30S.

TODAY INTO TONIGHT...FORECAST FOCUS IS ON THE UPPER LOW IN EASTERN 
SOUTH DAKOTA. MODELS ARE IN GOOD AGREEMENT TAKING THE LOW EASTWARD 
DURING THE NEXT 24 HOURS...MOVING INTO NORTHEAST IOWA AROUND 00Z... 
THEN INTO LOWER MICHIGAN BY 12Z FRIDAY. 500MB HEIGHT FALLS ARE NOT 
VERY IMPRESSIVE...ONLY 20 TO 40 METERS...AND MAINLY CONFINED TO 
AREAS ALONG AND SOUTH OF I-90. NONETHELESS...WITH INCREASED MID 
LEVEL MOISTURE ACCOMPANYING THE UPPER LOW...COMBINED WITH THE FACT 
LOW LEVELS ALREADY HAVE MOISTURE IN PLACE (REFLECTED BY THE STRATUS 
DECK)...THINK SOME LIGHT SNOW SHOULD MOVE INTO AND ACROSS MUCH OF 
THE FORECAST AREA. THIS WOULD BE ESPECIALLY TRUE IN THE AREAS OF 
GREATER HEIGHT FALLS. AN INTERESTING FIELD TO LOOK AT IS THE 
850-500MB RH PROGS...WHICH WOULD SUGGEST THE LIGHT SNOW AREA AROUND 
GRAND FORKS NORTH DAKOTA AT 00Z THU WOULD MOVE INTO SOUTHEAST 
MINNESOTA AND NORTHEAST IOWA BETWEEN 18-00Z TODAY...ACROSS SOUTHERN 
WISCONSIN BETWEEN 00-06Z TONIGHT...THEN INTO LOWER MICHIGAN BETWEEN 
06-12Z. HAVE FOLLOWED THIS IDEA IN THE FORECAST...INCREASING THE 
PROBABILITIES BETWEEN THE 18-06Z TIME WINDOW. IT IS CERTAINLY 
POSSIBLE TO HAVE A LITTLE LIGHT SNOW BEFORE AND AFTER THEN AS WELL. 
GIVEN THE SMALL HEIGHT FALLS AND OVERALL WEAK FRONTOGENESIS...NOT 
EXPECTING A LOT OF QPF/SNOW. 03.00Z NAM SEEMS A LITTLE HIGH ON QPF 
VERSUS THE GFS/ECMWF/SREF...SHOWING AROUND 0.15 OF AN INCH IN 
CRAWFORD AND RICHLAND COUNTIES. HOWEVER...WOULD NOT RULE OUT A 0.10 
OF AN INCH. GIVEN THAT A LOT OF THE FORCING THAT IS PRESENT IS IN 
THE DENDRITIC GROWTH ZONE...AN INCH OR TWO OF SNOW SEEMS POSSIBLE... 
MOST AGAIN SOUTH OF I-90. REGARDING TEMPERATURES...GIVEN EXTENSIVE 
CLOUD COVER...VERY LITTLE DIURNAL RANGE IS EXPECTED TODAY. 
TONIGHT...INDICATIONS ARE THAT COLDER AIR IS GOING TO ADVECT IN WITH 
850MB TEMPS FALLING FROM -10 TO -12C TODAY TO -15 TO -17C BY 12Z 
FRIDAY. AS A RESULT...DESPITE CLOUDS...COULD SEE READINGS FALLING 
INTO THE MID TEENS OVER SOUTHEAST MINNESOTA AND LOW 20S ELSEWHERE.

FRIDAY INTO FRIDAY NIGHT...UPPER TROUGH FROM HUDSON BAY INTO THE 
CENTRAL PLAINS LOOKS TO PARTIALLY LIFT OUT DURING THIS PERIOD... 
THOUGH IT SHOULD BE NOTED THAT THE PORTION NEAR HUDSON BAY ENDS UP 
HEADING WESTWARD AS A STRONG SHORTWAVE TOWARDS LAKE WINNIPEG. THE 
CAUSE FOR THE TROUGH PARTIALLY LIFTING OUT IS A MORE POTENT TROUGH 
DROPPING DOWN THROUGH WESTERN CANADA INTO THE NORTHERN ROCKIES... 
RESULTING IN HEIGHT RISES DOWNSTREAM. AS SUCH...DPVA FORCING FOR 
PRECIPITATION DURING THIS PERIOD IS BASICALLY GONE...WITH ACTUALLY 
SUBSIDENCE TAKING PLACE. HOWEVER...THIS SUBSIDENCE LOOKS TO ONLY 
HOLD THE STRATUS DECK OVER THE AREA IN PLACE UNTIL MAYBE VERY LATE 
FRIDAY NIGHT WHEN DRIER AIR STARTS TO ADVECT IN FROM THE 
WEST-SOUTHWEST. MAJORITY OF THE STRATUS DECK IS WITHIN THE DENDRITIC 
GROWTH ZONE TOO...SO IT WOULD NOT TAKE MUCH TO GET AT LEAST FLURRY 
PRODUCTION GOING. IN FACT...THE 270K SURFACE FROM THE NAM/GFS 
INDICATES SOME WEAK ISENTROPIC LIFT DEVELOPING IN THE AFTERNOON AND 
EVENING WITHIN THE STRATUS DECK...BASICALLY WHEN WINDS START BACKING 
WEST-SOUTHWEST. THEREFORE...HAVE DECIDED TO AT LEAST KEEP THE FLURRY 
MENTION GOING THROUGH 06Z FRIDAY NIGHT. AS FAR AS TEMPERATURES 
GO...WITH THE COLDER AIR OVER THE AREA AT 850MB ON FRIDAY AND PLENTY 
OF CLOUDS...READING SHOULD STRUGGLE TO RISE MUCH ABOVE MORNING LOWS. 
SURFACE TEMPS FROM THE NAM/GFS BARELY HAVE TEMPERATURES GETTING TO 
25. THIS IS SIMILAR TOO TO MET GUIDANCE...AND HAVE FOLLOWED THAT 
PLAN FOR HIGHS. FOR FRIDAY NIGHT...WITH CLOUDS POSSIBLY SCATTERING 
OUT LATE AND OVERALL LIGHT WINDS...COULD SEE TEMPERATURES TRY TO 
DROP LATE. THEREFORE...HAVE FOLLOWED THE MAJORITY OF GUIDANCE HAVING 
LOWS DROPPING INTO THE MID TO UPPER TEENS.

SATURDAY INTO SATURDAY NIGHT...POTENT TROUGH OVER THE NORTHERN 
ROCKIES IS PROGGED TO CONTINUE DROPPING SOUTH TOWARD THE FOUR 
CORNERS REGION...IN RESPONSE TO VERY HIGH AMPLITUDE RIDGING BUILDING 
FROM THE EAST PACIFIC INTO THE YUKON. THIS TROUGHING DEVELOPING IN 
THE WEST WILL CAUSE RIDGING TO BUILD DOWNSTREAM OVER THE OHIO 
VALLEY...AS WELL AS INTO THE UPPER MISSISSIPPI VALLEY AND WESTERN 
GREAT LAKES REGIONS. THE UPPER TROUGHING IS ALSO EXPECTED TO DEVELOP 
SURFACE TROUGHING FROM SOUTHEAST COLORADO INTO WESTERN MINNESOTA ON 
SATURDAY...WHICH GRADUALLY SLIDES EAST SATURDAY NIGHT. AHEAD OF THE 
SURFACE TROUGH...LOOK FOR A WARM ADVECTION PATTERN. INITIALLY FOR 
SATURDAY...THE AIRMASS IS QUITE DRY FROM THE SURFACE INTO THE MID 
LEVELS. SO AFTER ANY LINGERING STRATUS CLEARS IN THE MORNING... 
ANTICIPATE JUST SOME CIRRUS FOR THE AFTERNOON. 850MB TEMPS DO 
CLIMB...BUT ONLY REACH -7 TO -10C AT 18Z AT BEST. SO ALTHOUGH THE 
ADDITIONAL SUNSHINE WILL HELP TO MAKE TEMPERATURES WARMER THAN 
FRIDAY...HIGHS AT BEST SHOULD REACH CLOSE TO NORMAL. FOR SATURDAY 
NIGHT...INDICATIONS ARE THAT ISENTROPIC LIFTING INCREASES... 
ESPECIALLY ALONG AND NORTH OF I-90. THE RESULT IS THAT SOME 
ALTOSTRATUS DEVELOPS...AND HAVE INCREASED CLOUD COVER TO REFLECT IT. 
STILL...THE AIR BELOW THE ALTOSTRATUS IS QUITE DRY...SO NO 
PRECIPITATION CHANCES ARE MENTIONED. THE INCREASED CLOUDS WILL HELP 
TO HOLD TEMPERATURES UP SOME...AND HAVE RAISED LOWS A FEW DEGREES.

.LONG TERM...SUNDAY THROUGH THURSDAY

MEAN PATTERN FROM THE 03.00Z GFS/ECMWF/CANADIAN/GFS ENSEMBLES 
THROUGH AT LEAST TUESDAY FEATURES TROUGH FROM CENTRAL CANADA INTO 
THE UPPER MIDWEST AND WESTERN INTO THE WESTERN U.S.. WITHIN THIS 
TROUGHING...A PORTION OF THE POTENT TROUGH MENTIONED IN THE SATURDAY 
DISCUSSION IS PROGGED TO HEAD EAST ACROSS IOWA...NORTHERN MISSOURI 
AND ILLINOIS SUNDAY NIGHT. HEIGHT FALLS ASSOCIATED WITH THIS FEATURE 
AND SOME MOISTURE INFLOW SHOULD ALLOW FOR SOME LIGHT SNOW ON SUNDAY 
NIGHT...PERHAPS EVEN ENTERING FAR WESTERN SECTIONS OF THE FORECAST 
AREA SUNDAY AFTERNOON. RAISED CHANCES OF PRECIPITATION 10-20 PERCENT 
DURING THIS TIME-FRAME GIVEN TRENDS CENTERED ON THE SNOW MOVING 
ACROSS SUNDAY NIGHT. SOME LIGHT SNOW COULD LINGER BACK ACROSS THE 
AREA ON MONDAY...THOUGH WITH MUCH OF THE SHORTWAVE ENERGY PUSHING 
OFF TO THE EAST...LEFT THE GOING SLIGHT CHANCE WORDING IN PLACE. 
DRIER WEATHER SHOULD FOLLOW AT LEAST FOR MONDAY NIGHT. ON 
TUESDAY...A LARGE PIECE OF THE WESTERN TROUGH LOOKS TO EJECT OUT 
TOWARDS THE CENTRAL PLAINS. AN INVERTED SURFACE TROUGH IS FORECAST 
TO FORM IN THE PLAINS...PERHAPS BRINGING A LITTLE LIGHT SNOW TO THE 
AREA...THOUGH THERE IS MUCH DISAGREEMENT ON THE LOCATION OF THIS 
TROUGH. THEREFORE...HAVE LEFT THE GOING FORECAST IN PLACE WITH 20 
PERCENT CHANCES OF PRECIPITATION. REGARDING TEMPERATURES...PLAN ON 
READINGS TO REMAIN AT OR BELOW NORMAL DUE TO UPPER TROUGHING IN THE 
AREA.

BEYOND TUESDAY...MODELS AND ENSEMBLES HAVE BEEN CONSISTENT IN 
SHOWING A LARGE POLAR VORTEX DROPPING DOWN INTO WESTERN 
CANADA...WHILE THE FLOW SPEEDS UP ACROSS THE PACIFIC INTO THE U.S.. 
THE RESULT IS THAT THE FLOW SHOULD TURN ZONAL...CAUSING PACIFIC AIR 
TO BE BLOWN EASTWARD INTO THE U.S....WHILE ARCTIC AIR DROPS SOUTH 
ACROSS MUCH OF WESTERN CANADA. PACIFIC AIR WILL ONLY WARM UP MORE 
DOWNSLOPING OFF THE ROCKIES. THEREFORE A WARMING TREND LOOKS TO 
OCCUR...PROBABLY ON THURSDAY. 

&&

.AVIATION...TODAY AND TONIGHT

EXPECT CEILING HEIGHT TO BE THE PRIMARY ISSUE FOR AVIATION INTERESTS 
FOR THE NEXT 24-30 HOURS. CURRENTLY VFR ACROSS THE MAJORITY OF THE 
REGION...WITH HEIGHTS ABOVE 3 TO 4 THOUSAND FEET. BUT THERE ARE 
POCKETS OF MVFR CEILINGS...ESPECIALLY UPSTREAM OVER NORTHERN MN/WI. 
EXPECT SOME DIURNAL RISE...SO ANTICIPATE PRIMARY VFR CONDITIONS... 
BUT WILL ALSO HAVE TO BE COGNIZANT OF SNOW SHOWERS WHICH COULD 
REDUCE VISIBILITY TO A MILE OR TWO FOR BRIEF PERIODS. THERE IS AMPLE 
ISENTROPIC LIFT AND COLD ADVECTION TO HELP SQUEEZE SOME SNOW OUT OF 
THE SHALLOW CLOUD DEPTH.  VERY TOUGH TO TRY AND PIN THAT DOWN...BUT 
WILL ATTEMPT TO ADDRESS IT FOR THE 12Z ISSUANCE. 

&&

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

$$

SHORT/LONG TERM...AJ
AVIATION...MW