Expires:No;;287970
FXUS63 KARX 270913
AFDARX
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LA CROSSE WI
313 AM CST FRI NOV 27 2009
.SHORT TERM...TODAY THROUGH SUNDAY NIGHT
MAIN FORECAST CONCERNS ARE ON TEMPERATURES AND A BRIEF BOUT OF 
FLURRIES OR LIGHT SNOW SHOWERS SUNDAY MORNING.
CURRENTLY AS OF 09Z...WATER VAPOR AND RUC 500MB HEIGHT ANALYSIS 
SHOWED A NARROW BUT DEEP TROUGH FROM THE EASTERN GREAT LAKES INTO 
THE MID ATLANTIC...RIDGING ACROSS MUCH OF THE PLAINS INTO THE 
WESTERN U.S....AND A TROUGH NEAR THE WEST COAST. WITH THE UPPER 
MISSISSIPPI VALLEY REGION EXPERIENCING BUILDING HEIGHTS ALOFT... 
SUBSIDENCE CONTINUES TO OCCUR WHILE HIGH PRESSURE HAS BEEN BUILDING  
IN AT THE SURFACE. THE COMBINATION OF THE TWO HAVE HELPED KEEP THE 
NIGHT DRY...THOUGH CLOUDS HAVE BEEN A BIT OF AN ISSUE. A STRATUS 
DECK AS SEEN ON 11-3.9 UM SATELLITE IMAGERY AND SURFACE OBS PERSISTS 
ALONG AND EAST OF A LINE FROM NEAR MEDFORD TO RICHLAND CENTER. THESE 
CLOUDS SIT IN A CYCLONIC FLOW REGIME AROUND LOW PRESSURE OVER LAKE 
HURON. IN ADDITION...SCATTERED CIRRUS CLOUDS ARE STREAMING OVER THE 
AREA...FLOWING THROUGH THE NORTHWEST FLOW ALOFT. WATER VAPOR SHOWS 
THE MOISTURE STREAM REFLECTED BY THE CIRRUS EXTENDING BACK ON TOP OF 
THE WESTERN U.S. RIDGE AND SOUTHWEST TO HAWAII. DESPITE THE 
CIRRUS...THE LIGHT WINDS ASSOCIATED WITH THE SURFACE RIDGE AND 
DRYING AIR (00Z MPX SHOWED DEWPOINT DEPRESSIONS OF 20C BETWEEN 
750-900MB) HAS ALLOWED SOME SITES IN SOUTHEAST MINNESOTA AND 
NORTHEAST IOWA TO DROP INTO THE LOW 20S.
TODAY AND TONIGHT...IN RESPONSE TO THE TROUGH NEAR THE WEST COAST 
MOVING INLAND AND SPLITTING IN TWO...UPPER LEVEL RIDGING OVER THE 
PLAINS IS PROGGED TO PUSH EASTWARD...WITH ITS AXIS OVER LA CROSSE BY 
12Z SATURDAY. A SIMILAR EASTWARD RESPONSE IS EXPECTED AT THE 
SURFACE...WITH THE SURFACE RIDGE CROSSING THE FORECAST AREA TODAY... 
AND A LOW PRESSURE TROUGH FORMING OVER THE DAKOTAS RIGHT NOW MOVING 
INTO SOUTHEAST MINNESOTA LATE TONIGHT. THE COMBINATION OF THE 
EASTWARD MOVEMENT OF BOTH THE UPPER LEVEL AND SURFACE RIDGES WILL 
HELP IN CLEARING OUT CLOUDS TODAY...DUE TO CONTINUED SUBSIDENCE AND 
ADVECTION OF DRIER AIR IN FROM THE WEST. ALONG WITH THE DRYING... 
LOOK FOR WARMER AIR TO ADVECT IN FROM THE PLAINS...AS 850MB TEMPS 
CLIMB FROM THEIR INITIAL -2C TO 2C READINGS TO 2C TO 8C BY 00Z 
SATURDAY. WARMEST 850MB READINGS EXPECTED IN SOUTHEAST MINNESOTA AND 
NORTHEAST IOWA WHICH ARE FIRST TO EXPERIENCE RETURN FLOW. 
HOWEVER...THE LOW SUN ANGLE OF LATE NOVEMBER WILL PREVENT MIXING OF 
MUCH OF THIS WARM AIR. THEREFORE AT BEST HIGHS COULD TOP OUT IN THE 
MID 40S...AGAIN IN AREAS FIRST TO FEEL THE WARM AIR. TONIGHT... 
DESPITE THE APPROACH OF THE TROUGH...LITTLE IF ANY CLOUD COVER IS 
ANTICIPATED WITH A DRY AIRMASS IN PLACE AND MOST CIRRUS CLOUDS NORTH 
OF THE FORECAST AREA. WINDS LOOK LIGHT AS WELL...THEREFORE HAVE 
STAYED ON THE COOLER END OF GUIDANCE. 
SOMETHING OF NOTE FOR AFTER MIDNIGHT TONIGHT...MET GUIDANCE INSISTS 
ON DEVELOPING SOME FOG ACROSS NORTHEAST IOWA...SOUTHEAST MINNESOTA 
AND AT A FEW LOCATIONS ALONG/NEAR THE MISSISSIPPI RIVER. THE IDEA IS 
THAT TEMPERATURES CAN COOL DOWN ENOUGH THIS EVENING...AND WHEN 
RETURN FLOW MOISTURE WORKS INTO THE AREA...CONDENSATION OCCURS AND 
RESULTS IN FOG. AM NOT READY TO BUY INTO THIS YET...WITH THE GFS 
SHOWING A DRIER AND LACK OF FOG SCENARIO. 
SATURDAY AND SATURDAY NIGHT...ATTENTION SHIFTS SOMEWHAT TO THE SPLIT 
UP TROUGH OVER THE WESTERN U.S. AT 12Z SATURDAY. MODELS SHOW THE 
SOUTHERN PART FORMING A CLOSED UPPER LOW AND SHIFTING INTO NORTHWEST 
MEXICO SATURDAY NIGHT. MEANWHILE...THE NORTHERN PORTION ACTUALLY 
DEEPENS AS IT MOVES EASTWARD INTO THE NORTHERN PLAINS...APPARENTLY 
AIDED BY RIDGING BUILDING UP INTO ALBERTA BEHIND IT. AS THE TROUGH 
PUSHES EAST...500MB HEIGHTS FALL AROUND 90 METERS SATURDAY NIGHT. 
DESPITE THE HEIGHT FALLS...MOST MODELS ARE DRY IN THEIR QPF 
FIELDS...MAINLY DUE TO A LACK OF MOISTURE. WOULD NOT BE SURPRISED TO 
SEE A BAND OF ALTOSTRATUS...OR PERHAPS EVEN A LOWER STRATUS 
DECK...COME IN AND ACCOMPANY THE HEIGHT FALLS THOUGH. THESE CLOUDS 
WILL ALL FOLLOW WELL BEHIND A TROUGH PASSAGE...WHICH SHOULD OCCUR 
DURING THE AFTERNOON. PLENTY OF SUN IS EXPECTED ON SATURDAY...AND 
COMBINED WITH 850MB TEMPS UPWARDS BETWEEN 4-6C SHOULD RESULT IN 
ANOTHER ABOVE NORMAL DAY WITH TEMPERATURES AT LEAST 5 DEGREES WARM 
COMPARED TO TODAY. CLOUD COVER MOVING IN TONIGHT...ESPECIALLY IN THE 
CIRRUS DEPARTMENT...SHOULD HELP TO KEEP TEMPERATURES UP TO SOME 
DEGREE. HOWEVER...CIRRUS UNLESS REALLY THICK IS NEVER A GREAT 
INSULATOR. THEREFORE...STILL HAVE SOME COOL TEMPERATURES FORECAST.
SUNDAY INTO SUNDAY NIGHT...UPPER TROUGH OVER THE NORTHERN PLAINS IS 
PROGGED TO DROP SOUTHEAST INTO THE AREA SUNDAY NIGHT...BRINGING WITH 
IT COLDER AIR. 850MB TEMPS FALL TO -4 TO -8C BY 00Z MONDAY (AROUND A 
4C DROP DURING THE DAY)...THEN HOLD NEARLY STATIONARY SUNDAY NIGHT. 
NOT A WHOLE LOT OF PRECIPITATION EXPECTED WITH THIS TROUGH...THOUGH 
THERE MAY JUST BE ENOUGH FORCING TO ALLOW FOR A QUICK PERIOD (1-3 
HRS) OF FLURRIES OR LIGHT SNOW SHOWERS SUNDAY MORNING. DID NOT 
INCREASE PROBABILITIES OF PRECIPITATION AT THIS POINT...BUT DOES 
REQUIRE CONSIDERATION IN LATER FORECASTS. THE DEEPER MOISTURE AHEAD 
OF THE TROUGH MENTIONED ABOVE IN THE SATURDAY NIGHT DISCUSSION...IS 
PROGGED TO EXIT BY 18Z...LEAVING SOME THIN STRATUS IN ITS WAKE. IT 
APPEARS THERE SHOULD BE ENOUGH DRY ADVECTION SUNDAY NIGHT TO CAUSE 
PARTIAL CLEARING WEST TO EAST... ESPECIALLY IN SOUTHEAST MINNESOTA 
AND NORTHEAST IOWA WHERE LOW LEVEL FLOW TURNS ANTICYCLONIC WITH A 
SURFACE RIDGE BUILDING IN. BREEZY NORTHWEST WINDS ALSO LOOK LIKELY 
ON SUNDAY WITH THE COLD ADVECTION...ADDING AN EXTRA CHILL TO THE 
AIR. HAVE LOWERED HIGHS SOME...BUT WOULD NOT BE SURPRISED IF A 
LITTLE MORE LOWERING IS REQUIRED. IF WINDS CAN DECOUPLE SUNDAY 
NIGHT...SOME PARTIALLY CLEARED LOCATIONS COULD SEE LOWS IN THE 
TEENS. HOWEVER...UNCERTAINTY IS TOO HIGH SINCE SURFACE RIDGING IS 
PROGGED TO BUILD IN LATE. THEREFORE...KEPT GOING LOWS IN THE 20S.
.LONG TERM...MONDAY THROUGH FRIDAY
A LOT OF PATTERN CHANGE LOOKS TO OCCUR NEXT WEEK...GIVEN THE CURRENT 
AND PAST MODEL RUNS/ENSEMBLES. THE INITIAL TROUGHING OVER THE AREA 
SUNDAY NIGHT WILL RUSH EAST TO THE EAST COAST BY 12Z TUE...WHILE NEW 
TROUGHING DIGS DOWN INTO WESTERN MONTANA. IN-BETWEEN...RIDGING LOOKS 
TO BUILD OVER THE NORTHERN PLAINS...EVENTUALLY CROSSING THE UPPER 
MISSISSIPPI VALLEY REGION ON TUESDAY. DRY WEATHER LOOKS TO BE THE 
RULE FOR MONDAY AND TUESDAY WITH THIS RIDGE BUILDING IN AND 
SUBSIDENCE UNDERNEATH IT. MONDAY SHOULD REMAIN COOL...THOUGH NEAR 
NORMAL...WITH A LITTLE WARMER CONDITIONS FOR TUESDAY AS RETURN FLOW 
SETS UP. BEYOND TUESDAY...THERE IS A LITTLE MODEL DIVERGENCE BETWEEN 
THE 27.00Z GFS AND THE 27.00Z ECMWF/CANADIAN. THE LATTER CAMP WANTS 
TO DIG A FAIRLY DEEP TROUGH OVER THE CENTRAL PORTION OF NORTH 
AMERICA FOR WEDNESDAY THROUGH FRIDAY...WHEREAS THE GFS AND TO SOME 
DEGREE THE ENSEMBLES KEEP THE FLOW MORE PROGRESSIVE. DPROG/DT OF THE 
LAST 5 ECMWF RUNS SEEM TO SUPPORT THE SLOW...DEEP TROUGH. 
THEREFORE...AFTER A COLD FRONTAL PASSAGE PROGGED ON WEDNESDAY...THE 
COLDEST AIR SEEN SO FAR THIS SEASON LOOKS TO RUSH IN. THE ECMWF 
SHOWS 850MB TEMPS FALLING TO -12C BY 00Z FRIDAY. PRECIPITATION 
DETAILS OUT BEYOND THE FRONTAL PASSAGE LOOK QUESTIONABLE AT THIS 
POINT...BUT THERE SHOULD BE SOME LOW PRESSURE SYSTEM IN THE VICINITY 
OF THE GREAT LAKES WITH THE AREA PROBABLY ON THE SOUTHERN EDGE OF 
WRAP-AROUND SNOW. PLENTY OF TIME TO WATCH THIS TIME PERIOD.
&&
.AVIATION...TODAY AND TONIGHT
CONTINUING TO SEE MVFR/VFR STRATOCUMULUS CLOUD DECK ERODING SLOWLY 
EASTWARD EAST OF THE MISSISSIPPI RIVER EARLY THIS MORNING AS A RIDGE 
OF HIGH PRESSURE BUILDS IN FROM MN/IA/MO. MEANWHILE...A WARM FRONT 
EXTENDING FROM THE DAKOTAS INTO THE CENTRAL PLAINS WAS PRODUCING 
PATCHY CIRRUS IN THE CLEARING ALONG AND WEST OF THE MISSISSIPPI 
RIVER. AS THE HIGH PRESSURE RIDGE CONTINUES TO DRIFT INTO AND ACROSS 
THE AREA TODAY...VFR CONDITIONS WILL DOMINATE THE AREA WITH ONLY 
SOME PATCHY CIRRUS EXPECTED ACROSS THE KRST/KLSE TAF SITES. A WEAK 
COLD FRONT WILL BE APPROACHING THE AREA FROM THE NORTHERN PLAINS 
LATER TONIGHT. THEN NAM MODEL IS HINTING AT SOME POSSIBLE IFR LOW 
CLOUD/FOG AFTER 28.09Z IN WEAK SOUTHERLY AIRFLOW AHEAD OF THE 
APPROACHING FRONT. HOWEVER...APPEARS MOST OF THIS LOWER LEVEL 
MOISTURE TRANSPORT/BETTER IFR POSSIBILITY REMAINS SOUTH OF THE AREA. 
WILL CONTINUE TO ASSESS THIS IFR POSSIBILITY FOR THE 27.12Z TAF 
ISSUANCE.
&&
.ARX WATCHES/WARNINGS/ADVISORIES...
WI...NONE.
MN...NONE.
IA...NONE.
&&
$$
SHORT/LONG TERM...AJ
AVIATION...DAS