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
