894
FXUS63 KARX 072030
AFDARX
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LA CROSSE WI
229 PM CST SAT NOV 7 2009
SHORT TERM
TONIGHT THROUGH MONDAY NIGHT
MAIN FORECAST CONCERNS ARE LOW PRECIPITATION CHANCES FOR SUNDAY
THROUGH MONDAY NIGHT.
CURRENTLY AS OF 21Z...WATER VAPOR LOOP AND RUC 500MB HEIGHT ANALYSIS
SHOWED NEARLY ZONAL FLOW ACROSS THE WESTERN TWO THIRDS OF NORTH
AMERICA...THOUGH IT DOES APPEAR BROAD TROUGHING IS TRYING TO FORM IN
THE PACIFIC NORTHWEST. ALSO OF NOTE ARE A FEW SHORTWAVES IN NEVADA
AND ARIZONA RIDGING THROUGH THE FLOW. CLOSER TO HOME...MOSTLY SUNNY
SKIES PREVAIL WITH A DRY AIRMASS IN PLACE ACROSS MUCH OF THE
NORTHERN PLAINS AND UPPER MISSISSIPPI VALLEY. 12Z MPX AND ABR
SOUNDINGS BOTH SHOWED DEWPOINT DEPRESSIONS OF 15C OR GREATER BETWEEN
750-950MB. THESE SUNNY SKIES ARE ACCOMPANIED BY HIGH PRESSURE
BUILDING IN AT THE SURFACE BEHIND A PACIFIC COLD FRONT THAT CROSSED
EARLY THIS MORNING. THE CENTER OF THE HIGH IS OVER SOUTHWEST
MINNESOTA. DESPITE THE FRONTAL PASSAGE...SURFACE TEMPERATURES HAVE
RISEN WELL INTO THE 60S (EVEN 70 AT DECORAH AND PRAIRIE DU CHIEN)...
AIDED BY THE FACT THAT 850MB TEMPS DROPPED OFF SLOWLY BEHIND THE
FRONT. AT 12Z...THEY WERE STILL 15C AT MPX...COMPARED TO 6C AT ABR.
TONIGHT...UPPER LEVEL FLOW IS PROGGED TO TURN SOUTHWESTERLY AS THE
TROUGHING BECOMES A LITTLE DEEPER OVER THE NORTHERN ROCKIES. IN
ADDITION...ONE OF THE SHORTWAVES IN NEVADA IS PROGGED TO MOVE
ACROSS. HOWEVER...WITH A DRY AIRMASS IN PLACE AND ANTICYCLONIC FLOW
AT THE SURFACE...NO PRECIPITATION IS EXPECTED. THE SOUTHWEST FLOW
WILL BRING SOME HIGHER CLOUDS INTO THE AREA...THOUGH...WHICH CAN
ALREADY BE SEEN OVER WESTERN NEBRASKA. LOW TEMPERATURES TONIGHT WILL
DEFINITELY BE COOLER THAN LAST NIGHT DUE TO LESS WIND AND A DRIER
AIRMASS IN PLACE. NO REASONS EXIST TO STRAY MUCH FROM GUIDANCE.
SUNDAY INTO SUNDAY NIGHT...FOCUS OF ATTENTION TURNS TO THE COLD
FRONT THAT PASSED THROUGH THE AREA EARLIER THIS MORNING. TONIGHT...
THIS FRONT IS FORECAST TO RETURN NORTHWARD AS A WARM FRONT DUE TO
THE UPPER FLOW TURNING SOUTHWESTERLY...REACHING SOUTHERN IOWA AT 12Z
SUNDAY. AS THE TROUGHING IN THE NORTHERN ROCKIES LIFTS
EAST-NORTHEAST PER MODEL PROGS...AND THE SHORTWAVE IN ARIZONA MOVES
NORTHEAST THROUGH THE FLOW...THE WARM FRONT WILL CONTINUE LIFTING
NORTH. IN ADDITION...MODELS SHOW A SURFACE LOW TO ACCOMPANY THE
SHORTWAVE...WHICH WOULD MOVE ACROSS WINONA MN SUNDAY EVENING. THE
COLD FRONT ASSOCIATED WITH THE SURFACE LOW SHOULD CROSS MUCH OF THE
CWA BY 12Z MONDAY...WITH HIGH PRESSURE BUILDING IN BEHIND IT ACROSS
THE DAKOTAS. THE SYSTEM IMPACTS TO THE AREA IN THE PRECIPITATION
DEPARTMENT APPEAR TO BE MINIMAL...WITH PERHAPS A LITTLE LIGHT RAIN
OR DRIZZLE. THIS IS DUE TO SHALLOW MOISTURE AND WEAK FRONTOGENETICAL
FORCING. IF ANY PRECIPITATION FALLS...IT SHOULD BE CONFINED TO THE
SUNDAY AFTERNOON TO SUNDAY NIGHT TIME AND ONLY ACCUMULATE TO A FEW
HUNDREDTHS AT MOST. ALTHOUGH VERY LITTLE PRECIPITATION IS
EXPECTED...LOOK FOR A WIDESPREAD STRATUS DECK TO DEVELOP NORTH OF
THE WARM FRONT ON SUNDAY...DUE TO THE HIGHER DEWPOINTS ACROSS
OKLAHOMA AND MISSOURI MOVING INTO THE COOLER AIRMASS OVER THE AREA.
STRATUS WILL ALSO ACCOMPANY THE COLD FRONT...MORESO RELATED TO
FRONTOGENESIS ON TOP OF IT. SOME CLEARING MAY TRY TO ENCROACH INTO
THE FAR WESTERN PORTIONS OF THE FORECAST AREA TOWARDS 12Z MONDAY...
AS DRIER AIR ADVECTS IN ASSOCIATED WITH THE HIGH PRESSURE AREA. HIGH
TEMPERATURES ON SUNDAY ARE TRICKY... ESPECIALLY IN NORTHEAST IOWA
AND FAR SOUTHWEST WISCONSIN WHERE SOME PARTIAL CLEARING COULD OCCUR
IN THE AFTERNOON...AND THAT AREA IS FORECAST TO ENTER THE WARM
SECTOR. WENT WITH A BLEND OF GUIDANCE FOR NOW. MOST AREAS SHOULD END
UP WARMER SUNDAY NIGHT WITH CLOUDS AROUND...WITH LOWS IN THE MID 40S
TO LOW 50S.
MONDAY INTO MONDAY NIGHT...UPPER TROUGH COMING OUT THE NORTHERN
ROCKIES SUNDAY AND SUNDAY NIGHT IS CREATING SOME ISSUES AMONGST THE
MODELS...AND HAS IMPLICATIONS ON PRECIPITATION FOR PORTIONS OF THE
FORECAST AREA DURING THIS TIME FRAME. THE 07.12Z NAM/UKMET ARE BOTH
FAIRLY PROGRESSIVE...HAVING IT OVER NORTHERN ONTARIO BY 00Z TUESDAY.
THE 07.12Z ECMWF...AT THE SAME TIME...STILL HAS IT BACK OVER
NORTHERN MINNESOTA WHILE THE 07.12Z GFS IS EVEN FARTHER WEST OVER
EASTERN NORTH DAKOTA. HPC PREFERENCE PER THE PMDHMD IS A BLEND OF
THE GFS/ECMWF...WHICH WAS FOLLOWED FOR THIS FORECAST. FRONTOGENESIS
ZONE ALONG AND BEHIND THE COLD FRONT CROSSING THE AREA SUNDAY NIGHT
SHOULD BE SLOW TO MOVE TO THE SOUTHEAST DURING THE PERIOD...GIVEN
THE SLOWER UPPER TROUGH. AS A RESULT...HAVE HELD ONTO SOME 20-30
PERCENT CHANCES OF PRECIPITATION FOR BOTH MONDAY AND MONDAY NIGHT IN
NORTHEAST IOWA AND FAR SOUTHWEST WISCONSIN. OUTSIDE OF THIS AREA...
DRY ADVECTION FROM HIGH PRESSURE VERY SLOWLY BUILDING IN SHOULD
GRADUALLY HELP SCOUR OUT THE CLOUDS NORTHWEST TO SOUTHEAST. BY 12Z
TUESDAY...ENVISION MOSTLY CLEAR SKIES WILL ENCOMPASS MUCH OF THE
FORECAST AREA. HIGHS ON MONDAY LOOK TO STILL END UP MILD DESPITE THE
COLD FRONTAL PASSAGE...SINCE 850MB TEMPS AT 00Z TUESDAY ARE 4-6C.
THE CLEARING MONDAY NIGHT AND DIMINISHING WINDS WILL ALLOW READINGS
TO FALL INTO 30S...PERHAPS EVEN CLOSE TO 30 IN TAYLOR COUNTY AND FAR
WESTERN PORTIONS OF THE FORECAST AREA.
LONG TERM
TUESDAY THROUGH SATURDAY
MODEL/ENSEMBLE AGREEMENT ON THE UPPER LEVEL PATTERN ACROSS THE
COUNTRY DURING THE LONG TERM PORTION OF THE FORECAST IS REALLY GOOD
THROUGH ABOUT THURSDAY...THEN DROPS OFF FOR FRIDAY INTO THE WEEKEND.
UNTIL THURSDAY NIGHT...THE PATTERN SHOWN CONSISTS OF RIDGING
BUILDING INTO THE AREA ON TUESDAY IN THE WAKE OF MONDAY NIGHT\S
TROUGH...AND THEN THIS RIDGE FLATTENING OUT BUT HOLDING STRONG OVER
THE EASTERN TWO-THIRDS OF THE COUNTRY AS TROUGHING FORMS IN THE
WESTERN U.S.. THE IMPACT OF THIS PATTERN FOR THE FORECAST AREA IS
FOR DRY AND COOL CONDITIONS ON TUESDAY AS A SURFACE HIGH BUILDS
IN...THEN CONTINUED DRY CONDITIONS WITH GRADUAL WARMING THROUGH
THURSDAY AS THE HIGH PUSHES OFF TO THE EAST. ONE CAVEAT IS OVER
TAYLOR AND CLARK COUNTIES ON WEDNESDAY WHERE THE 07.00Z AND 07.12Z
ECMWF INDICATE SOME LIGHT PRECIPITATION. APPARENTLY THIS IS DUE TO
A ZONE OF ISENTROPIC LIFT AHEAD OF A SHORTWAVE TROUGH CROSSING
NORTHWEST ONTARIO. FOR NOW HAVE NOT INCLUDED ANY PRECIPITATION
CHANCES...GIVEN THE DRY 07.00Z/07.12Z CANADIAN AND 07.06Z/07.12Z
GFS...BUT IT IS SOMETHING TO WATCH.
FOR THURSDAY NIGHT INTO SATURDAY...HAVE DISCOUNTED THE 07.06Z GFS AS
AN OUTLIER AND FOLLOWED CLOSER TO THE 07.00Z ECMWF...WITH ITS
SUPPORT FROM THE 07.00Z GFS ENSEMBLES AND 06.12Z ECMWF ENSEMBLES.
THE 07.12Z GFS CAME IN LOOKING CLOSER TO THE 07.00Z ECMWF...
SUPPORTING THE FORECAST PLAN. HOWEVER...IT SHOULD BE NOTED THAT
THE THE ECMWF ENSEMBLES DO SHOW QUITE A BIT OF SPREAD ON SATURDAY SO
CONFIDENCE NEXT WEEKEND IS LOW. IN ANY EVENT...THE IDEA FOR THE
UPPER LEVEL PATTERN IS FOR THE WESTERN TROUGH TO GRADUALLY PUSH EAST
AND EJECT OUT...WHILE SIMULTANEOUSLY ANOTHER SHORTWAVE TROUGH DIGS
ACROSS SOUTHEAST CANADA. A FRONTAL BOUNDARY ASSOCIATED WITH THIS
NORTHERN STREAM SHORTWAVE IS PROGGED TO SLOWLY CROSS THE AREA FRIDAY
AND SATURDAY. THE SLOWNESS IS MOSTLY DUE TO THE BOUNDARY PARALLEL TO
THE UPPER FLOW. AHEAD OF THE BOUNDARY...PLAN ON MILD AND DRY
CONDITIONS WITH LACK OF FORCING AND 850MB TEMPS RUNNING 6-8C. BEHIND
THE BOUNDARY... FRONTOGENESIS WILL HELP DEVELOP CLOUDS AND
PRECIPITATION...KEEPING TEMPERATURES COOLER. AT THIS TIME...ONLY
FORECASTING LOW PROBABILITIES OF PRECIPITATION BEHIND THE BOUNDARY
GIVEN THE UNCERTAIN PATTERN. NEW 07.12Z ECMWF IS A BIT FASTER WITH
THE FEATURES FOR THIS TIME PERIOD...REFLECTING THE UNCERTAINTY.
AVIATION
REST OF THIS AFTERNOON THROUGH SUNDAY
NO APPRECIABLE AVIATION CONCERNS THROUGH 18Z SUNDAY...WITH ONLY SOME
HIGH CLOUDS MOVING IN DURING THE OVERNIGHT HOURS...AHEAD OF AN AREA
OF LOW PRESSURE APPROACHING FROM THE PLAIN STATES. EXPECT VFR
CONDITIONS THROUGH 18Z.
THE 07.12Z GFS/NAM WANT TO BRING IN A CONSIDERABLE AMOUNT OF LOW
LEVEL SATURATION BY 18Z SUN...WHICH WOULD LINGER OVER THE TAF SITES
THROUGH 12Z MON. THE IMPLICATIONS WOULD BE MVFR/IFR CEILINGS...WITH
POSSIBLY 3-5 SM BR. THIS WOULD BE ASSOCIATED WITH THE LOW SHIFTING
NORTHEAST ACROSS THE UPPER MISSISSIPPI RIVER VALLEY...AND A SLOPING
WARM FRONT. CONFIDENCE A LITTLE SHAKY ON THE MODELS/ ROBUST LOW
LEVEL RH...AS THEY DID POORLY WITH THIS A DAY AGO. HOWEVER...THERE
WILL BE DYNAMICS AND A MORE FAVORABLE SETUP FOR LOW STRATUS
DEVELOPMENT IN THIS STEAD...AND THAT ADDS SOME CREDIBILITY TO THE
MVFR/IFR POTENTIAL. LATER TAF ISSUANCES WILL HAVE TO ADDRESS THIS.
AS FOR PCPN CHANCES...THE DEEPER MOISTURE...THERMODYNAMICS AND UPPER
LEVEL ENERGY WILL BE ACROSS NORTHERN SECTIONS OF MN/WI...AND THIS IS
WHERE THE BETTER LIGHT RA- CHANCES WILL LIE. IF THE LOW STRATUS
SHOULD DEVELOP OVER KRST/KLSE SUNDAY AFTERNOON/EVENING...THERE
APPEARS TO BE ENOUGH ISENTROPIC UPGLIDE THROUGH THIS LAYER THAT
DRIZZLE WOULD BE POSSIBLE.
ARX WATCHES/WARNINGS/ADVISORIES
WI...NONE.
MN...NONE.
IA...NONE.
SHORT/LONG TERM...AJ
AVIATION..........REICK
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