000 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