RSS FeedsIssued by NWS La Crosse, WI
000 FXUS63 KARX 212110 AFDARX AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE LA CROSSE WI 310 PM CST SAT NOV 21 2009 .SHORT TERM...TONIGHT THROUGH MONDAY NIGHT MAIN FORECAST CONCERN IS ON CHANCES FOR LIGHT PRECIPITATION CHANCES THROUGH THE PERIOD. CURRENTLY AS OF 21Z...WATER VAPOR LOOP AND RUC 500MB HEIGHT ANALYSIS SHOWED RIDGING ACROSS THE UPPER MISSISSIPPI VALLEY AND GREAT LAKES...TROUGHING FROM ALBERTA SOUTH INTO UTAH...AND A MORE POTENT TROUGH JUST SOUTH OF THE GULF OF ALASKA. SUBSIDENCE UNDER THE UPPER RIDGE HAS KEPT MUCH OF THE AREA EAST OF THE ROCKIES AND SAY NORTH OF I-40 PRECIPITATION FREE TODAY. HOWEVER...NUMEROUS ISSUES EXIST THE CLOUD DEPARTMENT...WHERE A ZONE OF TWO DIFFERENT STRATUS LAYERS CAN BE SEEN OVER THE CENTRAL PLAINS AND OVER THE FORECAST AREA. BOTH AREAS HAVE MOSTLY EVOLVED FROM A COMBINATION OF MIXING OUT MORNING FOG...AND THEN THAT MOISTURE STAYING TRAPPED UNDER THE SUBSIDENCE INVERSION CAUSED BY THE UPPER RIDGE. THE CLOUDS OVER THE FORECAST AREA HAVE HELD TEMPERATURES DOWN...WITH READINGS MOSTLY IN THE MID TO UPPER 40S. RETURN LOW LEVEL FLOW...BETWEEN HIGH PRESSURE OVER THE OHIO VALLEY AND A LEE TROUGH IS BRINGING 40S DEWPOINTS NORTHWARD THROUGH THE PLAINS. TONIGHT...UPPER TROUGH OVER THE INTERMOUNTAIN WEST IS PROGGED TO PUSH EAST INTO THE WESTERN PLAINS...CAUSING UPPER LEVEL FLOW TO TURN SOUTHWESTERLY OVER THE AREA. IN RESPONSE IN THE LOW LEVELS... SOUTHERLY FLOW SHOULD INCREASE...AS THE LEE TROUGH/COLD FRONT MOVES INTO THE EASTERN DAKOTAS. THIS INCREASING SOUTHERLY FLOW WILL HELP BRING THE CLOUDS/MOISTURE OVER THE CENTRAL PLAINS INTO THE FORECAST AREA. PAST AND CURRENT GFS/ECMWF RUNS CONTINUE TO SHOW DEVELOPMENT OF LIGHT QPF ACROSS THE UPPER MISSISSIPPI VALLEY REGION...ESPECIALLY AFTER 06Z. HOWEVER...PAST AND CURRENT NAM RUNS REMAIN DRY. LOOKING AT GFS SOUNDINGS...THE LIGHT QPF LOOKS TO BE A RESULT OF DRIZZLE... SINCE SATURATION AND LIFT IS CONFINED TO AN AREA AT OR BELOW 5000 FT. PREVIOUS FORECAST WAS DRY...AND WILL CONTINUE THAT IDEA BUT MAINTAIN A MENTION OF PATCHY DRIZZLE. ONLY AREA THAT MAY NOT SEE DRIZZLE IS FROM JUNEAU/ADAMS COUNTIES SOUTHWEST INTO GRANT COUNTY... WHERE ENOUGH DRIER AIR INFLUENCE FROM CENTRAL ILLINOIS COMING THROUGH THE RETURN FLOW WILL PREVENT PRECIPITATION. ANOTHER ISSUE TO DEAL WITH TONIGHT IS THE MOISTURE COMING UP FROM THE SOUTH...RUNNING INTO A COOLER AIRMASS OVER THE FORECAST AREA. THIS MAY RESULT IN SOME FOG DEVELOPMENT...ESPECIALLY OVER THE RIDGES OF SOUTHEAST MINNESOTA AND NORTHEAST IOWA. HAVE MAINTAINED THE GOING AREAS OF DENSE FOG...BUT DID NOT ISSUE AN ADVISORY AT THIS TIME DUE TO UNCERTAINTY OF THE FOG BEING WIDESPREAD ENOUGH TO WARRANT THE ADVISORY. REGARDING LOW TEMPERATURES...WITH THE HIGHER DEWPOINTS MOVING INTO THE AREA FROM THE SOUTH...LOWS WILL LIKELY BE THIS EVENING WITH RISING TEMPERATURES AFTERWORDS. ONE CAVEAT TO WATCH FOR TONIGHT...WITH THE WINDS INCREASING... DEVELOPING LOW LEVEL SATURATION MAY BE TRICKY AS TEMPERATURES MAY NOT WANT TO FALL. IF THIS OCCURS...THEN ALL WE COULD END UP WITH IS SOME PASSING STRATUS CLOUDS WITH BASES AT 3000 FT OR SO...AS SEEN OVER EASTERN NEBRASKA AND NORTHEAST KANSAS. FUTURE SHIFTS WILL NEED TO MONITOR CLOUD TRENDS AND DEWPOINT DEPRESSIONS. SUNDAY...UPPER TROUGH IN THE WESTERN PLAINS AT 12Z WILL CONTINUE ITS MARCH EASTWARD...AIDED BY THE POTENT TROUGH CURRENTLY SOUTH OF THE GULF OF ALASKA DIGGING INTO IDAHO AND NORTHERN UTAH. ALTHOUGH THE UPPER TROUGH IS APPROACHING THE AREA...THE ASSOCIATED COLD FRONT AHEAD OF IT IS PROGGED TO FALL APART OVER WESTERN MINNESOTA... POSSIBLY DUE TO NEW LOW PRESSURE FORMING IN WYOMING. IN ANY EVENT...SOUTHERLY FLOW IS PROGGED TO CONTINUE OVER THE AREA... BRINGING MORE MOISTURE UP FROM THE SOUTH. SAME SCENARIO EXISTS AGAIN WITH THE GFS/ECMWF RUNS PRODUCING PRECIPITATION WHILE THE NAM IS DRY. SOUNDINGS FROM THE GFS OR NAM ARE NOT THAT IMPRESSIVE...WITH LITTLE LIFT AND HAVE CEILINGS CLIMBING THROUGH THE DAY TO HELP PREVENT DRIZZLE. ISENTROPIC LIFT PROGS ARE FAIRLY BENIGN TOO WITH LITTLE IF ANY LIFT. TEMPTED TO DRY OUT THE FORECAST...BUT WITH THE CONSISTENT SIGNAL BY THE GFS/ECMWF...HAVE MAINTAINED THE LOW PROBABILITIES OF PRECIPITATION. ANY FOG FROM TONIGHT SHOULD DISSIPATE BY NOON WITH HEATING AND WARM ADVECTION. REGARDING THAT WARM ADVECTION...TEMPERATURES SHOULD BE ABLE TO CLIMB INTO THE 50S IN MOST LOCATIONS. SUNDAY NIGHT AND MONDAY....UPPER TROUGH THAT WAS MARCHING EASTWARD TOWARDS THE FORECAST AREA ON SUNDAY SHOULD MOVE THROUGH MONDAY MORNING...WHILE THE POTENT TROUGH DIGS DOWN INTO NEBRASKA AND KANSAS. ALTHOUGH THE UPPER TROUGH IS MOVING THROUGH THE AREA...THE TROUGH IS MUCH WEAKER THAN WHAT IT LOOKS LIKE ON WATER VAPOR...DUE TO HEIGHTS TRYING TO RISE AHEAD OF THE POTENT TROUGH. THEREFORE... THE OVERALL DPVA FORCING IS QUITE WEAK OVER THE FORECAST AREA...AS WELL AS THE ISENTROPIC LIFT. IF ANYWHERE COULD SEE PRECIPITATION DURING THIS PERIOD...IT LOOKS LIKE NORTHEAST IOWA...SOUTHEAST MINNESOTA AND INTO CENTRAL WISCONSIN WHERE MOST OF THE WEAK FORCING IS LOCATED. ELSEWHERE...THIS PERIOD LOOKS MAINLY CLOUDY...THOUGH WOULD NOT BE SURPRISED TO SEE SOME CLEARING OF THE CLOUDS FROM GRANT COUNTY NORTHEAST INTO ADAMS. THIS CLEARING IS IN RESPONSE TO A WEDGE OF DRY AIR IN THE LOW TO MID LEVELS THAT ADVECTS IN ON SOUTHEAST WINDS. WITH PLENTY OF CLOUDS AND RELATIVELY HIGH DEWPOINTS FOR LATE NOVEMBER...TEMPERATURES SHOULD REMAIN ON THE MILD SIDE AT NIGHT AND NOT RISE A WHOLE DURING THE DAY MONDAY. PREVIOUS FORECAST LOWS SEEMED REASONABLE. MONDAY NIGHT...TROUGH OVER NEBRASKA AND KANSAS IS PROGGED TO SLOWLY SHIFT EAST...REACHING A POSITION NEAR KANSAS CITY BY 12Z TUESDAY. HEIGHT FALLS...ISENTROPIC LIFT AND AN OPEN GULF OF MEXICO FEED OF MOISTURE AHEAD OF IT WILL ALLOW FOR THE FORMATION OF BOTH WARM AND COLD CONVEYOR BELTS WITH THE UPPER LOW. MODELS PROG THAT THE WARM CONVEYOR BELT MAY GET INTO NORTHEAST IOWA AND SOUTHEAST MINNESOTA...AND HAVE RAISED CHANCES OF PRECIPITATION UP IN THIS AREA. FARTHER NORTH AND EAST...HAVE MAINTAINED GOING LOW CHANCES OF PRECIPITATION...MOSTLY DUE TO CONCERNS THAT THE UPPER LOW COULD PROGRESS A LITTLE BIT FASTER AS THE 21.12Z GFS SHOWS. IMAGINE OVER TIME THE CHANCES OF PRECIPITATION CAN BE REFINED SUCH THAT LESS AREA IS AFFECTED...THOUGH. ALONG WITH THE INCOMING WARM CONVEYOR BELT SHOULD BE A MASS OF AT LEAST HIGH AND MID CLOUDS...IF NOT LOW CLOUDS TOO. THEREFORE...ANOTHER CLOUDY TIME PERIOD IS EXPECTED...WHICH SHOULD HELP KEEP LOW TEMPERATURES WELL ABOVE NORMAL. .LONG TERM...TUESDAY THROUGH SATURDAY FOCUS OF THIS PERIOD REMAINS ON THE SYSTEM FORECAST IN THE CENTRAL U.S. TUESDAY MORNING AND ITS IMPACTS INTO THE THANKSGIVING HOLIDAY. PROBLEM IS...MODELS ARE YET TO COME TO FULL AGREEMENT ON THE TIMING OF THE SYSTEM. THE 21.06Z GFS AND 21.00Z CANADIAN...ALONG WITH THEIR RESPECTIVE ENSEMBLES...ALL MOVING THE SYSTEM ALONG FAIRLY QUICKLY. BY 12Z WEDNESDAY...THEY SHOW THE SYSTEM IN LOWER MICHIGAN. OF NOTE IS THAT THE CANADIAN...WHICH A FEW DAYS AGO WAS ONE OF THE SLOWEST...HAS NOW TURNED TO THE FASTEST SCENARIO. THE 21.00Z ECMWF AS WELL AS THE 20.12Z ECMWF ENSEMBLES...ARE THE SLOWEST...REACHING ONLY THE CHICAGO AREA AROUND 00Z THURSDAY. THE TIMING OF THIS SYSTEM IS CRITICAL TOO...SINCE ALL MODELS SHOW A FAIRLY POTENT SHORTWAVE TROUGH IN THE NORTHERN STREAM FLOW DROPPING DOWN INTO THE UPPER MISSISSIPPI VALLEY LATE WEDNESDAY. THIS TROUGH WILL HELP BRING COLDER AIR IN...AND IF THE SYSTEM IS STILL LINGERING AROUND LIKE THE 21.00Z ECMWF SHOWS...A BETTER CHANCE EXISTS FOR AN ACCUMULATING SNOW. FOR NOW...THE PREFERRED OPTION IS TO STAY CLOSER TO THE 21.00Z ECMWF...AS PER HPC PROGS. INTERESTING THAT THE 21.12Z ECMWF HAS COME IN FASTER. THEREFORE...WITH THE WIDE MODEL VARIETY...THE TUESDAY THROUGH THURSDAY TIME FRAME IS DEFINITELY SUBJECT TO CHANGE. ENOUGH CONFIDENCE EXISTS TO RAISE CHANCES OF PRECIPITATION TO 60-70 ON TUESDAY...DESPITE THE TIMING DIFFERENCES...AS MAJORITY OF MODELS SHOW THE AREA AFFECTED BY A WARM CONVEYOR BELT. AFTER TUESDAY...HAVE KEPT THE PRECIPITATION CHANCES NO HIGHER THAN 50 UNTIL CERTAINTY INCREASES. WILL START CHANGING THE PRECIPITATION TYPE OVER TO SNOW TOWARDS WEDNESDAY NIGHT...THOUGH...AS THAT NORTHERN STREAM TROUGH DROPS IN. HAVE EXTENDED CHANCES OF PRECIPITATION INTO THANKSGIVING AS WELL...BUT ONLY LOW PROBABILITIES AT THIS POINT. FOR THURSDAY NIGHT THROUGH SATURDAY...MODELS AND ENSEMBLES DEPICT RIDGING OVER THE WESTERN U.S. PUSHING EAST INTO THE UPPER MISSISSIPPI VALLEY...ALBEIT FLATTENING OUT. THIS RIDGE IS BEING PUSHED EAST BY A TROUGH MOVING INTO THE WESTERN U.S.. LOOKS OF THIS TROUGH APPEARS TO BE ONE OF A MORE PROGRESSIVE NATURE...ESPECIALLY GIVEN THE FLATTENING RIDGE AHEAD OF IT. HOWEVER...IT APPEARS ANY PRECIPITATION ASSOCIATED WITH IT SHOULD NOT ARRIVE UNTIL AFTER SATURDAY. THEREFORE...THE FORECAST FOR THIS TIME FRAME WILL STAY DRY. LOOKING FOR A COOL THURSDAY NIGHT AS A SURFACE RIDGE BUILDS IN...AND COULD BE EVEN COOLER IF SNOW COVER EXISTS. GRADUAL WARMING IS THEN PLANNED FOR FRIDAY INTO SATURDAY AS RETURN FLOW SETS UP IN THE WAKE OF THE RIDGE. IT SHOULD BE NOTED THAT A COLD FRONTAL BOUNDARY MAY BE ENTERING THE PICTURE FOR SATURDAY...AHEAD OF THE WESTERN TROUGH PUSHING EASTWARD. && .AVIATION...TONIGHT AND SUNDAY NOT A HIGH CONFIDENCE FORECAST FOR THE NEXT 24 HOURS WHICH BEGINS WITH POORLY MODELED STRATUS ACROSS THE AREA THIS AFTERNOON. RUC HAS BEEN THE ONLY MODEL EVEN CLOSE TO INDICATE THIS LOW LEVEL MOISTURE...HOWEVER IT ALSO HAS NOT HAD A REAL GOOD HANDLE ON IT. VISIBILITY RESTRICTIONS CONTINUE ACROSS MUCH OF THE AREA TODAY...HOWEVER FINALLY SEEING IMPROVEMENTS EARLY THIS AFTERNOON. WINDS HAVE ALSO BEGUN TO PICK UP A BIT WITH TIGHTENING PRESSURE GRADIENT THIS AFTERNOON WHICH SHOULD CONTINUE THROUGH TONIGHT. ANOTHER AREA OF FOG/STRATUS CURRENTLY OVER EASTERN NEBRASKA/KANSAS AND MISSOURI WILL PUSH NORTHEAST TOWARDS THE AREA FOR TONIGHT AND SUNDAY MORNING. THIS SIGNAL HAS BEEN PRETTY CONSISTENT OVER THE PAST FEW DAYS WITH BRINGING INCREASED MOISTURE INTO LOCATIONS MAINLY WEST OF THE MISSISSIPPI RIVER. WITH CLOUDS INCREASING AND WINDS STAYING UP...RATHER THAN RADIATION FOG DEVELOPMENT...THIS LOOKS TO BE AN ADVECTION FOG EPISODE AFFECTING MORE OF THE HIGHER TERRAIN AREAS WEST OF THE MISSISSIPPI RIVER...WITH STRATUS ELSEWHERE. STRATUS/FOG LOOKS TO PUSH INTO THE KRST/KLSE AROUND 22.03-04Z. HELD OFF ON SIGNIFICANTLY LOWERING VISIBILITIES OVER KRST UNTIL AFTER MIDNIGHT WHEN T/TD SPREAD IS LOWEST. DID LOWER VISIBILITIES AT KLSE...HOWEVER KEPT CEILINGS/VISIBILITIES IN MVFR CATEGORY. && .ARX WATCHES/WARNINGS/ADVISORIES... WI...NONE. MN...NONE. IA...NONE. && $$ SHORT/LONG TERM...AJ AVIATION...JLB
