000 FXUS63 KARX 232108 AFDARX AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE LA CROSSE WI 308 PM CST MON NOV 23 2009 .SHORT TERM...TONIGHT THROUGH WEDNESDAY NIGHT MAIN FORECAST CONCERN IS ON SYSTEM TO AFFECT THE AREA TUESDAY INTO THANKSGIVING. CURRENTLY AS OF 21Z...WATER VAPOR AND RUC 500MB HEIGHT ANALYSIS SHOWED TROUGHING ALONG THE LEE OF THE U.S. ROCKIES...RIDGING AHEAD OF IT OVER THE GREAT LAKES AND RIDGING BEHIND IT OVER THE PACIFIC NORTHWEST...AND A POTENT TROUGH MOVING INTO BRITISH COLUMBIA. ALSO OF NOTE IS A VERY WEAK SHORTWAVE TROUGH OVER THE FORECAST AREA... WHICH WHEN COMBINED WITH LOW LEVEL ISENTROPIC LIFT HELPED PRODUCE THE RAIN OBSERVED LAST NIGHT INTO THIS MORNING. SUBSIDENCE FOLLOWING IN THE WAKE OF THE TROUGH AND LACK OF ISENTROPIC LIFT HAS DIMINISHED THE RAIN...AS WELL AS CLEARED OUT SOME OF THE CLOUDS WEST OF A LINE FROM EAU CLAIRE TO DUBUQUE. ALONG AND EAST OF THIS LINE...A LARGE AMOUNT OF VERY LOW STRATUS EXISTS. A LOT OF THIS RESULTED FROM FOG WHICH MIXED OUT...BUT THE MOISTURE GOT TRAPPED UNDER A SUBSIDENCE INVERSION...SIMILAR TO SATURDAY. 12Z GRB SOUNDING REFLECTS THIS WELL. FARTHER TO THE WEST...DESPITE THE TROUGH PRESENT OVER THE WESTERN PLAINS...VERY LITTLE PRECIPITATION ACTIVITY IS OCCURRING. HOWEVER...LOW PRESSURE HAS FORMED AHEAD OF THE TROUGH FROM EASTERN SOUTH DAKOTA INTO THE OKLAHOMA PANHANDLE...WITH THE MAIN LOW IN THE OKLAHOMA PANHANDLE. TONIGHT...UPPER TROUGH OVER THE WESTERN PLAINS IS EXPECTED TO DIG DOWN INTO KANSAS AND OKLAHOMA...WITH A CLOSED LOW FORMING WITHIN IT NEAR KANSAS CITY BY 12Z TUESDAY. AHEAD OF THE UPPER TROUGH...RIDGING WILL CONTINUE TO BUILD OVER THE FORECAST AREA...MAINTAINING SUBSIDENCE. HOWEVER...AFTER 06Z...23.12Z GFS/NAM BOTH DEPICT ISENTROPIC LIFT INCREASING ON THE 290K AND 295K SURFACES OVER SOUTHEAST MINNESOTA AND NORTHEAST IOWA. THIS LIFT IS AHEAD OF THE LOW PRESSURE TROUGH IN THE EASTERN DAKOTAS...WHICH MODELS PROG REACHING I-35 BY 12Z TUESDAY. IN ADDITION...AN UPPER LEVEL JET STREAK CURRENTLY OVER THE OKLAHOMA PANHANDLE IS INDICATED TO MOVE NORTHEAST INTO EASTERN IOWA BY 12Z...IN RESPONSE TO THE DIGGING UPPER TROUGH. THEREFORE AT LEAST NORTHEAST IOWA COULD GET INTO SOME UPPER LEVEL DIVERGENCE IN THE LEFT EXIT REGION OF THE GET. NET RESULT OF ALL THIS IS TO MAINTAIN AT LEAST THE GOING 20-40 PERCENT CHANCES OF RAIN AFTER 06Z. DO BELIEVE THAT THE BULK OF RAIN WILL END UP WEST OF THE FORECAST AREA...THOUGH...CLOSER TO THE LOW PRESSURE TROUGH WHERE MORE LOW TO MID-LEVEL FRONTOGENESIS EXISTS. OTHER CONCERN TONIGHT IS THE LOW STRATUS DECK HANGING AROUND OVER THE EASTERN TWO THIRDS OF WISCONSIN. WITH LOW LEVEL SOUTHEAST FLOW... NIGHT TIME COOLING AND HIGH DEWPOINTS...THE STRATUS MAY JUST ADVECT WESTWARD THIS EVENING. THEREFORE...FORECAST TONIGHT IS MOSTLY CLOUDY TO CLOUDY. HAVE ALSO INCLUDED PATCHY FOG UNDERNEATH THE LOW STRATUS DECK...GIVEN THAT MOST SITES UNDERNEATH IT RIGHT NOW HAVE 1 TO 3 MILE VISIBILITIES. IF STRATUS DOES INDEED SPREAD WESTWARD THIS EVENING...THE PATCHY FOG MAY NEED EXPANSION WESTWARD. THE CLOUDS AND HIGH DEWPOINTS SHOULD HELP HOLD TEMPERATURES UP...WITH LOWS IN MOST LOCATIONS IN THE LOW TO MID 40S. TUESDAY...UPPER LOW NEAR KANSAS CITY...AS WELL AS ITS ASSOCIATED LOW PRESSURE AREA WHICH SHOULD BE JUST NORTH OF THE LOW...IS PROGGED TO HEAD NORTHEAST TO A POSITION NEAR KIRKSVILLE MISSOURI BY 00Z. THE PUSH NORTHEASTWARD IS CAUSED BY THE CURRENT POTENT TROUGH ENTERING BRITISH COLUMBIA TRYING TO DIG DOWN INTO NORTHEAST MONTANA AND WESTERN NORTH DAKOTA. AS THE UPPER AND SURFACE LOWS MOVE NORTHEASTWARD...ISENTROPIC LIFT AND FRONTOGENESIS FORCING INCREASES IN A ZONE ABOUT 100 MILES NORTH OF THE UPPER LOW...I.E. WHERE THE WARM CONVEYOR BELT IS PROGGED. MANY QUESTIONS EXIST NOW IF THIS ZONE WILL MAKE IT UP INTO THE FORECAST AREA WITH THE 23.12Z MODEL CYCLE. MOST MODELS SHOW THE ZONE EITHER MOVING INTO THE NORTHEAST IOWA AND FAR SOUTHWEST WISCONSIN COUNTIES...OR JUST STAYING TO THE SOUTH. AS FOR THE LOW PRESSURE TROUGH NEAR I-35 IN THE MORNING AND ITS ASSOCIATED LOW TO MID LEVEL FRONTOGENESIS...MODELS BASICALLY SHOW THIS STAYING STATIONARY. WITH ALL OF THIS INFORMATION...DECIDED TO REDUCE CHANCES OF PRECIPITATION FROM CENTRAL WISCONSIN INTO THE SOUTHEAST MINNESOTA COUNTIES BORDERING THE MISSISSIPPI RIVER...AS WELL AS THE FIRST TIER OF NORTHEAST IOWA COUNTIES BORDERING MINNESOTA. FURTHER LOWERING OF PRECIPITATION CHANCES MAY BE NECESSARY...AS THE NEW 23.18Z NAM INDICATES. ALTHOUGH PRECIPITATION MAY NOT BE WIDESPREAD...CLOUD COVER DOES...LIMITING DAYTIME HEATING. IN FACT...THERE MAY NOT BE MUCH RISE OFF TONIGHT\S LOWS...ESPECIALLY WHERE IT PRECIPITATES. TUESDAY NIGHT AND WEDNESDAY...POTENT UPPER TROUGH DROPPING DOWN INTO THE FAR NORTHERN PLAINS AT 00Z WEDNESDAY IS FORECAST BY MODELS TO DIVE SOUTH THROUGH THE DAKOTAS TUESDAY NIGHT...THEN MOVE INTO IOWA BY 00Z THURSDAY. THE DIGGING NATURE OF THIS TROUGH WILL HELP KICK THE UPPER LOW NEAR KIRKSVILLE AT 00Z NORTHEASTWARD QUICKLY...AND MODELS SHOW IT REACHING THE EASTERN GREAT LAKES WEDNESDAY AFTERNOON. HOWEVER...ITS SURFACE LOW IS GOING TO HANG UP OVER UPPER MICHIGAN IN RESPONSE TO THE DIGGING TROUGH. AS FOR THE IMPACTS OF THE UPPER LOW ON THE FORECAST AREA...SHOULD SEE THE DEFORMATION ZONE OF RAIN ON THE NORTH SIDE OF THE LOW MOVE ACROSS NORTHEAST IOWA...WESTERN WISCONSIN AND POSSIBLY SOUTHEAST MINNESOTA TUESDAY NIGHT. SOUTHEAST MINNESOTA AND NORTHEAST IOWA COULD ALSO BE AFFECTED BY THE DIMINISHING FRONTOGENESIS ZONE NEAR I-35...WHICH SHOULD SLIDE SLOWLY EAST. OVERALL THE PREVIOUS FORECAST HANDLED THIS WELL...HAVING 60-70 PRECIPITATION PROBABILITIES. MAJORITY OF THE RAIN MAY TEND TO DIMINISH FOR A TIME WEDNESDAY MORNING...UNTIL THE POTENT UPPER TROUGH NEARS THE AREA...CAUSING MID LEVEL HEIGHTS TO FALL AND THE DEVELOPMENT OF A NEW BAND OF FRONTOGENETICAL PRECIPITATION. THIS AREA OF PRECIPITATION LOOKS TO MOVE INTO MOST OF THE FORECAST AREA LATE IN THE MORNING AND THEN REMAIN THROUGH THE REST OF THE DAY. AGAIN...THE PREVIOUS FORECAST HAD A GOOD HANDLE WITH THE PRECIPITATION PROBABILITIES...GENERALLY AROUND 60 PERCENT. MOST OF THE PRECIPITATION LOOKS TO BE LIGHT...UNLESS THE 23.12Z ECMWF IS CORRECT...AND THEREFORE WAS NOT CONFIDENT ENOUGH TO ADJUST THE PROBABILITIES. PRECIPITATION TYPE DOES BECOME A CONCERN AS THE TROUGH MOVES CLOSER...RESULTING FROM 850MB TEMPS FALLING TO -2C IN CENTRAL WISCONSIN TO -6C IN NORTHEAST IOWA. THIS COLD AIR ADVECTION SHOULD ALLOW RAIN TO SLOWLY CHANGE TO SNOW...ESPECIALLY IN NORTHEAST IOWA AND SOUTHWEST MINNESOTA...CLOSER TO THE CORE OF THE UPPER TROUGH. THE COLD AIR ADVECTION MAY NOT BE STRONG ENOUGH TO IMPACT THE DIURNAL CURVE...AS NOTED IN THE NAM/GFS. NONETHELESS...BOTH LOWS ON TUESDAY NIGHT AND HIGHS ON WEDNESDAY ARE EXPECTED TO BE COOLER THAN AS OF LATE. WEDNESDAY NIGHT...BEST SHOT OF ACCUMULATING SNOW EXISTS DURING THIS PERIOD...THOUGH THERE IS TOO MUCH UNCERTAINTY AS INDICATED BY QPF FIELDS TO SPECULATE ON HOW MUCH SNOW. MODELS SHOW THE UPPER TROUGH OVER IOWA HEADING EAST INTO ILLINOIS...WITH A MID-LEVEL FRONTOGENESIS ZONE ON ITS NORTH AND NORTHWEST SIDE MOVING THROUGH THE FORECAST AREA. NIGHT-TIME AND DYNAMIC COOLING SHOULD HELP MANY AREAS SWITCH OVER TO SNOW...ESPECIALLY WEST OF THE MISSISSIPPI RIVER. RAIN OR A RAIN/SNOW MIX COULD LINGER EAST OF THE RIVER... THOUGH...DUE TO WARMER AIR WRAPPING AROUND THE LOW IN UPPER MICHIGAN INTO THE FORECAST AREA. STAY TUNED TO LATER FORECASTS FOR AMOUNTS... BUT RIGHT NOW IT SEEMS LIKE THEY SHOULD BE NO HIGHER THAN ADVISORY CRITERIA. IN THE CASE OF THE 23.12Z NAM/GFS...THERE MAY NOT BE MUCH SNOW AT ALL. ONLY THE 23.12Z ECMWF WOULD SUGGEST A POSSIBLE NEED FOR AN ADVISORY. .LONG TERM...THANKSGIVING THROUGH MONDAY MODELS AND ENSEMBLES ARE IN FAIRLY GOOD AGREEMENT AT THE BEGINNING OF THE PERIOD...SAY THROUGH FRIDAY...THEN THE DETERMINISTIC MODELS (23.00Z ECMWF...23.06Z GFS AND 23.00Z CANADIAN) BEGIN TO DIVERGE. THEREFORE CONFIDENCE DECREASES FOR THE WEEKEND INTO EARLY NEXT WEEK...THOUGH THE 22.12Z ECMWF ENSEMBLES AND 23.00Z CANADIAN ENSEMBLES PROVIDE QUITE A BIT OF HELP. THE PATTERN STARTS OFF WITH TROUGHING OVER MOST OF THE MISSISSIPPI VALLEY THANKSGIVING MORNING...WHICH PROGRESSES TO THE EAST COAST BY LATE FRIDAY IN RESPONSE TO RIDGING MOVING INTO THE NORTHERN PLAINS. ALSO LATE ON FRIDAY...THERE SHOULD BE SOME TROUGH IN THE WESTERN U.S....BUT ITS DEPTH AT THAT TIME HAS A LOT OF SPREAD. THE 23.00Z CANADIAN SEEMS TOO DEEP AND FAST AT THAT TIME COMPARED TO ENSEMBLES...AND THEREFORE HAS BEEN DISCOUNTED. HOWEVER...ECMWF...GFS AND CANADIAN ENSEMBLES DO SHOW THAT THE TROUGH IN THE WEST WILL LIKELY DEEPEN OVER THE WEEKEND...AND COULD SPLIT INTO TWO. THIS APPEARS TO BE THE FAVORED SCENARIO...WHICH ALSO THE 23.00Z ECMWF (AS WELL AS IT PREVIOUS RUNS) INDICATES. THEREFORE FOR OUR AREA...INTEREST IS ON THE NORTHERN PORTION OF THE TROUGH...WHICH LOOKS TO CROSS THE UPPER MISSISSIPPI VALLEY SUNDAY NIGHT. RIDGING IS THEN EXPECTED TO BUILD ACROSS THE NORTHERN ROCKIES ON MONDAY. REGARDING THE DETAILS...SOME LINGERING DEFORMATION PRECIPITATION IS EXPECTED ON THANKSGIVING...ESPECIALLY EAST OF THE RIVER. WET-BULB ZERO HEIGHTS SUGGESTS MOST OF THE PRECIPITATION IN THE FORM OF SNOW...THOUGH COULD NOT COMPLETELY RULE OUT SOME RAIN MIXING IN EAST OF THE RIVER. CONFIDENCE IS TOO LOW TO INCLUDE RAIN...THOUGH. CONDITIONS SHOULD DRY OUT FOR THURSDAY NIGHT...BUT CONCERN STILL REMAINS THAT THE DEFORMATION PRECIPITATION COULD BE AROUND. FOR NOW HAVE FOLLOWED THE DRIER 23.00Z/23.12Z ECMWF AND 23.12Z NAM/GFS. DRY CONDITIONS THEN LOOK TO FOLLOW FOR FRIDAY THROUGH SATURDAY AS UPPER LEVEL RIDGING BUILDS OVER THE FORECAST AREA. THE SATURDAY NIGHT THROUGH SUNDAY NIGHT FORECAST REVOLVES AROUND THE NORTHERN PORTION OF THE WESTERN TROUGH IMPACTING THE AREA. AT LEAST FOR SATURDAY NIGHT AND SUNDAY...THE 23.00Z ECMWF DOES NOT SHOW ANY PRECIPITATION...DESPITE A COLD FRONTAL PASSAGE AHEAD OF THE TROUGH. THE 22.12Z ECMWF WAS SIMILAR AS WELL. THEREFORE HAVE KEPT THE PERIOD DRY. SOME PRECIPITATION IS INDICATED SUNDAY NIGHT AS THE TROUGH CROSSES THE AREA...WHICH THERMAL PROFILES WOULD SUGGEST AS SNOW (850MB TEMPS DOWN TO -6C). THEREFORE HAVE INCLUDED SOME LOW PROBABILITIES. WILL KEEP THESE LOW PROBABILITIES GOING INTO MONDAY GIVEN TIMING ISSUES WITH THE TROUGH. IT SHOULD BE NOTED AGAIN THAT THE PERIOD FROM SATURDAY NIGHT AND BEYOND IS VERY UNCERTAIN...SO THE PERIOD IS SUBJECT TO CHANGE. && .AVIATION...TONIGHT AND TUESDAY LITTLE OVERALL CHANGE IN THE PATTERN IS EXPECTED FOR MUCH OF THE PERIOD. A WEAK SURFACE BOUNDARY WILL REMAIN WEST OF THE REGION MAINTAINING THE SOUTH FLOW OVER THE AREA...WITH GENERAL VFR CONDITIONS EMBEDDED IN THIS FLOW. THE CONCERN COMES FOR LATE TONIGHT INTO TUESDAY MORNING. A DEVELOPING CUT OFF UPPER LEVEL LOW OVER WESTERN NEBRASKA THIS MORNING WILL SHIFT EAST INTO NORTHERN MISSOURI BY 18Z TUESDAY. WITH THIS TRACK...BOTH THE 23.12Z NAM AND GFS INDICATE THE BEST PV ADVECTION IN THE 500-300 MB LAYER WILL STAY TO THE SOUTH OF THE TAF SITES WITH ONLY SOME WEAK OMEGA DEVELOPING TUESDAY MORNING DUE TO SOME UPPER LEVEL DIFLUENCE IN THE NORTHEAST QUADRANT OF THE UPPER LEVEL LOW. THE SURFACE LOW IS ANTICIPATED TO MOVE INTO CENTRAL IOWA BY 18Z TUESDAY ALLOWING THE WEAK BOUNDARY TO PROGRESS EAST INTO CENTRAL MINNESOTA. THIS WILL MAINTAIN THE SOUTH TO SOUTHEAST FLOW AND THE LOW LEVEL WARM AIR ADVECTION. THE WARM AIR ADVECTION WILL MAINTAIN 1-2 UBARS/S OF UP GLIDE ON THE 290K ISENTROPIC SURFACE...SO THE QUESTION BECOME WHETHER THIS COMBINED FROM THE WEAK LIFT FROM THE UPPER LEVEL LOW WILL BE ENOUGH TO PRODUCE LOW CEILINGS AND SOME RAIN. THE 12Z GFS LOOKS TO SUFFER FROM CONVECTIVE FEEDBACK PRODUCING A SPURIOUS SHORT WAVE TROUGH AND PRECIPITATION BULLS EYE OVER THE AREA LATE TONIGHT AND TUESDAY MORNING. THE NAM SUGGEST THE POSSIBILITY OF SOME LIGHT RAIN AND MVFR/IFR CEILINGS FOR KRST WHILE THE LOCAL WRF KEEPS ALL THE PRECIPITATION SOUTH OF THE TAF SITES. FOR NOW HAVE BLENDED THE NAM AND WRF SOLUTIONS BY BRINGING IN VICINITY SHOWERS WITH MVFR CEILINGS AT KRST AFTER 14Z TUESDAY. HAVE CONTINUED THE IFR VISIBILITY FROM THE PREVIOUS FORECAST ALTHOUGH CONFIDENCE IS NOT HIGH GIVEN LACK OF IFR CONDITIONS THAT DEVELOPED THIS MORNING IN SIMILAR CONDITIONS. THINK THE RAIN WILL NOT GET TO KLSE SO KEPT THE CEILINGS VFR WHILE MAINTAINING MVFR VISIBILITY IN SOME FOG. && .ARX WATCHES/WARNINGS/ADVISORIES... WI...NONE. MN...NONE. IA...NONE. && $$ SHORT/LONG TERM...AJ AVIATION...04