Issued by NWS La Crosse, WI
000 FXUS63 KARX 242109 AFDARX AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE LA CROSSE WI 309 PM CST TUE NOV 24 2009 .SHORT TERM...TONIGHT INTO FRIDAY MAIN FORECAST CONCERNS ARE ON PRECIPITATION TRENDS TONIGHT THROUGH WEDNESDAY NIGHT...AS WELL AS PRECIPITATION TYPE FOR WEDNESDAY INTO WEDNESDAY NIGHT. CURRENTLY AS OF 21Z...WATER VAPOR LOOP AND RUC 500MB HEIGHT ANALYSIS SHOWED RIDGING ALONG BOTH COASTS OF THE U.S. AND TWO TROUGHS FROM SOUTH CENTRAL CANADA INTO THE CENTRAL U.S.. THE FIRST TROUGH OVER THE CENTRAL PLAINS WITH AN UPPER LOW IN FAR SOUTH CENTRAL IOWA IS CO-LOCATED WITH A SURFACE LOW UNDER THE UPPER LOW. HEIGHT FALLS ASSOCIATED WITH THE UPPER LOW AS WELL AS ISENTROPIC LIFT IS HELPING TO PRODUCE TWO AREAS OF PRECIPITATION AS SEEN ON RADAR. THE FIRST IS FROM WESTERN ILLINOIS INTO EASTERN IOWA...PRETTY MUCH THE WARM CONVEYOR BELT OF THE SYSTEM. THE SECOND IS FROM CENTRAL IOWA INTO NORTHWEST WISCONSIN...ASSOCIATED WITH A MID-LEVEL FRONTOGENESIS ZONE AND AN INVERTED TROUGH OFF THE SURFACE LOW...THOUGH THIS HAS GREATLY WEAKENED OVER THE COURSE OF THE DAY. WIDESPREAD CLOUD COVER EXISTS TOO ASSOCIATED WITH THE FIRST TROUGH...PRIMARILY EAST OF I-29. A ZONE OF UPPER AND SURFACE RIDGING DOMINATES THE DAKOTAS... WHICH HAS KEPT ANY CLOUD COVER TO CIRRUS. 12Z BIS SOUNDING SHOWS WHY TOO WITH A FAIRLY DRY PROFILE THROUGHOUT AND PRECIPITABLE WATER AROUND 0.25 INCH. JUST ACROSS THE BORDER IN SOUTHERN SASKATCHEWAN IS A BAND OF LIGHT SNOW...ASSOCIATED WITH HEIGHT FALLS FROM THE SECOND TROUGH. TONIGHT...MAIN FOCUS IS ON THE UPPER LOW IN SOUTH CENTRAL IOWA. ALL MODELS ARE IN GOOD AGREEMENT TAKING THE LOW EASTWARD TO JUST SOUTHEAST OF CHICAGO BY 12Z WEDNESDAY...IN RESPONSE TO THE TROUGH IN SOUTH CENTRAL CANADA DROPPING SOUTH THROUGH THE DAKOTAS. GIVEN THE UPPER LOW TRACK...THE BULK OF THE HEIGHT FALLS ALOFT OCCUR THIS EVENING. IN FACT...THAT AREA OF RIDGING ACROSS THE DAKOTAS SHOULD END UP BUILDING OVER THE FORECAST AREA AROUND 12Z. THEREFORE... THINKING IS THAT THE BEST PRECIPITATION CHANCES WOULD END UP THIS EVENING...AS THE WARM CONVEYOR BELT CONTINUES WRAPPING INTO NORTHEAST IOWA AND WESTERN WISCONSIN. UNSURE ABOUT HOW PRECIPITATION THE AREA FROM SOUTHEAST MINNESOTA INTO CENTRAL WISCONSIN WILL RECEIVE...SINCE THE INVERTED TROUGH AND ITS ASSOCIATED FRONTOGENESIS ALOFT IS FALLING APART. THAT TREND IS LIKELY TO CONTINUE TOO BECAUSE OF THE RIDGING BUILDING IN. THEREFORE...LOWEST PROBABILITIES ARE IN THIS AREA. WITH THAT RIDGING BUILDING IN ALOFT...CONCERN WOULD EXIST FOR SOUTHEAST MINNESOTA TO TRY TO CLEAR OUT. HOWEVER...SOUNDINGS SEEM TO SUGGEST PLENTY OF LOW LEVEL MOISTURE LINGERING AROUND TO MAINTAIN A CLOUD DECK. THEREFORE... CLOUDY SKIES ARE ANTICIPATED EVERYWHERE...HELPING TO HOLD READINGS UP. THERE SHOULD BE SOME COLD ADVECTION TONIGHT AS THE LOW PASSES BY...BUT OVERALL...PLAN ON ANOTHER ABOVE NORMAL NIGHT. WEDNESDAY INTO WEDNESDAY NIGHT...FOCUS SHIFTS TO THE TROUGH THAT IS DIGGING DOWN INTO THE NORTHERN PLAINS TONIGHT. MODELS TRACK THE TROUGH...WHICH SHOULD ALSO CONTAIN AN UPPER LOW...DOWN INTO IOWA AND MISSOURI BY 00Z THU...AND THEN INTO INDIANA BY 12Z THU. BEST 500MB HEIGHT FALLS OVER THE FORECAST AREA WITH THIS TROUGH LOOK TO OCCUR DURING THE AFTERNOON AND EVENING...AND ARE SOMEWHAT IMPRESSIVE...ON THE ORDER OF 70-120 METERS PER 12 HOURS. ASSOCIATED WITH THE TROUGH IS ALSO SOME DECENT COOLING...CAUSING 850MB TEMPS TO FALL FROM AROUND 0C AT 12Z WED TO -2 TO -5C AT 00Z THU...COLDEST WEST OF THE MISSISSIPPI RIVER. 850MB TEMPS THEN STAY NEARLY STATIONARY THROUGH 12Z THU WHILE THE BULK OF THE COOLING GOES TO OUR SOUTH WITH THE UPPER LOW. THE COOLING DOES MAKE FOR INTERESTING PRECIPITATION TYPE ISSUES AS WELL AS A POSSIBLE NON-DIURNAL TEMPERATURE TREND FOR THE AFTERNOON. FIRST...REGARDING THE PRECIPITATION...A DEFORMATION BAND AIDED ALSO BY HEIGHT FALLS SHOULD MOVE FROM THE DAKOTAS INTO THE FORECAST AREA LATE WEDNESDAY MORNING. THIS BAND THEN LOOKS TO RE-ORIENT ITSELF OVER THE AREA BETWEEN THE AFTERNOON AND EVENING... DUE TO THE UPPER LOW/TROUGH SWITCHING FROM DIVING SOUTH TO MOVING EAST. AFTER MIDNIGHT...PRECIPITATION IS EXPECTED TO DIMINISH WEST TO EAST AS THE UPPER LOW/TROUGH PULLS AWAY AND 500MB HEIGHTS BEGIN TO RISE. LOOKING AT A CROSS-SECTION OF THE BAND...MUCH OF THE EPV IS ABOVE 0.25...SUGGESTING LITTLE IN THE WAY OF CONVECTIVE POSSIBILITY WITH THE BAND. THIS WILL HELP PREVENT RATES FROM GETTING TOO HEAVY. ANOTHER FACTOR THAT COULD LIMIT AMOUNTS IS THE LACK OF MOISTURE THE SYSTEM HAS TO WORK WITH...NOTED BY THE 12Z BIS SOUNDING. NONETHELESS...THE AREA ALONG AND WEST OF THE RIVER COULD SEE 0.25 TO 0.50 INCH OF PRECIPITATION DURING THIS PERIOD AS INDICATED BY 24.12Z GFS/NAM AND 24.09Z SREF. WET-BULB ZERO HEIGHTS...REFERENCED ABOVE BY THE 850MB TEMPS...ARE TRICKY IN DETERMINING HOW MUCH OF THE PRECIPITATION FALLS AS RAIN AND THAT OF SNOW. THE NAM HAS BEEN PERSISTENT OF HAVING SOUTHEAST MN...NORTHEAST IA AND FAR SOUTHWEST WI TURN TO SNOW DURING THE DAY WEDNESDAY. HOWEVER...THE GFS AS WELL AS THE SREF MEAN ARE QUITE A BIT WARMER... AND ALSO SHOW A WARM TONGUE WRAPPING IN WEDNESDAY NIGHT. NOW THE 24.18Z NAM SHOWS THAT WARM TONGUE TOO. THEREFORE MUCH OF THE FORECAST IS WORDED RAIN OR RAIN/SNOW AND SNOW ACCUMULATIONS MAY REACH 2 INCHES AT MOST. ABOVE NORMAL GROUND TEMPS WILL ALSO LIMIT ACCUMULATIONS...ESPECIALLY IF THERE ARE NO CONVECTIVE ELEMENTS. THE AREA FOR MOST ACCUMULATION IS FROM SOUTHEAST MN INTO FAR SOUTHWEST WI. THANKSGIVING INTO FRIDAY...PLAN ON DRYING CONDITIONS DURING THIS PERIOD AS RIDGING CURRENTLY OVER THE PACIFIC NORTHWEST BUILDS EASTWARD INTO THE NORTHERN PLAINS. MODELS HAVE REALLY SPED UP THE DEPARTURE OF THE DEFORMATION BAND FROM WEDNESDAY NIGHT...WITH MANY MODELS SUGGESTING THANKSGIVING COULD END UP DRY. TRENDED THIS WAY IN THE GRIDS AS WELL...WITH ONLY SOME 20 PERCENT CHANCES OF PRECIPITATION OVER CENTRAL WISCONSIN. THESE MIGHT NEED REMOVAL IN THE NEXT FORECAST. OTHERWISE...ALONG WITH THE DEPARTING PRECIPITATION...WE SHOULD SEE A CLEARING TREND AS WELL AS DRIER AIR MOVES IN FROM THE PLAINS. HOWEVER...COOL 850MB TEMPS FLOWING IN ON THANKSGIVING AS UPPER TROUGHING SWINGS ACROSS (COULD BE -5 TO -8C)...WILL RESULT IN A MUCH COOLER DAY THAN AS OF LATE. MOST PLACES SHOULD STRUGGLE TO EVEN GET TO 40. WITH CLEARING AND THE LOWER HIGH TEMPERATURES...AS WELL AS A SURFACE RIDGE MOVING ACROSS...EXPECT THURSDAY NIGHTS LOWS TO END UP CLOSER TO NORMAL (MID 20S). A RETURN FLOW LOOKS TO SETUP DURING THE DAY FRIDAY...HELPING TO BOOST 850MB TEMPS BACK UP AT LEAST TO 3-7C BY 00Z SATURDAY AS SHOWN BY THE GFS/ECMWF. HOWEVER...MUCH OF THIS WARMING LOOKS TO OCCUR ALOFT...AS TO BE EXPECTED THIS TIME OF YEAR...AND THEREFORE ONLY HAVE HIGHS ABOUT 3-5 DEGREES WARMER ON FRIDAY COMPARED TO THURSDAY. .LONG TERM...FRIDAY NIGHT INTO TUESDAY MAIN FORECAST CONCERNS ARE PRECIPITATION CHANCES SUNDAY AND TUESDAY. THE 24.00Z GFS AND ECMWF ARE IN DECENT AGREEMENT WITH OVERALL UPPER LEVEL PATTERN THROUGH 00Z SUNDAY THEN BEGIN TO DIVERGE ON TIMING AND STRENGTH OF UPPER LEVEL LOW/TROUGH OVER SOUTHWEST U.S. AND NORTHERN PLAINS STATES UPPER LEVEL TROUGH. THIS WILL HAVE IMPACTS ON THE TIMING OF SURFACE LOW/FRONT MOVING ACROSS THE UPPER MIDWEST AND GREAT LAKES REGION SUNDAY. WHERE THE 24.00Z GFS IS MORE PROGRESSIVE THAN THE 24.00Z ECMWF. THE 24.00Z GEFS SUGGEST A MORE PROGRESSIVE UPPER LEVEL FLOW PATTERN...AND THE 24.00Z ECMWF ENSEMBLES SUGGEST A SLIGHTLY SLOWER PROGRESSIVE SOLUTION. AT THIS TIME...CONFIDENCE ON EVOLUTION OF THE OVERALL UPPER LEVEL PATTERN AFTER 00Z SUNDAY IS LOW...BASED ON THIS WILL TAKE A BLEND OF THE DETERMINISTIC 24.00Z GFS AND ECMWF RUNS. BOTH MODELS ARE IN AGREEMENT IN MOVING SURFACE LOW/COLD FRONT ACROSS THE CENTRAL U.S. SUNDAY...THOUGH TIMING OF THE SURFACE FRONT/LOW IS STILL IN QUESTION. HAVE INTRODUCE PRECIPITATION CHANCES ACROSS FORECAST AREA SUNDAY. BOTH THE 24.00Z GFS AND ECMWF INDICATE UPPER LEVEL LOW TO CUT-OFF OVER THE CENTRAL AND EASTERN U.S. AFTER 00Z TUESDAY. CONFIDENCE IN FORECAST IS LOW ON PLACEMENT AND TIMING OF UPPER LEVEL CUT-OFF LOW MOVING INTO THE UPPER MIDWEST AND GREAT LAKES REGION AND OVERALL UPPER LEVEL PATTERN BY 12Z TUESDAY. WILL LEAVE FORECAST AREA DRY FOR TUESDAY. && .AVIATION...TONIGHT AND WEDNESDAY TRICKY AVIATION FORECAST NEXT 24 HOURS WITH ONE SHORT WAVE TROUGH MOVING BY TODAY AND ANOTHER IMPULSE DROPPING DOWN FROM SOUTHERN CANADA. GENERALLY IFR CONDITIONS PREVAIL WITH BANDS OF MAINLY LIGHT RAIN ROTATING AROUND FIRST PASSING SYSTEM. RAIN SHOULD FILL IN A BIT THIS EVENING AS WAVE PASSES BY JUST TO THE SOUTH. ONCE THIS HAPPENS UPSTREAM SHORT TERM RIDGING MAY ALLOW BREAK IN PRECIPITATION AHEAD OF NEXT SYSTEM. HAVE ATTEMPTED TO CREATE SOME TIMING IN AVIATION FORECASTS TO ACCOUNT FOR THIS. SECOND SHORT WAVE TROUGH DROPS INTO MIDWEST WEDNESDAY MORNING. BETTER FORCING NOW PROGGED JUST TO THE WEST AND SOUTH OF THE AREA BUT AS SYSTEM PINWHEELS ACROSS SOUTHWEST GREAT LAKES...ANY RAIN WILL TRANSITION OVER TO SNOW FAIRLY QUICKLY AS COLDER AIR ADVECTS IN. WHILE LATE IN AVIATION FORECAST PERIOD...WILL BEGIN WITH MIX OF RAIN AND SNOW ACROSS MINNESOTA AND QUICKLY TRANSITION OVER TO SNOW. WILL LIKELY START AS RAIN EAST OF MISSISSIPPI RIVER WITH TRANSITION MORE LIKELY BY WEDNESDAY EVENING AND MAINLY IFR CONDITIONS PREVAILING THROUGH THIS PERIOD. && .ARX WATCHES/WARNINGS/ADVISORIES... WI...NONE. MN...NONE. IA...NONE. && $$ SHORT TERM...AJ LONG TERM...DTJ AVIATION...TJS
