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000 FXUS63 KLSX 231156 AFDLSX AREA FORECAST DISCUSSION...UPDATED AVIATION NATIONAL WEATHER SERVICE ST LOUIS MO 556 AM CST MON NOV 23 2009 .DISCUSSION... /444 AM CST MON NOV 23 2009/ STILL WATCHING THE TROF OF LOW PRESSURE NOW DIGGING INTO THE GREAT PLAINS THIS MORNING AS OUR FIRST FORECAST CHALLENGE OF THIS PACKAGE. THE 23/00Z GFS/GEM/ECMWF WERE IN VERY GOOD AGREEMENT IN HANDLING THIS STORM SYSTEM THIS MORNING...WHILE THE NAM WAS THE OUTLIER. NOT ONLY WAS THE NAM THE OUTLIER...IT WAS SUBSTANTIALLY SLOWER THAN THE GOING FORECAST...WHILE THE OTHER THREE WERE ACTUALLY VERY CONSISTENT WITH PREVIOUS PACKAGE. THEREFORE I IGNORED THE NAM THIS MORNING AND WENT WITH THE CONSENSUS. THAT BEING SAID...I ONLY MADE SMALL COSMETIC CHANGES TO THE FORECAST THIS MORNING. THE BIGGEST CHANGE WAS TO SPLIT TONIGHT`S POPS TO START THE PRECIP AFTER MIDNIGHT OVER CENTRAL MISSOURI. REMAINDER OF THE FORECAST IS VIRTUALLY THE SAME AS THE LAST PACKAGE. STILL EXPECTING SHOWERS TO OVERSPREAD THE AREA LATE TONIGHT/TUESDAY MORNING. NOT EXPECTING ANY HEAVY RAIN THIS TIME...WITH MOST AREAS PROBABLY SEEING AROUND 1/4 INCH AHEAD OF THE COLD FRONT. A FEW COUNTIES UP IN NORTHEAST MO/WEST CENTRAL IL COULD GET AROUND 1/2 INCH AS THEY`LL BE CLOSER TO THE LOW CENTER AND HAVE A LONGER PERIOD OF RAIN. THE TROF QUICKLY TAKES ON A NEGATIVE TILT TUESDAY NIGHT...LIFTING OUT AS ANOTHER SHORTWAVE DROPS IN BEHIND IT ON WEDNESDAY. THIS SECOND WAVE IS STILL LOOKING PRETTY COLD...WITH 850MB TEMPS FALLING TO AROUND -5 DEGREES BY 06-12Z THURSDAY MORNING. THIS WOULD ALMOST CERTAINLY BE COLD ENOUGH FOR SNOW...IF THERE WERE ANY MOISTURE LEFT OVER TO LIFT IN THE WAKE OF TUESDAY`S WAVE. I CERTAINLY WOULDN`T RULE OUT SOME SPRINKLES OR FLURRIES ON THANKSGIVING...BUT I THINK THE CHANCE FOR MEASURABLE PRECIP IS PRETTY LOW. TEMPS WILL BE SUBSTANTIALLY COLDER TOWARD THE END OF THE WEEK BEHIND THE SECOND LOW. WITH CLOUDS AND NORTHWEST FLOW WE`LL BE LUCKY TO MAKE IT TO THE LOW 40S ON THURSDAY. AND WHILE IT LOOKS LIKE WE`LL SEE SOME SUN ON FRIDAY IT`LL STAY IN THE 40S SINCE WE`LL BE IN THE HEART OF THE COLD AIRMASS AT THAT TIME. CARNEY && .AVIATION... /541 AM CST MON NOV 23 2009/ FOR THE 12Z TAFS...THE NAM LO LVL RH PROGS AND FORECAST SOUNDINGS APPEAR TO BE DOING THE BEST JOB EMULATING THE GRADUAL NWD EXPANSION OF LOW CLOUDS INTO SE MO FROM ARKANSAS AND W KENTUCKY DURING THE PREDAWN HOURS. JUDGING BY THE 11-3.9 SATELLITE IMAGERY THE GFS APPEARS TO BE TOO AGGRESSIVE/TOO FAST WITH THE NWD EXPANSION OF THE LOW VFR/MVFR CIGS...WITH THE LATEST RUC OUPUT ALSO GIVING MORE CREDENCE TO THE SLOWER NWD EXPANSION AS DEPICTED BY THE NAM. GENERAL TREND SHOULD BE FOR THE SC TO OUR S TO MAKE A SLOW NWD EXPANSION DURING THE DAY AND INTO THE EVENING...WITH THE ADVECTION OF DEEPER MOISTURE GRADUALLY OVERCOMING THE EROSION OF THE N EDGE OF THE CLOUD MASS DUE TO DIURNAL MIXING. GFS AND NAM DO AGREE THAT INCREASING MOISTURE OVERNIGHT SHOULD CAUSE CIGS TO LOWER TO AT LEAST THE LOW END OF THE MVFR RANGE...AND HAVE TRENDED FORECASTS THIS DIRECTION IN THE 06-09Z TIME FRAME. TRUETT && .LSX WATCHES/WARNINGS/ADVISORIES... MO...NONE. IL...NONE. && $$ WFO LSX
