Issued by NWS St. Louis, MO
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000 FXUS63 KLSX 221203 AFDLSX AREA FORECAST DISCUSSION...UPDATED AVIATION NATIONAL WEATHER SERVICE ST LOUIS MO 603 AM CST SUN NOV 22 2009 .DISCUSSION... /438 AM CST SUN NOV 22 2009/ LITTLE CHANGE TO THE GOING FORECAST FOR THIS CYCLE. THE PRIMARY FOCUS CONTINUES TO BE THE STORM FOR TUESDAY WHICH WILL BRING SOME RAIN AND COOLER TEMPERATURES TO THE REGION. HOWEVER...FIRST IN LINE IS A WEAK SHORTWAVE THAT WILL BRING CLOUDS AND PERHAPS SOME SPRINKLES TO PARTS OF CENTRAL AND NORTHEAST MISSOURI AND WEST CENTRAL ILLINOIS TODAY. RH PROGS ARE PRETTY PESSIMISTIC FOR ANY CLEARING TODAY OVER CENTRAL MISSOURI SO HAVE KEPT CLOUDY/MOSTLY CLOUDY CONDITIONS IN THE FORECAST. IT`S DEBATABLE WHETHER OR NOT TO INCLUDE ANY POPS OR MENTION OF PRECIP AT ALL TODAY. SOUNDINGS SHOW A VERY DRY LAYER AROUND 900 MB WITH DEWPOINT DEPRESSIONS OF NEARLY 30 DEGREES. WILL KEEP AN EYE ON THE RADAR AND SURFACE OBSERVATIONS THIS MORNING AND UPDATE AS NECESSARY...BUT IF THOSE SOUNDINGS ARE CORRECT ANYTHING SHORT OF A HEAVY THUNDERSTORM WILL HAVE TROUBLE PRECIPITATING THROUGH THAT DRY LAYER. HAVE INTRODUCED SOME SLIGHT CHANCE POPS FOR TONIGHT THO AS THE MAIN SHORTWAVE RIPPLES BY. LOW LEVELS STILL LOOK PRETTY DRY...BUT THERE COULD BE ENOUGH ISENTROPIC LIFT COUPLED WITH SHORTWAVE ENERGY TO KICK OUT A FEW LIGHT RAIN SHOWERS. THE MAIN SHOW WILL BEGIN EARLY TUESDAY MORNING AS A STRONG WAVES DROPS OFF THE ROCKIES AND CLOSES OFF OVER THE PLAINS. LATEST GUIDANCE APPEARS TO BE A BIT SLOWER THAN PREVIOUS FORECASTS WITH RESPECT TO THE SURFACE FEATURES ASSOCIATED WITH THIS SYSTEM. HAVE ACCORDINGLY SLOWED THE PROGRESSION OF THE COLD FRONT BY ABOUT 6 HOURS. HAVE BUMPED POPS UP A BIT OVER PORTIONS OF THE AREA FOR TUESDAY DUE TO THE SLOWER PROGRESSION OF THE SYSTEM...BUT THE BASIC IDEA IS THE SAME...LIKELY POPS JUST ABOUT EVERYWHERE EXCEPT CENTRAL MISSOURI WHERE HIGH CHANCE IS THE RULE. TUESDAY`S SHORTWAVE TAKES ON A NEGATIVE TILT AND LIFTS RAPIDLY NORTHEAST INTO THE GREAT LAKES REGION ON WEDNESDAY...JUST IN TIME FOR ANOTHER SHORTWAVE TO DROP INTO THE MIDWEST AND DIG ALL THE WAY INTO THE LOWER MISSISSIPPI VALLEY. THIS WAVE WILL BRING A REINFORCING SHOT OF COLD AIR INTO THE REGION...AND WE MIGHT ACTUALLY GET OUR FIRST FREEZE OF THE SEASON IN ST. LOUIS FRIDAY MORNING. THERE`S SOME CONCERN FOR LIGHT RAIN OR EVEN LIGHT SNOW ON THANKSGIVING DAY...BUT IF THE GFS IS CORRECT THERE WON`T BE ENOUGH MOISTURE TO SUPPORT PRECIPITATION...SO HAVE LEFT THANKSGIVING DAY DRY FOR NOW. CARNEY && .AVIATION... /543 AM CST SUN NOV 22 2009/ FOR THE 12Z TAFS...RATHER THICK BLANKET OF MID CLOUDS HAS BEEN DRAPED OVER MOST OF MISSOURI DURING THE PREDAWN HOURS...HOWEVER EASTERN EDGE OF THIS CLOUD DECK HAS BEEN ERODING WWD. EDGE CURRENTLY EXTENDS FROM NEAR PPQ TO SUS...AND EXTRAPOLATION OF THIS EDGE PLACES IT OVER UIN IN THE 15-16Z TIME FRAME AND BY COU BY EARLY AFTERNOON...IN VERY GOOD AGREEMENT WITH THE MID LVL RH PROGS FROM BOTH THE GFS AND NAM. IN THE STL AND SUS AREAS WHERE SKIES HAVE NOW CLEARED RESIDUAL LO LEVEL MOISTURE MAY LEAD TO SEVERAL HOURS OF MVFR VSBYS EARLY THIS MORNING...BUT ANY FOG WILL QUICKLY LIFT BY 15Z. MID LEVEL CLOUDINESS WILL ATTEMPT TO OVERSPREAD THE AREA ONCE AGAIN TONIGHT...WITH WEDGE OF DRIER AIR AOB 850MB HOLDING LOWER CIGS AT BAY THROUGH 12Z. TRUETT && .LSX WATCHES/WARNINGS/ADVISORIES... MO...NONE. IL...NONE. && $$ WFO LSX
