FXUS63 KLSX 252323
AFDLSX

AREA FORECAST DISCUSSION...UPDATED AVIATION
NATIONAL WEATHER SERVICE ST LOUIS MO
523 PM CST FRI DEC 25 2009

.DISCUSSION...
/317 PM CST FRI DEC 25 2009/

A WHITE CHRISTMAS HAS BEEN DELIVERED FOR MANY PARTS OF THE FCST RGN 
DURING THE OVERNIGHT INTO THE AFTERNOON HRS.  SFC LOW WAS DEEPENING 
AND ROTATING THRU NWRN IA AS OF 20Z. WITH ATTENDANT SHORTWAVE ENERGY 
ALOFT SNOW DEVELOPED DURING THE COURSE OF TODAY RESULTING IN 
ACCUMULATIONS OF SEVERAL TENTHS TO UPWARDS OF AN INCH ACRS MUCH OF 
THE BI-STATE RGN.  SNOW IS EXPECTED TO CONTINUE OVERNIGHT WITH 
STRONG AND GUSTY WLY WINDS AROUND 30 MPH AS LOW-MID LVL COLD AIR 
ADVECTION WILL CONTINUE TO ALLOW FOR FAIRLY STEEP LAPSE RATES AND 
ADIABATIC MIXING OF HIGHER MOMENTUM ALOFT THROUGH THE BNDRY LYR.
TEMPS TO THE EAST OF THE MS-RVR VLY HAVE FALLEN AROUND 10 DEGREES
SINCE MORNING WITH LOCATIONS ONLY VARYING A FEW DEGREES UNDER A
LOW DECK OF STRATUS CLOUDS. ONLY EXPECT A FEW DEGREES OF TEMP
MODERATION THIS EVENING FROM PRESENT TEMPS.

SHORT-TERM (FRI NGT - SUN)...

DETERMINISTIC MDLS IN FAIRLY GOOD AGREEMENT OF NEARLY STACKED LOW 
PRES ROTATING THRU WRN INTO ERN IA SAT BEFORE LIFTING NEWD THRU THE 
GRT LKS RGN BY SUN.  AS THE SYS CONTINUES ITS OCCULUSION PROCESS 
ASSOC SHORTWAVE ENERGY WILL ROTATE THRU THE MID-MS VLY RGN ALLOWING 
FOR SCATTERED LIGHT SNOW THROUGHOUT THE FCST AREA.  

BUT THE TRICKY PART OF THIS FCST IS WHERE AND WHEN LIGHT SNOW WILL 
DEVELOP...AND WHETHER IT WILL ACCUMULATE. MDLS FAIRLY SUCCINCT IN 
KEEPING REGIONS OF MID-LVL FRONTOGENESIS AND -PV ANOMALIES...ALONG 
WITH ATTENDANT SHORTWAVE ENERGY PROVIDING ENCHANCED LIFT ESP WITHIN 
THE DENDRITIC GROWTH ZONE...THROUGHOUT THE MID-MS VLY. IN ADDITION 
WITH THE CONTINUED OCCLUSION PROCESS HIGHER THETAE AIR WITHIN THE 
LOW-MID LVLS SHUD WRAP TO THE WEST OF THE SFC LOW AND ADVECT SWD 
THRU NRN MO INTO THEMID-MS VLY RGN.  WITH ENHANCED MOISTENING OF 
ATMOSPHERIC BNDRY LYR WITHIN RGNS OF FORCED ASCENT BETTER CONFIDENCE 
FOR SNOWBAND DEVELOPMENT LIES ACRS NRN MO INTO W-CNTRL IL AND THUS 
HAVE KEPT LIKELIES FOR TNGT INTO SAT WITH SCATTERED SNOW SHOWERS 
FURTHER SOUTH.  HAVE TAPERED CHCS INTO SAT NGT INTO SUN AS SYS 
WEAKENS AND PULLS EWD.

WITH THE DEEP OCCLUDED SYS AND ATTENDANT SFC COLD AIR ADVECTION ON 
THE BACKSIDE LAPSE RATES WILL REMAIN FAIRLY STEEP AT LOW LVLS 
THEREBY ALLOWING FOR BNDRY LYR MIXING AND CONTINUING STRONG AND 
GUSTY WINDS FOR THE REGION INTO THE OVRNGT PERIOD INTO SATURDAY 
SLOWLY TAPERING INTO SUN. 

AFORMENTIONED LOW-MID LVL THETAE ADVECTION AND RECENT PRECIP SHUD 
KEEP LOW LVLS FAIRLY MOIST AND WITH AFOREMENTIONED ADIABATIC MIXING 
AND ENHANCED LIFT VIA ROTATING SHORTWAVE ENERGY WILL KEEP LOW LVL 
STRATUS ACRS THE RGN THRU THE WEEKEND.  AS SUCH SFC TEMPS WILL BE 
LIMITED IN MODERATING SUBSTANTIALLY AND HAVE ONLY DIURNAL CHANGES ON 
THE ORDER OF 5 DEGREES OVER THE WEEKEND WITH TEMPS HOVERING AROUND 
THE MID 20S.

FOR EXTENDED (SUN NGT - SAT)...

BETTER RUN-TO-RUN CONSISTENCY IS NOTED IN THE GFS/ECMWF SOLNS FOR 
THE MID-RANGE AND THUS HAVE FOLLOWED A BLEND OF THE TWO FOR THE 
EXTENDED FCST.  UPR LVL ENERGY ASSOC WITH THE DEEP SYNOPTIC H5 
STACKED LOW LIFTING THRU THE GRT LKS RGN WILL KEEP THE CHCS FOR 
FLURRIES INTO MON MORNING AS LOW LVL MOISTURE WILL BE SUFFICIENT 
ALONG WITH STEEP LOW LVL LAPSE RATES.  NWLY WINDS WILL CONTINUE TO 
USHER IN COLD AIR ACRS THE BI-STATE RGN BUT TAPER INTO TUE AS SFC 
HIGH PRESSURE ADVECTS EWD ACRS THE MID-MS VLY.  EXPECT THE COLDEST 
TEMPS FOR TUE MORN INTO THE TEENS AND AROUND 20 DEGREES AS WINDS 
WILL BE LGHT AND VRB UNDER MOSTLY CLEAR SKIES.  

ENERGY ASSOC WITH A DEEP SYNOPTIC SYS ACRS PAC NW/SRN AK WILL DIVE 
SWD ENHANCING CYCLOGENESIS ACRS THE SWRN CONUS AND NRN MEXICO TUE 
NGT.  THIS SYS AND ASSOC SHORTWAVE ENERGY AND H3 JET STREAM WILL 
TRASLATE EWD AND PUSH EWD THRU THE TXARLA RGN INTO WED MORN.  FEEL 
ECMWF HAS THE BETTER HANDLE ON THE SYS.  ISENTROPIC FLOW ALONG THE 
290-295K SFCS ILLUSTRATE A TONGUE OF HIGHER THETAE AIR LIFTING INTO 
SRN MO ALONG AND AHEAD OF THE H5 TROF.  COLLOCATED QVECT CONV AND 
UPR LVL DIFFLUENCE IS PREVALENT ACRS ERN OK.  THUS HAVE KEPT SLIGHT 
POPS FOR LIGHT SNOW BEGINNING TUE NGT INTO WED WITH BEST CHCS EARLY 
IN THE PERIOD ESP ACRS THE SRN FCST AREA WHERE BETTER DYNAMICS AND 
MOISTURE LAY. THEREAFTER FEEL THE SYS WILL BE MOISTURE STARVED AS 
TONGUE OF HIGH THETAE AND BETTER LIFT SHIFTS MORESO OVER THE SRN 
CONUS.  TEMPS TO RISE INTO THE MID-UPR 30S WITH LOWS AROUND THE 
MID-20S WITH ASSOC SLY FLOW AND ANTICIPATED LOW-MID CLOUD COVER. 

INTO THURS GFS/ECMWF SUPPORTIVE OF A STRONG COLD FRONT THOUGH 
MOISTURE DEPRIVED MOVING SEWD THRU THE BI-STATE RGN USHERING MUCH 
COLDER AIR AND BELOW NORMAL TEMPS.  ECMWF PRESENTLY IS MORE ROBUST 
IN BRINGING SINGLE DIGITS FOR THE AREA...BUT FOR NOW HAVE BLENDED 
WITH GFS.  NWLY WINDS AND DRY WEATHER WILL BE THE NORM IN WAKE OF 
THE SFC COLD FRONT.

SIPPRELL
&&

.AVIATION...
/523 PM CST FRI DEC 25 2009/

FOR THE 00Z TAFS...STRONG SURFACE LOW OVER NORTHWEST IOWA NOW WITH 
STRONG GUSTY SW WINDS PERSISTING OVER FORECAST AREA. WILL SEE 
SEVERAL ROUNDS OF LIGHT SNOW/FLURRIES ROTATE AROUND THIS LOW FOR THE 
NEXT DAY OR TWO. AS FOR CIGS...VFR CIGS WILL EVENTUALLY LOWER BACK 
DOWN TO MVFR AROUND 03Z AT ALL SITES AND REMAIN THAT WAY FOR THE 
REST OF THE FORECAST PERIOD. GUSTY SOUTHWEST WINDS WILL DIMINISH 
JUST A BIT AND TURN TO THE WEST BY TOMORROW AFTERNOON AS ANOTHER 
SHORTWAVE MOVES THROUGH. 

BYRD
&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...NONE.
IL...NONE.
&&

$$
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