FXUS63 KLSX 220513
AFDLSX

AREA FORECAST DISCUSSION...UPDATED AVIATION
NATIONAL WEATHER SERVICE ST LOUIS MO
1115 PM CST MON DEC 21 2009

.DISCUSSION...
/309 PM CST MON DEC 21 2009/

CLEARING HAS MOVED INTO THE WRN AND SRN PARTS OF THE CWA EARLY THIS 
AFTERNOON AND WILL CONTINUE TO WORK NEWD INTO THE EVENING HOURS.
NAM/LOCAL WRF SOUNDINGS AT UIN/COU SHOW ENOUGH LOW LEVEL MOISTENING
LATE TONIGHT CAUSE THE REDEVELOPMENT STRATUS TOWARD DAWN. THEN BY
EARLY TUESDAY...MAY HAVE PATCHY DRIZZLE/FREEZING DRIZZLE OVER THE
NRN AND WRN PARTS OF THE CWA IN THE MORNING. NAM/GFS IS THEN
SHOWING SOME MODEST LOW LEVEL MOISTURE CONVERGENCE MOVING INTO
SWRN PART OF THE CWA BY LATE TOMORROW...SO HAVE LOW CHANCE POPS
HERE.

MAIN CONCERN CONTINUES TO BE HOW THE MAJOR WINTER STORM THAT WILL 
MOVE ACROSS THE CENTRAL CONUS IN THE NEXT FEW DAYS WILL AFFECT THE 
CWA.  HAVE KEPT THINKING OF PREVIOUS FORECAST IN THAT THIS WILL BE A 
RAIN EVENT WITH SOME CHANCE OF THUNDERSTORMS.  WHILE THERE ARE SOME 
DIFFERENCES IN THE DETAILS...MOST OF THE MODELS AGREE THAT UPPER LOW 
WILL BOTTOM OUT OVER THE SRN PLAINS EARLY ON THURSDAY AND THEN TRACK 
NEWD INTO ERN IA BY FRIDAY EVENING.  850MB LOW TRACK AND OTHER
GUIDANCE FAVORS KEEPING THE HEAVY SNOW BAND OVER ERN KS/FAR NW
MO/CNTRL IA. OVER OUR REGION...925-850MB MOISTURE CONVERGENCE WILL
BEGIN TO RAMP UP BY TOMORROW NIGHT AS 40KT LOW LEVEL JET SETS UP
IN ADVANCE OF UPPER LOW. THIS LOW LEVEL FORCING REMAINS OVER THE
AREA UNDER INCREASING LARGE SCALE ASCENT AS UPPER LOW BECOMES
NEGATIVELY TILTED BY MIDDAY THURSDAY. ATMOSPHERE WILL BECAUSE
SLIGHTLY UNSTABLE DURING THIS PERIOD BRINGING THE THREAT OF
THUNDERSTORMS. ALL OF THIS IN CONJUNCTION WITH PWATS CLIMBING TO
AROUND 1 INCH...WHICH IS 2 STANDARD DEVIATIONS ABOVE NORMAL...
WILL ALLOW FOR AN EXTENDED PERIOD OF MODERATE TO HEAVY RAINFALL
AT TIMES. FORECAST QPF AMOUNTS FROM TOMORROW NIGHT INTO THURSDAY
WILL BE IN THE 1.5-2.5 INCH RANGE...SO THIS COULD CAUSE SOME
LOCALIZED FLOODING CONSIDERING THE CURRENTLY FROZEN GROUND. WILL
MENTION THIS POTENTIAL IN THE UPDATED SPS AND HWO. LOW WILL DRAG
THE COLD FRONT ACROSS THE REGION ON THURSDAY NIGHT ALLOWING FOR
THE CHANGEOVER TO SNOW FROM WEST TO EAST. THERE MAY STILL BE
ENOUGH FORCING FROM THE EXITING DEFORMATION ZONE FOR SOME
ACCUMULATING SNOW OVER PARTS OF CNTRL AND NERN/W CNTRL IL ON
THURSDAY NIGHT INTO FRIDAY WITH SCT SNSH ELSEWHERE. TIGHT PRESSURE
GRADIENT AROUND DEEP LOW MEANS ANOTHER ROUND OF WINDY CONDITIONS
THURSDAY INTO FRIDAY.

BY NEXT WEEKEND INTO EARLY NEXT WEEK...HAVE LEFT A CHANCE OF 
FLURRIES GOING FRIDAY NIGHT INTO SATURDAY AS LOW LEVEL CYCLONIC FLOW 
WILL REMAIN OVER THE AREA IN WAKE OF THE SURFACE LOW.  ECMWF DOES 
SHOWS A SECOND, INTENSE UPPER LOW MOVING ACROSS MO/IL AT THE END OF 
THE WEEKEND.  THE CONTINUITY ON THIS SYSTEM IS POOR AND IS NOT 
SUPPORTED BY THE GFS...SO HAVE CHOSEN TO KEEP FORECAST DRY FOR 
SUNDAY INTO MONDAY.  USED A BLEND OF PERSISTENCE...HPC MANUAL AND 
GEFS MEAN MOS GUIDANCE FOR TEMPERATURES.

BRITT
&&

.AVIATION...
/1115 PM CST MON DEC 21 2009/

FOR THE 06Z TAFS...MID AND HIGH CLOUDS REMAIN OVER THE AREA THROUGH 
THE FIRST 6 HOURS OF THE TAF VALID TIME. IN THE MEANTIME...WARM 
FRONT WILL BEGIN TO DEVELOP TO OUR SOUTH AND LIFT NORTHWARD INTO THE 
AREA BY MIDDAY TODAY. AS THE WARM FRONT APPROACHES WILL SEE SC 
REDEVELOP ...MAINLY BETWEEN 12Z AND 13Z...WITH KUIN AND KCOU SEEING 
MVFR CIGS AND KSUS/KSTL LOW END VFR. AS FRONT LIFTS NORTHWARD WILL 
SEE SC SCATTER OUT AT KCOU...KSUS AND KSTL BY 16Z/17Z. THEN AS WARM 
FRONT REMAINS STATIONARY OVERHEAD...WILL SEE SC/IFR CIGS MOVE BACK 
INTO CENTRAL AND EAST CENTRAL MO BY THIS EVENING. COULD SEE SOME 
PCPN DEVELOP LATE DURING THE DAY TODAY...BUT FEEL THAT BEST CHANCES 
WILL BE AFTER 06Z...SO JUST ADDED MENTION TO KSTL AFTER 09Z TONIGHT. 
WINDS WILL REMAIN EAST TO EAST SOUTHEAST THROUGH THE PERIOD.

BYRD
&&

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

$$
WFO LSX