FXUS63 KLSX 052342
AFDLSX

AREA FORECAST DISCUSSION...UPDATED AVIATION
NATIONAL WEATHER SERVICE ST LOUIS MO
542 PM CST SAT DEC 5 2009

.DISCUSSION...
/300 PM CST SAT DEC 5 2009/

MAIN CONCERN CONTINUES TO FOCUS ON TWO SYSTEMS THAT WILL AFFECT THE 
AREA. ONE WILL QUICKLY PASS THROUGH THE MO/IL ON SUNDAY NIGHT AND
ANOTHER LATE TUESDAY INTO WEDNESDAY THAT PROMISES TO BE A MAJOR
WINTER STORM FOR PARTS OF THE MIDWEST.

TIMING IS GOOD WITH THE LEAD SHORTWAVE TROUGH EJECTING FROM THE WEST 
COAST CLOSED LOW THE NEXT 36-48 HOURS AND PLAN NO SIGNIFICANT 
CHANGES TO GOING FORECAST.  LARGE SURFACE CYCLONE EXTENDING FROM THE 
OHIO RIVER VALLEY INTO THE LOWER MISSISSIPPI VALLEY WILL CONTINUE TO 
PROVIDE DRY SELY LOW LEVEL FLOW THROUGH MOST OF TOMORROW.  TIME 
CROSS SECTIONS ARE SHOWING NARROW WINDOW OF DECENT ASCENT/DEEP 
ATMOSPHERIC MOISTURE MOVING EWD ACROSS THE REGION ON SUNDAY
NIGHT...SO HAVE GONE WITH LIKELY CHANCES OF LESS THAN A TENTH OF
AN INCH OF PRECIPITATION. QUICK MOVEMENT OF THE SYSTEM WILL KEEP
PRECIP AMOUNTS LIGHT. HAVE KEPT THE IDEA OF GOING WITH FLURRIES OR
LIGHT FZDZ LATE TOMORROW NIGHT/MONDAY IN THE WAKE OF THE BEST
ASCENT AS LOWER ATMOSPHERE REMAINS MOIST.

THINK MAIN INTEREST IS WHAT KIND AND HOW MUCH WINTER WEATHER WE WILL 
HAVE MIDWEEK.  SEEING GOOD AGREEMENT BETWEEN THE GFS/ECMWF/CANADIAN 
GEM WITH OPENING UP THE CLOSED LOW OVER THE WEST COAST ON MONDAY AND 
MOVING QUICKLY EWD ACROSS THE MIDWEST.  THERE ALSO HAS BEEN GOOD 
CONTINUITY WITH THE THREE MODELS IN FAVORING THE MOST LIKELY TRACK 
FOR HEAVY SNOW FROM NERN KS...ACROSS NRN MO/SRN IA INTO NRN IL.  
HAVE EXPANDED LIKELY POPS ACROSS MOST OF THE AREA ON TUESDAY...WITH 
CATEGORICAL POPS OVER NERN MO/W CNTRL IL TUESDAY NIGHT AND LIKELY 
POPS ELSEWHERE WHEN THERE WILL BE STRONG ASCENT AHEAD OF THE MID 
LEVEL TROUGH/LOW LEVEL MOISTURE CONVERGENCE.  FORECAST SOUNDINGS 
FAVOR A RAIN/SNOW MIX OVER THE SERN 2/3RDS OF THE CWA ON TUESDAY 
WITH MAINLY SNOW OVER THE NORTH...AND THEN RAIN CHANGING TO SNOW 
FROM NW TO SE ON TUESDAY NIGHT.  SFC LOW IS SHOWING SIGNIFICANT 
DEEPENING AS IT MOVES THROUGH ON TUESDAY NIGHT WHICH WILL TIGHTEN 
THE PRESSURE GRADIENT CAUSING WINDY CONDITIONS AS THE SYSTEM EXITS 
THE AREA LATE TUESDAY NIGHT INTO WEDNESDAY.  HAVE ADDED AREAS OF 
BLSN OVER NERN MO/W CNTRL IL DURING THE TIME BASED ON THE EXPECTATION 
OF ACCUMULATING SNOW. 

MEDIUM RANGE MODELS ARE IN GENERAL AGREEMENT THAT MIDWEEK TROUGH 
WILL MOVE QUICKLY EWD LEAVING MAINLY ZONAL FLOW ACROSS THE CONUS 
WITH A DEEP POLAR VORTEX OVER CNTRL CANADA.  MODELS ARE SHOWING 
ANOTHER WEAK SHORTWAVE TROUGH RACING ACROSS THE MIDWEST LATE IN THE 
WEEK...WITH THE ECMWF MOVING IT THROUGH QUICKER THAN THE GEM OR THE 
GFS.  TRYING TO TIME THIS SYSTEM IN THE ZONAL FLOW WILL BE 
DIFFICULT...SO HAVE JUST SLIGHT CHANCES FOR -R/-S GOING THURSDAY 
INTO FRIDAY FOR NOW.  TEMPERATURES ARE A MIX OF PERSISTENCE...GEFS 
MEAN MOS AND HPC GUIDANCE.

BRITT

&&

.AVIATION...
/518 PM CST SAT DEC 5 2009/

FOR THE 00Z TAFS...GENERALLY SLY WINDS WILL CONTINUE THROUGH SUN
MORNING AS AN AREA OF HIGH PRES OVR THE OHIO VALLEY MOVES EWD.
WINDS WILL THEN TURN MORE SELY ON SUN AFTN AS A LOW PRES SYS OVER
THE SRN PLAINS APPROACHES THE BISTATE REGION. MID AND HIGH LVL
CLDS WILL INCREASE ON SUN AS THE SYSTEM APPROACHES.

KANOFSKY

&&

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

$$

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