FXUS63 KLSX 152319
AFDLSX

AREA FORECAST DISCUSSION...UPDATED AVIATION
NATIONAL WEATHER SERVICE ST LOUIS MO
519 PM CST TUE DEC 15 2009

.DISCUSSION...
/341 PM CST TUE DEC 15 2009/

COLD DECEMBER NIGHT IS ON TAP FOR THE REGION AS CENTER OF THE ARCTIC 
AIRMASS SLOWLY BUILDS INTO THE HEART OF THE CWA.  GIVEN THE FACT 
THAT TEMPS HAVE STRUGGLED TO WARM THIS AFTERNOON AND THAT CONDITIONS 
SHOULD BE IDEAL FOR RADIATIONAL COOLING...THE TRENDS OF THE CURRENT  
FORECAST TO GO AOB THE COLDEST MOS GUIDANCE STILL LOOKS LIKE THE WAY 
TO GO.  IN PARTS OF THE N OZARKS...HAVE GONE AN ADDITIONAL 2-3 
DEGREES BELOW THE COLDEST MET NUMBERS.

GOING FORECAST TRENDS WHICH INDICATE A GRADUAL MODERATION IN TEMPS 
FROM WED-THU ARE STILL SUPPORTED BY 12Z NWP OUTPUT...AS THE ARCTIC 
HIGH SLOWLY LOOSENS ITS GRIP ON THE AREA.   UPCOMING FORECAST 
PACKAGE WILL CONTINUE WITH THESE TRENDS...WITH ONLY MINOR 
MODIFICATIONS.

FORECAST BECOME INCREASINGLY DIFFICULT HEADING INTO FRIDAY AND 
SATURDAY...AND INTO THE START OF NEXT WEEK.   FIRST ITEM OF CONCERN 
IS THE THREAT OF PRECIPITATION ON FRIDAY...ASSOCIATED WITH THE FIRST 
IN A SERIES OF SHORTWAVES THAT ARE FORECAST TO DIG INTO AREA.  GFS 
IS THE MOST AGGRESSIVE WITH THE PRECIP ASSOCIATED WITH THIS SYSTEM 
...PRIMARILY DUE TO THE FACT THAT IT IS MOST AGRRESSIVE WITH THE 
MOISTURE ADVECTING INTO THE SYSTEM.  MEANWHILE...THE NAM REMAINS 
EXTREMELY DRY...WHILE THE GEM AND ECMWF ARE SOMEWHERE IN BETWEEN.  I 
HAVE STAYED WITH SLIGHT CHC POPS FOR THIS SYSTEM ON FRIDAY... 
PRIMARILY DUE TO THE FACT THAT WE WILL BE IN THE "WARM SECTOR" OF 
THE SYSTEM...WITH THE BEST BAROCLINICITY AND LIFT OCCURRING WELL N 
OF OUR CWA.  HOWEVER...HAVE STAYED WITH LOW CHANCE POPS ON FRIDAY 
NIGHT AND SATURDAY...AS SWD SURGE OF COLDER AIR AND ADDITIONAL 
SHORTWAVE ENERGY SHOULD BE ABLE TO SQUEEZE OUT SOME LIGHT 
PRECIPITATION.   

FORECAST CONFIDENCE QUITE LOW AS WE HEAD INTO NEXT WEEK. THERE IS 
STILL CONSENSUS THAT MEAN TROF WILL BE FORMING OVER THE EASTERN HALF 
OF THE CONUS...WHICH CERTAINLY SUGGESTS THAT THE THEME OF BELOW 
NORMAL TEMPERATURES IN CURRENT FORECAST IS STILL VERY APPROPRIATE. 
HOWEVER...MODELS ARE WILDLY DIVERGENT ON SPECIFICS OF THIS 
EVOLUTION.  EARLIER SOLUTIONS...GENERATED ON SUNDAY AND 
MONDAY...SUGGESTED A LARGE ELONGATED VORTEX FORMING OVER THE GREAT 
LAKES/NE US WITH LOWS SPIRALLING AROUND THIS FEATURE...BUT TODAYS 
RUNS ARE BREAKING OFF THE LOWS INTO MORE DISTINCT ENTITIES.   THIS 
MAKES THE NUANCES OF DAY TO DAY TEMPERATURE TRENDS ARE ALMOST 
IMPOSSIBLE TO PIN DOWN WITH ANY RELIABILITY.  OTHER QUESTION DURING 
THE MEDIUM RANGE IS PRECIPITATION THREAT AFTER THE PASSAGE OF THE 
FRI/SAT SYSTEM.  THERE WOULD BE ANOTHER CHANCE OF SNOW BY LATE IN 
THE PERIOD IF AN UPPER LOW DOES DROP INTO THE REGION AS SUGGESTED BY 
THE GFS/ECWMF...BUT GIVE THE EXTREME FLIP-FLOP IN SPECIFICS OF FLOW 
REGIME DON'T HAVE ENOUGH CONFIDENCE TO INTRODUCE PRECIP AT THIS 
TIME.

TRUETT

&&

.AVIATION...
/516 PM CST TUE DEC 15 2009/

FOR THE 00Z TAFS...WINDS WILL BECOME LIGHT AND VRB TONIGHT UNDER
CLEAR SKIES AS HIGH PRESSURE MOVES OVERHEAD. WINDS WILL BECOME S
TO SSE ON WED AT AROUND 5 TO 8 KTS AS THE HIGH MOVES E. CI WILL
MOVE INTO THE REGION WED AFTERNOON. OTHERWISE...SKIES WILL REMAIN
CLEAR. 

TILLY

&&

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

$$

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