FXUS63 KLSX 051108
AFDLSX

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

.DISCUSSION...
/444 AM CST SAT DEC 5 2009/

TRANSITION TO A ZONAL FLOW PATTERN AT H500 WILL OCCUR TODAY WITH 
THIS ZONAL FLOW PATTERN TO REMAIN LARGELY UNHINDERED UNTIL THE 
MIDDLE PART OF NEXT WEEK. THIS WILL PLACE US ON A FAVORED PATH TO 
GET IMPACTED BY QUICK MOVING STORM SYSTEMS EVERY COUPLE OF DAYS OR 
SO...WITH THE FIRST SLATED FOR SUNDAY NIGHT AND THE SECOND FROM LATE 
TUESDAY THRU EARLY WEDNESDAY. THE LONGWAVE H500 TROF THAT HAD BEEN 
FOCUSED OVER THE CNTRL CONUS WILL RETROGRADE TO THE WRN CONUS AS A 
RESULT...TEMPORARILY...BEFORE PUSHING BACK TO THE CNTRL CONUS AGAIN 
DURING THE MIDDLE OF NEXT WEEK AND CONTG THRU LATE WEEK. THIS 
FORETELLS TEMPS MODERATING TOWARDS SEASONABLE LEVELS FOR EARLY NEXT 
WEEK WITH ANOTHER SIGNIFICANT SHOT OF COLD AIR BEHIND THE SECOND 
STORM ON WEDNESDAY. 

LOOK FOR SUNNY SKIES TODAY WITH TEMPS STRUGGLING TO EXIT THE 30S. 
METMOS TEMPS ONCE AGAIN APPEAR TO HAVE A GOOD HANDLE ON THE 
SITUATION AS VERY WEAK DECEMBER SUN WILL HAVE LITTLE IMPACT ON 
EXCEEDING TYPICAL CLIMO UPSWINGS OF ABOUT 20F FROM MORNING MIN 
TEMPS. 

CLOUDS WILL BEGIN TO INCREASE LATE TONIGHT WITH THE FA GOING MOCLDY 
ON SUNDAY WITH APPROACH OF FIRST STORM. THIS WILL BE AN OPEN MID 
LEVEL WAVE THAT WILL MOVE THRU FAST...AND WHILE MOISTURE WILL RACE 
NWD TO INTERACT WITH IT...THERE WILL BE A RELATIVELY SMALL WINDOW 
FOR PCPN TO ACTUALLY FORM AND THREATEN TO MEASURE AT THE SFC...WHICH 
WILL LARGELY BE THIS EVENING. WHILE THERE IS STILL SOME MODEL SPREAD 
ON THE EXACT TRACK OF THE MID LEVEL SYSTEM...GENERAL CONSENSUS IS 
FOR A NRN ONE THRU NWRN MO AND SERN IA. AREAS TO THE S OF THESE 
QUICK MOVING SYSTEMS OF SIMILAR STRUCTURE USUALLY STRUGGLE TO GET 
MEASURABLE PCPN...AND FOR THIS REASON...HAVE HEAVILY FAVORED THE 
DRIER MAVMOS POPS AND IN MANY CASES WENT EVEN BELOW THAT TO AROUND 
50PCT. BELIEVE PCPN WILL FALL OVER MOST AREAS...BUT IT WILL STRUGGLE 
TO ACTUALLY MEASURE AND ANY PCPN SHOULD BE VERY LIGHT. BEST CHC FOR 
ACCUMULATING SNOW WILL BE IN NERN MO AND WCNTRL IL WHICH WILL BE 
CLOSER TO THE BETTER UPPER SUPPORT AND FURTHER IN THE COLDER AIR... 
WHERE AROUND A HALF INCH MAY FALL...WITH A DUSTING OR NOTHING 
ELSEWHERE. HAVE ADDED IN DZ/FZDZ INTO THE FORECAST FOR LATE SUNDAY 
NIGHT AND MONDAY MORNING...AS MODELS HAVE STRENGTHED THEIR SIGNAL 
THEY WERE SHOWING LAST NIGHT FOR A DRYING OF THE MID LEVELS OF THE 
ATMOSPHERE WHILE BRINGING IN ABUNDANT LO CLOUD IN A WEAK LIFT 
ENVIRONMENT WITH CYCLONIC FLOW ON THE BACKSIDE OF THE LO PRES WAVE 
AND TRAILING FRONT. MODEL SOUNDINGS SHOW CLOUD TOP TEMPS TO RESIDE 
BETWEEN -5 AND -7C WHICH WILL MAKE IT DIFFICULT FOR MUCH IN THE WAY 
OF ICE NUCLEI. LO CLOUDS SHOULD FINALLY RECEDE BY MONDAY AFTN. 

LITTLE IN THE WAY OF A BREAK WILL OCCUR BEFORE SYSTEM NUMBER TWO 
ARRIVES BY TUESDAY AFTN. THE 00Z/06Z RUNS OF THE MODELS HAVE COME 
INTO BETTER CONSENSUS REGARDING THE TRACK AND TIMING OF THE SYSTEM. 
LIKE WITH THE FIRST SYSTEM...THIS CONSENSUS FAVORS A MORE NLY TRACK 
THAN SOME PREVIOUS RUNS HAD INDICATED...WITH THE H700 LO TRACKS THRU 
FAR NRN MO AND FAR NWRN IL. THIS H700 TRACK WILL INDICATE WHERE WE 
ARE EXPECTING THE HEAVIEST SNOW TO FALL...WITH SEVERAL INCHES 
POSSIBLE. S OF THIS LINE...WHICH INCLUDES VIRTUALLY ALL OF THE 
FA...IT IS APPEARING MORE LIKELY FOR GREATER PERIODS OF LIQUID PCPN 
WITH PERHAPS A BRIEF CHANGEOVER TO SNOW AT THE END. THE INTITIAL 
ONSET OF THE SYSTEM ALSO PRESENTS A CHALLENGE AS LOWER ATMOSPHERE 
INTITALLY SATURATES...AND MAY ALSO SEE AN EXPANDED COVERAGE OF MIXED 
OR FROZEN PCPN LATE TUESDAY BEFORE WAA FROM MAIN SYSTEM WARMS THE 
ATMOSPHERE ENOUGH TO GO ALL LIQUID. 

ONE THING WE ARE REASONABLY CERTAIN ABOUT...SOMEONE IN THE MIDWEST 
WILL GET A LOT OF SNOW AND IT WILL GET QUITE COLD AND WINDY IN THE 
WAKE OF THIS STORM ON WEDNESDAY AND INTO THURSDAY.

TES

&&

.AVIATION...
/505 AM CST SAT DEC 5 2009/

FOR THE 12Z TAFS...VFR CONDITIONS WILL PERSIST THRU THE NEXT 24
HOURS AT ALL TAF SITES. WINDS WILL REMAIN SLY TO SSWLY THRU END OF
THE FORECAST PERIOD AS THE NEXT SYSTEM APPROACHES THE REGION SUN
MORNING. AS THIS SYSTEM NEARS...CLOUDS WILL SPREAD OVER THE AREA.
ONLY CI TO MID LEVEL CLOUDS ARE EXPECTED THRU 12Z SUN. 

TILLY

&&

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

$$

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