FXUS63 KLSX 012155
AFDLSX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ST LOUIS MO
355 PM CST TUE DEC 1 2009

.DISCUSSION...
/355 PM CST TUE DEC 1 2009/

THE MODELS CONTINUE TO CONVERGE ON A PREFERRED TRACK FOR THE MID 
LEVEL PORTION OF THE SYSTEM FOR OUR FIRST WINTER STORM OF THE 
SEASON...AND IT IS VERY CLOSE TO THE 12Z GEM TRACK FROM YESTERDAY... 
WHICH WILL TAKE THE CENTER THRU SERN MO AND SERN IL. THE GFS REMAINS 
THE OUTLIER FURTHER TO THE S AND E AND HAS BEEN TOSSED. A COUPLE OF 
CHANGES TO THIS SET OF MODEL GUIDANCE FROM PREVIOUS RUNS IS THAT 
THEY HAVE AGAIN BECOME A LITTLE SLOWER WITH THE INITIAL ONSET...NWD 
DRIVE...AND THE LO LEVEL COLD AIR IS A TAD SLOWER IN MOVING THRU 
BEHIND THE EXPECTED SFC CD FROPA FOR MOST AREAS ON WED MORNING. 
OTHERWISE...MUCH OF WHAT WE WERE EXPECTING FROM BEFORE REMAINS THE 
SAME OTHER THAN A TAD SLOWER RAIN TO SNOW CHANGEOVER WHICH WILL 
ULTIMATELY REDUCE EXPECTED SNOW ACCUMS A BIT. 

WE ARE EXPECTING TWO AREAS OF SNOW ACCUMULATION...ONE SYNOPTIC SCALE 
WHICH WILL FOLLOW ROUGHLY ALONG AND JUST TO THE LEFT OF THE H700 LO 
TRACK AND THIS WILL TAKE IT THRU SERN MO AND SWRN IL AND PARTS OF 
STL METRO EAST IN ILLINOIS. BUT THE SLOWER CHANGEOVER TO 
SNOW...WHICH APPEARS MORE LIKELY NOW AROUND SS OR EARLY TO MID 
EVENING...DELAYED BY ABOUT 3HRS FROM YESTERDAY...WILL LIMIT SNOW 
ACCUMS TO LESS THAN AN INCH. DUE TO THE LACK OF COLD AIR...IT 
APPEARS THAT THIS WILL END UP BEING LARGELY A DISAPPOINTMENT FOR 
SNOW ACCUMULATION. 

THE OTHER AREA OF SNOW ACCUMULATION WILL BE MORE MESOSCALE...OF 
WHICH THE MODELS HAVE BEEN DEPICTING TO THE BEST OF THEIR ABILITY 
FOR THE PAST TWO DAYS NOW DEPENDING ON THE MESH OF THEIR GRID. THIS 
BAND WILL BE THE RESULT OF STRONG DIVERGENCE FROM H300 JET COUPLING 
AND STRONG MID LEVEL FRONTOGENESIS...ALTHOUGH THE FRONTOGENESIS 
MAXES OUT MUCH HIGHER THAN WHAT WE NORMALLY WOULD EXPECT...BTWN 
H500-400. GIVEN THIS...SHOULD SEE NARROW BUT INTENSE BAND OF PCPN 
DEVELOP ONCE THE LO LEVELS SATURATE. THERE IS NOT ENOUGH SKILL AT 
THIS TIME TO ACCURATELY NAIL DOWN THE AREA...BUT SHOULD BE NEAR A 
KPPQ-KJEF LINE...PERHAPS A TAD SE. THIS SHOULD END UP BEING THE 
BIGGER SNOW PRODUCER THAN THE SYNOPTIC SCALE DEF ZONE DUE TO IT 
BEING FURTHER BACK IN THE COLD AIR. WE HAVE PENCILED IN UP TO A 
COUPLE INCHES OF SNOW...BUT WOULD NOT BE SURPRISED A FEW ISOLATED 
SPOTS MAY SEE 3 OR 4. 

IN BETWEEN THESE AREAS...RIGHT NOW WE ARE LOOKING AT THE WRN SUBURBS 
OF STL METRO...THERE WILL BE A LULL IN THE LIFT AND DRY AIR TO 
OVERCOME AND THIS AREA MAY NOT SEE MUCH AT ALL AND A DELAYED ONSET 
FROM OTHER AREAS. 

THINGS SHOULD ALL WIND UP BY EARLY EVENING FOR MOST PLACES...AND 
LINGER A BIT LONGER OVER PARTS OF SWRN IL TIL MIDNIGHT. THEN THE 
COLD AIR DIGS IN FOR A FEW DAYS...AND FORECASTED AOB THE COLDEST MOS 
TEMPS DURING THE DAY...AND LEANED A BIT CLOSER AT NIGHT FOR MINS. 

FOR THE EXTENDED PERIOD (FRIDAY NIGHT - TUESDAY)...MODELS ARE 
FINALLY BEGINNING TO CONVERGE ON A PREFERRED SOLUTION FOR THE 
WEEKEND AND THIS REASONABLE AGREEMENT EVEN CONTINUES INTO EARLY NEXT 
WEEK ALTHOUGH SOME DIVERGENCE DOES OCCUR. THE MODELS HAVE NOW 
SETTLED ON MAINTAINING THE TROF OVER THE GRTLKS REGION THRU MUCH OF 
SATURDAY WHICH WILL MEAN ONE MORE COLD DAY. FEEL THAT THE TROF WILL 
FADE ENOUGH DURING THE DAY TO NOT WARRANT FLURRIES MENTION HOWEVER. 
FROM SUNDAY ON...ZONAL FLOW WILL BE THE RULE WITH A SERIES OF 
SHORTWAVES MAINLY SKIRTING THE FA TO THE N. THIS WILL TEND TO 
MODERATE TEMPS BACK TO NORMAL AND WILL MAINTAIN DRY FCST...AS WE 
WOULD LIKE TO SEE SOME CONTINUITY BEFORE INTRODUCING MENTIONABLE 
POPS.

TES

&&

.AVIATION...
/1100 AM CST TUE DEC 1 2009/

FOR THE 18Z TAFS...SLY WINDS WITH CLEAR SKIES WILL PERSIST FOR
THE AFTERNOON. CI WILL MOVE INTO THE REGION WITH CIGS GRADUALLY
LOWERING OVERNIGHT AND INTO WED MORNING. WINDS WILL BACK TO BECOME
NRLY WED MORNING. LATEST GUIDANCE SUGGESTS PRECIP WILL HOLD OFF
UNTIL AFTER 18Z WED AT ALL SITES...WITH PRECIP CHANGE OVER TO SN
AT STL AROUND OR JUST AFTER 00Z THURSDAY. SINCE ANY PRECIP IS NOT
EXPECTED TO START UNTIL LATE WED AFTERNOON...WILL NOT MENTION ANY
PRECIP AT THIS TIME FOR STL. 

TILLY

&&

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

$$

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