FXUS63 KLSX 230527
AFDLSX

AREA FORECAST DISCUSSION...UPDATED AVIATION
NATIONAL WEATHER SERVICE ST LOUIS MO
1125 PM CST TUE DEC 22 2009

.UPDATE...
/830 PM CST TUE DEC 22 2009/

02Z OBSERVATIONAL DATA SHOWS QUASI-STNRY BDRY ARRAYED W-E FROM
WCNTRL MO THRU CNTRL MO TO THE ERN OZARKS OF SERN MO. THIS FRONT
HAS MOVED LITTLE AND WITH SFC CAA KNOCKING AT ITS GATES THANKS TO
PERSISTENT ELY FLOW TO ITS N...THIS SHOULD KEEP IT AT BAY FOR THE
NEXT SEVERAL HOURS RESULTING IN NEAR STEADY TEMPS FOR MUCH OF THE
FA. THE FRONT IS NOT EXPECTED TO GET RENEWED IMPETUS TO MOVE NEWD
UNTIL MID LEVEL FLOW BACKS MORE IN RESPONSE TO RIDGE SLIDING MORE
TO OUR E...WHICH SHOULD HAPPEN BY 12Z/WED...AND SO HAVE TEMPS
EDGING UPWARD BY SUNRISE.

PCPN TRENDS ARE ALSO SLOWER TO EVOLVE THAN PREVIOUSLY THOUGHT.
SHRA DEVELOPING NICELY IN SERN MO IN RESPONSE TO INCRSG ISENTROPIC
LIFT AND LO LEVEL JET NOSING INTO REGION. THIS REGION SHOULD
ENABLE SHRA ACTIVITY TO SPREAD NEWD FIRST...BEFORE MAIN SURGE
COMES LATE TONIGHT WITH ADVANCING FRONT TO OVERSPREAD MUCH OF THE
BI-STATE.

TES
&&

.DISCUSSION...
/252 PM CST TUE DEC 22 2009/

MAIN CONCERN WILL BE POTENTIAL FOR FLOODING THE NEXT FEW DAYS 
FOLLOWED BY A CHRISTMAS AND DAY AFTER THAT CERTAINLY WILL FEEL LIKE 
WINTER.  HAVE ENOUGH CONFIDENCE AT THIS TIME TO ISSUE A FLOOD WATCH 
FROM MIDDAY WEDNESDAY THROUGH LATE THURSDAY.  UPPER LOW NOW OVER THE 
WRN CONUS WILL DIG SEWD INTO THE FOUR CORNERS BY TOMORROW AND THEN
EJECT OUT INTO THE PLANS BY THURSDAY NIGHT. WHILE THERE ARE SOME
DIFFERENCES BETWEEN THE MODELS...EACH AGREE THAT THERE WILL BE A
STRONG COLD FRONT MOVE ACROSS MO/IL ON THURSDAY AT THE SAME TIME
THE UPPER TROUGH BECOMES NEGATIVELY TILTED. GENERALLY FOLLOWED THE
GFS GUIDANCE WHICH FALLS IN THE MIDDLE OF THE SPREAD.

CHANCES OF RAIN WILL INCREASE FROM SOUTH TO NORTH TONIGHT AS 925-850
MOISTURE CONVERGENCE DEVELOPS BY LATE THIS EVENING. LARGE SCALE
ASCENT WILL ALSO INCREASE AS WEAK SHORTWAVE TROUGH CURRENTLY OVER
THE CENTRAL HIGH PLAINS MOVES INTO THE AREA. THIS FORCING WILL
ONLY INCREASES AS MAIN TROUGH BECOMES NEGATIVELY TILTED...SO
INTENSITY OF RAINFALL WILL INCREASE WEDNESDAY AFTERNOON AND
CONTINUE INTO EARLY THURSDAY. SCATTERED TSRA IS STILL A GOOD IDEA
AS COVERAGE OF CONVECTION TENDS TO INCREASE WITH SUCH STRONG
DYNAMIC FORCING EVEN THOUGH THE ATMOSPHERE WILL ONLY BE SLIGHTLY
UNSTABLE. TSRA CERTAINLY WILL BE ABLE TO GENERATE LOCALLY HEAVY
RAINFALL. GIVEN PWATS INCREASING TO ANOMALOUSLY HIGH 1+ INCHES...
EXPECT RAINFALL AMOUNTS WILL RANGE FROM 1.5" OVER CNTRL AND NERN
MO TO AROUND 3" OVER PARTS OF SERN MO INTO S CNTRL ILLINOIS. THE
GROUND IS FROZEN OVER ALL BUT THE CNTRL/SERN MO COUNTIES...SO ANY
RAINFALL WILL BE IMMEDIATE RUNOFF IN THESE LOCATIONS. RAINFALL
AMOUNTS OVER SERN MO WILL APPROACH FLASH FLOOD GUIDANCE...SO THINK
THESE COUNTIES NEED ALSO TO BE INCLUDED IN THE WATCH. KEPT PART OF
THE CNTRL MO COUNTIES OUT FOR NOW AS THE GROUND IS NOT FROZEN AND
RAINFALL AMOUNTS ARE NOT EXPECT TO REACH FFG.

FRONT WILL RACE THROUGH THE AREA ON THURSDAY NIGHT CAUSING THE RAIN 
TO CHANGE TO SNOW AS IS IT DOES.   ONE DIFFERENCE THAT THE 12Z 
MODELS INTRODUCED WAS TO SPLIT THE 850MB LOW TRACK...CAUSING ONE
INITIAL TRACK TO MOVE NORTHWARD THROUGH ERN KS/NE ON THURSDAY...AND
A SECOND ONE FARTHER SE THAT WILL NWD MOVE ACROSS CNTRL MO ON
THURSDAY NIGHT. THIS MEANS THAT THE FAR WRN/NWRN PART OF THE CWA
WILL SEE SOME SMALL SNOW ACCUMULATION FROM THIS SECOND DEFORMATION
ZONE.

FRIDAY INTO THE WEEKEND WILL DEFINITELY FEEL LIKE WINTER AS THERE IS 
DECENT AGREEMENT BETWEEN THE ECMWF/GFS THAT SECOND UPPER LOW WILL 
DIVE DOWN INTO MEAN TROUGH AND LINGER OVER MO/IL.  FORECAST 
SOUNDINGS FROM THE ECMWF/GFS SHOW STEEP LOW LEVEL LAPSE RATES ALONG 
WITH ABUNDANT MOISTURE AND CYCLONIC TURNING IN THE LOW LEVELS.  HAVE 
GONE WITH SCATTERED SNOW SHOWERS FRIDAY INTO SATURDAY WITH FLURRIES 
LINGERING INTO SUNDAY.  UPPER LOW THEN PULLS OUT AND RIDGING BUILDS 
IN EARLY NEXT WEEK...SO HAVE GONE DRY.  HAVE GONE WITH COOLER HPC 
GUIDANCE AND GEFS MEAN MOS FOR HIGH TEMPS OVER THE WEEKEND GIVEN THE 
CLOUDS.

BRITT
&&

.AVIATION...
/1125 PM CST TUE DEC 22 2009/

FOR THE 06Z TAFS...SHOWERS...WITH SOME ISOLATED TRWS...HAVE BEEN 
DEVELOPING OVER SOUTHERN MISSOURI THIS EVENING AND SPREADING TO THE 
NORTH TOWARDS THE TAF FORECAST SITES. AS FOR TIMING OF HEAVIER 
PCPN...BEST CHANCES WILL BE AFTER 08Z...THOUGH MORE TOWARDS 12Z FOR 
KUIN. SINCE NOT SURE WHERE TRWS WILL POP UP...ADDED CB MENTION TO 
FORECAST. MVFR CIGS AT KCOU...KSUS AND KSTL WILL LOWER TO IFR BY 08Z 
WITH THE HEAVIER RAIN AND REMAIN THERE THROUGH THE REST OF THE VALID 
TAF TIME. KUIN CIGS TO REMAIN LIFR THROUGH THE FORECAST PERIOD. WILL 
SEE RAIN LET UP SOME DURING THE LATE MORNING AND AFTERNOON HOURS 
TODAY...WITH ANOTHER HEAVY ROUND BY THIS EVENING...AFTER 01Z/02Z 
TIMEFRAME. WINDS TO PERSIST FROM THE EAST EARLY ON THEN VEER TO THE 
SOUTHEAST AND INCREASE TO AROUND 15 KTS.

BYRD
&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...FLOOD WATCH FROM WEDNESDAY AFTERNOON THROUGH THURSDAY AFTERNOON 
     FOR AUDRAIN-CRAWFORD-FRANKLIN-GASCONADE-IRON-JEFFERSON-KNOX-
     LEWIS-LINCOLN-MADISON-MARION-MONROE-MONTGOMERY-PIKE-RALLS-
     REYNOLDS-SHELBY-ST. CHARLES-ST. FRANCOIS-ST. LOUIS-ST. 
     LOUIS CITY-STE. GENEVIEVE-WARREN-WASHINGTON.

IL...FLOOD WATCH FROM WEDNESDAY AFTERNOON THROUGH THURSDAY AFTERNOON 
     FOR ADAMS IL-BOND IL-BROWN IL-CALHOUN IL-CLINTON IL-FAYETTE 
     IL-GREENE IL-JERSEY IL-MACOUPIN IL-MADISON IL-MARION IL-
     MONROE IL-MONTGOMERY IL-PIKE IL-RANDOLPH IL-ST. CLAIR IL-
     WASHINGTON IL.
&&

$$
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