FXUS63 KLSX 250528
AFDLSX

AREA FORECAST DISCUSSION...UPDATED AVIATION
NATIONAL WEATHER SERVICE ST LOUIS MO
1128 PM CST THU DEC 24 2009

.UPDATE...
/1006 PM CST THU DEC 24 2009/

STRONG LO PRES WINDING UP OVER SCNTRL MO AS IT BEGINS TO TAKE MORE
OF A TRUE NWD TURN THRU THE SHOW-ME STATE. THE NEW 00Z NAM/GFS
SEEM TO PICK UP ON THIS VERY WELL AND ARE ALSO IN LINE WITH
PREVIOUS MODEL GUIDANCE FROM TODAY. RADAR SHOWS DEFORMATION ZONE
WELL DEVELOPED NOW STRADDLING THE MO/KS BORDER WITH SOME FILLING
IN EXPECTED OVERNIGHT ON THE ERN PERIPHERY AS FAR BACK AS CNTRL
AND NERN MO BY 12Z. THE REMAINDER OF THE AREA WILL GRADUALLY GET
DRY SLOTTED AS STEADY PCPN GIVES WAY TO OFF-AND-ON PCPN. 

PREVIOUS FORECAST HAS SFC TEMP FORECAST WELL IN HAND AND ONLY HAD
TO MAKE MINOR ADJUSTMENTS IN LIGHT OF LATEST TRENDS...WHICH HAVE
COLDER AIR BEHIND LO STRONGER. 2M NAM TEMPS SEEM TO HANDLE THIS
VERY WELL AND HAVE TRENDED IN THIS DIRECTION. 

LOOK FOR A SHARP TRANSITION ZONE TO PCPN...FROM RAIN TO SNOW...AS
COLDER AIR RAPIDLY SPILLS NEWD BEHIND LO BUT A BRIEF PERIOD OF
FZRA/PL IS EXPECTED AS WELL AS WARM AIR ALOFT BEING WRAPPED AROUND
THE TIGHLY WOUND STORM SYSTEM IS SQUEEZED OUT BUT AT A SLOWER RATE
THAN THE SFC AIR. NO CHANGES TO SNOW FORECAST WERE MADE EXCEPT IN
WCNTRL IL WHERE DRY SLOT SHOULD KEEP DEF ZONE TO THE W OVER NERN
MO AND LIMIT ANY SNOW ACCUMS THERE UNTIL DISTURBANCES ROTATE
AROUND THE BACKSIDE FRIDAY NIGHT AND INTO SATURDAY. 

TES
&&

.DISCUSSION...
/322 PM CST THU DEC 24 2009/

SHORT RANGE (TONIGHT - SAT NIGHT)...

MAIN ISSUES DURING THIS PERIOD CONTINUE TO REVOLVE AROUND RAIN AND
FLOODING POTENTIAL THIS EVENING THEN WINTER WEATHER. FORTUNATELY
TODAY'S MODEL RUNS ARE IN VERY CLOSE AGREEMENT THROUGH THE PERIOD
EJECTING THE NORTH TEXAS UPPER LOW NEWD INTO ERN MO BY 12Z FRIDAY IN
RESPONSE TO THE UPPER LOW DIGGING SEWD INTO THE CENTRAL PLAINS.
THE UPPER LOWS THEN CONSOLIDATE OVER MO ON FRIDAY EVENTUALLY
MOVING INTO THE WESTERN GREAT LAKES REGION/UPR MS VLY BY SAT
EVENING. THE CMC GEM WHICH HAD BEEN AN EASTERN OUTLIER HAS NOW
SHIFTED WEST AND THE ENVELOPE OF SOLUTIONS OF THE SFC/85H LOW ARE
TIGHTLY CLUSTERED WITHIN 45 OR SO MILES. THE ATTENDANT SFC LOW
CURRENTLY IN WEST CENTRAL AR IS FORECAST TO MOVE VERY CLOSE TO A
JEFFERSON CITY TO NOVELTY LINE FROM 06-12Z TONIGHT. THERE IS NOT
ONLY VERY GOOD AGREEMENT WITH THIS TRACK IN THE NWP GUIDANCE BUT
THE LATEST SFC PRESSURE FALLS SUPPORT THIS TRACK WITH THE MAX
FALLS FROM NE ARKANSAS INTO WEST CENTRAL MO. ANOTHER PRESSURE FALL
MAX FROM SRN IL INTO SRN IN IS IN RESPONSE TO STRONG WAA.

GIVEN THIS TRACK THE COLD AIR ARRIVAL WILL BE DELAYED TIL THE SFC
LOW PASSAGE WITH THE CAA COMING FROM THE SOUTHWEST AS COLD AIR IS
WRAPPED CYCLONICALLY ABOUT THE NEWD LIFTING LOW. RAIN WILL
CONTINUE TO BE THE MAIN STORY THIS EVENING WITH RADAR SHOWING A
WALL OF WATER ACROSS ERN AR INTO SE MO HEADING NORTH. STRONG LOW
LEVEL MCON...STRONG LARGE SCALE ASCENT AND DIVERGENCE ALOFT ARE
ALL FOCUSED ACROSS ERN MO THIS EVENING SUPPORTING THE RADAR TRENDS
AND POTENTIAL FOR HEAVY RAIN THIS EVENING. WARM SECTOR FORCING
THEN SHIFTS TO THE EAST AND NORTHEAST OF THE CWA OVERNIGHT AND
THIS SHOULD END THE HEAVY RAIN THREAT. THUS THE CURRENT
CONFIGURATION OF THE FFA LOOKS ON TRACK ENDING AT 06Z.

DEFORMATION ZONE PCPN WILL THEN BE THE MAIN ELEMENT OVERNIGHT. THE
GREATEST FORCING WILL BE FOCUSED ACROSS WESTERN MO GIVEN THE
AFOREMENTIONED LOW TRACKS. THE POSITION OF THIS FORCING ALONG
WITH THE SLOWER ARRIVAL OF DEEPER COLD AIR TIL AFTER 09Z IN MID
MO...SHOULD REDUCE THE THREAT OF ACCUMULATING SNOWS THROUGH 12Z
SAT. BASED ON MODEL SOUNDINGS THE CHANGEOVER AT KCOU MAY NOT BE
UNTIL 10-11Z OR SO...WITH MAYBE 1-2 INCHES ACROSS PORTIONS OF
MONITEAU....BOONE AND COLE COUNTIES. THERE COULD BE A BRIEF PERIOD
OF MIXED PCPN BUT THE SOUNDINGS SUGGEST IT WOULD VERY SHORT LIVED. 

STRONG CAA WILL ENSUE THROUGHOUT CHRISTMAS DAY WITH FALLING TEMPS
AND BLUSTRY CONDITIONS DUE TO GUSTY SWLY WINDS. FORCING WILL BE
DIMINISHING AS THE UPPER LOWS CONSOLIDATE WITH GREATEST POTENTIAL
FOR ACCUMULATING SNOW REMAINING ACROSS CENTRAL AND NE MO WHERE
ANOTHER 1 INCH OR SO IS REASONABLE. GIVEN THAT THE FORECAST
AMOUNTS IN THE TONIGHT AND FRIDAY TIME FRAME FALL BELOW ADVISORY
CRITERIA...THE WATCH WILL BE CANCELLED AND WON'T BE CONVERTED TO
ANOTHER HEADLINE. LATER SHIFTS CAN ADDRESS NEED FOR AN ADVSRY IF
THAT NEED MATERIALIZES.

THE PATTERN LOOKS QUITE FAVORABLE FOR SNOW SHOWERS ON SATURDAY
INTO SAT EVENING. DEEP COLD AIR WITH PLENTY OF MOISTURE AND ICE
NUCLEI...CYCLONIC FLOW...AND STEEP LOW LEVEL LAPSE RATES IN THE
1000-850 MB LAYER. SNOW AMOUNTS IN THIS TYPE OF PATTERN ARE
TYPICALLY QUITE VARIABLE BY SOME LOCATIONS COULD SEE SEVERAL
INCHES. 

GLASS

EXTENDED (SUN - FRI MORNING)...

DETERMINISITC MODELS IN AGREEMENT THAT BY SUNDAY NRN HEMISHPERIC 
BLOCK OVER THE NORTH ATLANTIC SHUD WEAKEN ALLOWING THE UPR LVL FLOW 
TO PROGRESS EWD.  NEARLY STACKED LOW PRES OVER THE SRN GRT LKS WILL 
ROTATE QUICKLY ACRS THE ERN CONUS AND WITH ITS DEPARTURE EXPECT 
ASSOC UPR LVL ENERGY TO INDUCE ENHANCED LIFT CONTINUING CHCS FOR 
SNOW SHOWERS INTO SUN NGT.  IN ADDITION SFC PRES GRADIENT WILL STILL 
REMAIN TIGHT AND THUS HAVE CONTINUED GUSTS AS WELL.  EXPECT LOW 
STRATUS TO LINGER INTO MON NGT CLEARING THEREAFTER AS HIGH PRES 
BUILDS THRU THE RGN BY TUE WITH WEAKENING WESTERLY WINDS.  HAVE 
TRENDED WITH COLDEST TEMPS FOR TUE MORNING WITH LGT AND VRB WINDS 
AND CLEAR SKIES. 

BEGINNING TUE NGT AND THRU THE REMAINDER OF THE PERIOD FOLLOWED 
CLOSELY WITH ECMWF.  UPR LVL ENERGY ASSOC WITH A DEEP LOW PRES 
CENTER OVER SRN AK SHUD BIFURCATE SWD ALONG THE LONGWAVE FLOW THRU 
THE SWRN CONUS ENHANCING A H5 TROF TO IMPACT THE FCST RGN BY 
MIDWEEK.  HAVE INCREASED MAX AND MINS FOR THE LATE PERIOD WITH 
ANTICIPATION OF SLY FLOW AHEAD OF THE H5 TROF AND INCREASING MID LVL 
CLOUDS.  ISENTROPIC ANALYSES OF 290-295K SFCS HIGHLIGHT BEST LIFT 
AND ADVECTION OF THETAE OF 300-305K DURING THE DAY WED ALONG WITH 
SIGNIFICANT QVECT CONVERGENCE.  HAVE SLGT POPS FOR LIGHT SNOW FROM 
12Z WED-0Z THURS TIMEFRAME CONTINUING INTO 12Z THURS. THEREAFTER 
ATMOS APPEARS MOISTURE DEPRIVED AND WILL FOREGO ANY FURTHER MENTION 
OF PRECIP LATE IN THE PERIOD.

SIPPRELL
&&

.AVIATION...
/1128 PM CST THU DEC 24 2009/

FOR THE 06Z TAFS...SURFACE LOW IS NOW JUST WEST OF KVIH AS OF 05Z 
AND WILL CONTINUE TO TRACK TO THE NORTH NORTHEAST EARLY THIS MORNING 
...INTO NORTHEAST MO THEN INTO IOWA. MILD TEMPERATURES PERSISTING 
OVER TAF SITES...SO PCPN ALL IN THE FORM OF RAIN FOR NOW. AS LOW 
PULLS NORTHWARD...PCPN WILL CHANGE OVER TO ALL SNOW BY 10Z AT 
KCOU...14Z AT KSUS/KSTL AND BY 15Z AT KUIN. AS FOR CIGS...WILL SEE 
MVFR CIGS OVER THE FIRST COUPLE OF HOURS...THEN THEY WILL SINK BACK 
DOWN TO IFR/LIFR TIL THIS AFTERNOON THEN LIFT TO MVFR. AS SURFACE 
LOW LIFTS NORTHWARD THROUGH CENTRAL MO IN THE NEXT COUPLE OF HOURS 
...GUSTY EAST WINDS TO GRADUALLY SHIFT TO THE SOUTHEAST THEN 
EVENTUALLY TO THE WEST TO SOUTHWEST BY LATE THIS MORNING...AND 
REMAIN GUSTY THROUGH THE REST OF THE FORECAST PERIOD.

BYRD
&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...FLOOD WATCH UNTIL MIDNIGHT CST TONIGHT FOR AUDRAIN-BOONE-
     CALLAWAY-COLE-CRAWFORD-FRANKLIN-GASCONADE-IRON-KNOX-LEWIS-
     LINCOLN-MADISON-MARION-MONITEAU-MONROE-MONTGOMERY-OSAGE-
     PIKE-RALLS-REYNOLDS-SHELBY-ST. FRANCOIS-WARREN-WASHINGTON.

     FLOOD WATCH UNTIL 6 AM CST FRIDAY FOR JEFFERSON-ST. CHARLES-ST. 
     LOUIS-ST. LOUIS CITY-STE. GENEVIEVE.

IL...FLOOD WATCH UNTIL 6 AM CST FRIDAY 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.
&&

$$
WFO LSX