FXUS63 KLSX 281126
AFDLSX

AREA FORECAST DISCUSSION...UPDATED AVIATION
NATIONAL WEATHER SERVICE ST LOUIS MO
526 AM CST SAT NOV 28 2009

.DISCUSSION...
/310 AM CST SAT NOV 28 2009/

A MILD FALL DAY IS ON TAP WITH WELL ABOVE NORMAL TEMPERATURES
EXPECTED AS SWLY FLOW AND WAA DOMINATE THE LOWER TROP AND RIDGING
DOMINATES THE MID AND UPPER LEVELS. EARLY MORNING SKIES WILL BE
RELATIVELY CLOUD FREE WITH SOME HIGH CLOUDS INCREASING INTO THE
AFTERNOON AS A WEAK SHORT WAVE PASSES TO THE SOUTH. UPSTREAM TEMPS
YDA SUPPORT THE IDEA THAT HIGHS WILL BE NEAR THE WARMER END OF THE
MOS GUIDANCE...NEARLY 20 DEGS ABOVE AVERAGE. THINGS BEGIN TO
CHANGE TONIGHT AS A COLD FRONT MOVES INTO THE NRN CWA LATE. THE
MODELS ARE SHOWING A GENERAL INCREASE IN MOISTURE LATE HOWEVER THE
MOISTURE IS SLOW RETURNING AND REALLY WON'T GET TAPPED BY THIS
SYSTEM UNTIL IT IS IMPACTING THE SE THIRD OF THE CWA ON SUNDAY.
ANTICIPATING THAT AS SUNDAY PROGRESSES THE COMBINATION OF THE
INCREASING MOISTURE...LARGE SCALE ASCENT AND LOW LEVEL
FRONTOGENETICAL FORCING SHOULD LEAD TO THE DEVELOPMENT AND
EXPANSION OF RAIN/SHRA WITHIN THE WARM SECTOR AHEAD OF THE SEWD
PROGRESSING COLD FRONT. THE MODELS ARE ALSO SHOWING SOME MODESTLY
STEEP MID LEVEL LAPSE RATES AND NEGATIVE DELTA THETAE VALUES IN
THE 800-600 MB LAYER SUGGESTIVE OF THE POTENTIAL FOR SOME ISOLATED
THUNDERSTORMS. GIVEN UNCERTAINTIES IN THE QUALITY OF LOW LEVEL
MOISTURE RETURN AND VALIDITY OF THESE PARAMETERS...I HAVE LEFT
THUNDER OUT OF THE FCST FOR NOW...BUT IT CERTAINLY MAY NEED TO BE
ADDED IN FUTURE FCSTS. THE PCPN THREAT WILL CONTINUE ACROSS THE
EASTERN OZARKS AND SOUTHERN IL DURING THE EVENING ALONG AND IN
ADVANCE OF THE FRONT WITH NWLY WINDS AND DECENT CAA IN ITS WAKE.
TRANQUIL WEATHER AND SEASONABLE BUT MUCH COOLER TEMPERATURES ARE
THEN EXPECTED THROUGH THE FIRST HALF OF THE UPCOMING WEEK.  

THE FUTURE OF THE UPPER LOW NOW DIGGING THROUGH CENTRAL CA IS
SOMETHING WE ARE CAREFULLY AND CAUTIOUSLY WATCHING FOR MID WEEK. BOTH
THE CMC GEM AND ECMWF ARE INTERESTING BRINGING THIS SYSTEM OUT OF
NRN MEXICO AND IMPACTING PORTIONS OF THE REGION WITH LIQUID AND
WINTRY PCPN. GIVEN THE CURRENT UNCERTAINTIES IN THE MODEL FCSTS AND
THE AMOUNT OF TIME INTO THE FUTURE...THE SILENT SLIGHT CHANCE POPS
PRESENTLY IN THE FCST HAVE THE SITUATION ADEQUATELY COVERED.

GLASS

&&

.AVIATION...
/500 AM CST SAT NOV 28 2009/

FOR THE 12Z TAFS...MAY INCLUDE LLWS IN THE COU TAF TIL 15Z SAT AS
THE NAM MODEL FORECAST SOUNDING HAS WLY WNDS AT 1500 FT AROUND 40
KTS FOR THE INITIAL COUPLE OF HOURS OF THE TAF FORECAST. OTHERWISE
SWLY SFC WNDS WILL INCREASE TO NEAR 10 KTS THIS MRNG WITH A CLR
SKY CONTINUING. CIRRUS CLOUDS SHOULD BE INCREASING ACROSS THE AREA
BY THIS EVNG. MORE SIGNIFICANT WILL BE THE LOW LEVEL MOISTURE THAT
THE MODELS ADVECT E-NEWD INTO THE CWA LATE TGT AND EARLY SUN MRNG
AHEAD OF AN APPROACHING CDFNT WHICH WILL BE DROPPING SEWD INTO THE
CWA LATE TGT...AND PASSING THRU THE CWA ON SUN. MODEL MOS
GUIDANCE FORECASTS CIG HGTS OF ONLY 1000-2000 FT IN COU BY 12Z SUN
AND IN STL/SUS BY 15Z SUN...CIG HGTS MAYBE A LTL HIGHER IN UIN.
THIS FRONT SHOULD MAKE IT SEWD TO NEAR UIN AND COU BY 12Z SUN AND
TO STL/SUS AROUND 15Z SUN. THE SWLY SFC WND WILL VEER AROUND TO A
NWLY DIRECTION AFTER THE FRONTAL PASSAGE SUN MRNG. MAY INCLUDE A
PROB30 GROUP OF LIGHT RAIN IN THE STL TAF FROM 16-18Z SUN.

GKS

&&

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

$$

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