FXUS63 KLSX 061140
AFDLSX
AREA FORECAST DISCUSSION...UPDATED AVIATION
NATIONAL WEATHER SERVICE ST LOUIS MO
540 AM CST SUN DEC 6 2009
.DISCUSSION...
/525 AM CST SUN DEC 6 2009/
THE LONGWAVE H500 TROF THAT HAS BEEN ADVERTISED TO RETROGRADE HAD
THIS PROCESS WELL UNDERWAY FOR THE 00Z RAOBS LAST EVENING AND BASED
ON THE LATEST WATER VAPOR IMAGERY...IT HAS BEEN ACHIEVED. THIS IS
IMPORTANT BECAUSE THE DEGREE OF DEEPENING WILL PLAY A KEY FACTOR IN
THE MAJOR WINTER STORM THAT IS EXPECTED TO TAKE SHAPE ACROSS THE
MIDWEST TUESDAY AND WEDNESDAY. THE MODELS SHOULD BEGIN TO GET A LOCK
ON THE TRACK LATER TODAY OR MORE LIKELY...BY MONDAY. IN THE
MEANTIME...WE ARE STILL EXPECTING A QUICK MOVING SHORTWAVE TO IMPACT
OUR AREA TONIGHT THRU A MEAN ZONAL FLOW PATTERN. A STRONGER...AND
WELL ADVERTISED...SHORTWAVE WILL THEN DRIVE THRU THE MIDWEST ON
TUESDAY AND INTO WEDNESDAY...AND WILL BRING WITH IT...AN EWD
PROGRESSION OF THE LONGWAVE H500 TROF AS IT RE-ESTABLISHES ITSELF
OVER THE CNTRL CONUS. THIS LONGWAVE TROF WILL THEN RETREAT SOMEWHAT
HEADING INTO THE END OF THE WORK WEEK.
HAVE SIDED WITH THE COOLER METMOS TEMPS ONCE AGAIN FOR TODAY/S MAXES
AS TEMPS ALOFT ARE NOT MUCH DIFFERENT THAN PERSISTENCE...BUT THE
FACTOR THAT HAS CHANGED IS THICKENING CLOUDS OVERHEAD AND WILL
PROBABLY RESULT IN TEMPS NEAR PERSISTENCE.
THE SYSTEM FOR TONIGHT WILL BE QUICK MOVING AND PRESENT LIFT IN TWO
STAGES. THE FIRST WILL BE ASSOCIATED WITH THE UPPER LEVEL SYSTEM AND
PRIMARILY ACT ON THE AIR OVER THE NRN FA...WHILE THE SECOND WILL BE
ASSOCIATED WITH WAA AT THE LOWER LEVELS AND WILL ALSO CARRY WITH IT
A TONGUE OF BETTER MOISTURE. NE'ER TWAIN THE TWO SHALL MEET WHICH
WILL LEAVE THEM WITH THEIR OWN OBSTACLES TO OVERCOME...THE DRY AIR
AT THE LO LEVELS IN THE NRN FA...AND THE LACK OF DEEPER LIFT IN THE
SERN FA. BELIEVE THE SERN FA WILL HAVE THE BEST CHANCE FOR
MEASURABLE PCPN DUE TO LIFT CENTERED LOWER IN THE ATMOSPHERE BUT
BRIEF PERIOD WHEN EVERYTHING COMES TOGETHER WILL RESULT IN A LOW
QPF. MODEL SOUNDINGS HAVE COME IN A BIT COOLER THAN 24HRS AGO AND
FAVOR MORE OF A SNOW P-TYPE WHEN THE ATMOSPHERE DOES SATURATE WHICH
WILL LIKELY NOT HAPPEN IN MANY AREAS UNTIL CLOSE TO MIDNIGHT. BY
LATE TONIGHT AND CONTINUING INTO MONDAY MORNING...WE ARE STILL
EXPECTING MORE FAVORABLE CONDS FOR DZ/FZDZ DEPENDING ON THE PRECISE
SFC TEMPS.
MODELS CONTINUE TO SHOW A PREFERRED CONSENSUS FOR A NRN TRACK TO THE
TUESDAY/WEDNESDAY STORM. AS A MATTER OF FACT...THE H850 LO TRACK
WILL BARELY PUSH THRU THE FAR NWRN FA TUESDAY NIGHT...AND THIS
STRONGLY SUGGESTS LITTLE IN THE WAY OF ACCUMULATING SNOW EXCEPT NEAR
THIS TRACK AND EVEN THEN IT IS ON SHAKY GROUND BECAUSE OF TEMP
PROFILE ISSUES...MEANING IT MAY NOT BE ENTIRELY ALL SNOW EVEN THERE.
THE HEAVY SNOW TRACK APPEARS IT WILL BE STILL FURTHER NW INTO IOWA.
HOWEVER...THE RETROGRADING LONGWAVE H500 TROF HAS NOT COMPLETED YET
AND SO THIS TRACK COULD CERTAINLY CHANGE AND MEAN BIG CHANGES.
ANOTHER ITEM IS THE STRENGTH OF THE BUILDING COLD AIRMASS FOR MONDAY
AND HOW EASILY IT WILL GIVE WAY TO THIS STORM...WHICH MAY NUDGE IT
FURTHER S. AS IT STANDS NOW...THOUGH...WE ARE LOOKING AT PRIMARILY
SNOW IN THE FAR NWRN CORNER OF THE FA...NW OF KUIN...WHERE SEVERAL
INCHES OF SNOW COULD ACCUMULATE. A PERIOD OF SNOW EARLY CHANGING TO
RAIN FOR MUCH OF THE STORM THEN BACK TO SNOW LATE WILL BE IN ORDER
FOR AREAS FURTHER S TO THE I-70 CORRIDOR. HOWEVER SFC TEMPS WILL
LIKELY BE ABOVE FRZG FOR MUCH OF THE STORM IN AREAS THAT ARE S OF
KUIN AND LIMIT ANY SNOW ACCUMS. WHAT MAKES THE P-TYPES CHALLENGING
WILL BE THE WARM AIR SURGING OUT AHEAD OF THE STORM...EVEN THROUGH
MUCH OF TUESDAY NIGHT...LIKELY RESULTING IN SLOWLY RISING TEMPS THRU
TUESDAY EVENING WITH THE WEDGE OF WARM AIR ALOFT THAT WILL ALSO
DEVELOP. WITH THE MORE NLY TRACK THE STORM APPEARS TO BE TAKING...
CANNOT EVEN REALLY RULE OUT THUNDER FOR MANY AREAS TUESDAY NIGHT.
ONCE THE STORM EXITS BY MIDDAY WEDNESDAY...MUCH COLDER AND WINDY
CONDS WILL MOVE IN. LOOK FOR TEMPS TO FALL THRU THE DAY WITH MAX
TEMPS AROUND 12Z/WED WITH GUSTS AS HI AS 40MPH. BLSN WILL BE AN
ISSUE FOR AREAS THAT DO ACCUMULATE SEVERAL INCHES OF SNOW UP N. THE
AIR WILL EVEN BE COLD ENOUGH BY LATE WED NIGHT AND THURSDAY MORNING
TO BE TALKING WIND CHILLS APPROACHING 10 BELOW.
TES
&&
.AVIATION...
/539 AM CST SUN DEC 6 2009/
FOR THE 12Z TAFS...TAFS ARE SPLIT INTO TWO PARTS FOR THIS CYCLE. FIRST
HALF OF TAF IS A WIND FCST FOR TODAY AS SLY WINDS BACK TO BCMG ESELY
THIS AFTERNOON INTO THE EVE HOURS AS A SURFACE LOW APPROACHES THE REGION.
WINDS WILL THEN BCMG SLY AGAIN LATE TONIGHT...THEN SWLY TOWARD THE END
OF THE FORECAST PERIOD AS THE WEAK LOW DRIFTS NEWD THRU THE REGION.
THE SECOND HALF OF THE FORECAST DEALS WITH CIGS AND THE POTENTIAL OF
SOME LIGHT PRECIP LATE TONIGHT...MAINLY AT COU AND UIN. HAVE NOT MENTIONED
ANY PRECIP AT ANY SITE ATTM AS VISBYS ARE NOT EXPECTED TO BE REDUCED.
ANY PRECIP THAT DOES OCCUR IS EXPECTED TO START AS SN. AS DRY AIR IS
ADVECTED IN ABOVE THE CLOUD LAYER WITH THE TOP OF THE CLOUD LOWERING
BELOW THE DENDRITIC ZONE...PRECIP SHUD SWITCH OVER TO FZDZ. ANY PRECIP
WILL BE SHORT AS THE SYSTEM WILL BE MOVING QUICKLY THRU THE REGION.
TILLY
&&
.LSX WATCHES/WARNINGS/ADVISORIES...
MO...NONE.
IL...NONE.
&&
$$
WFO LSX
