FXUS63 KLSX 302332
AFDLSX
AREA FORECAST DISCUSSION...UPDATED AVIATION
NATIONAL WEATHER SERVICE ST LOUIS MO
532 PM CST MON NOV 30 2009
.DISCUSSION...
/400 PM CST MON NOV 30 2009/
WE NOW HAVE THE TWO KEY COMPONENTS TO THE POTENTIAL WINTER STORM FOR
WEDNESDAY AND WEDNESDAY NIGHT OVER LAND IN ORDER TO BEGIN TO GET
BETTER SAMPLING. AS A RESULT...WE HAVE SEEN A SHIFT NWWD TO THE
EXPECTED STORM TRACK AS WELL AS A TIMING ADJUSTMENT TO HAVE THE
STORM MOVE IN A BIT QUICKER THAN PREVIOUS MODEL RUNS. THIS HAS
OCCURRED ON ALL MODELS...BUT THERE STILL REMAINS A DECENT SPREAD IN
STORM TRACKS ALTHOUGH THE TIMING IS ROUGHLY THE SAME. THE GFS HAS
BEEN THE MOST SLY TRACK WITH THIS STORM ALMOST SINCE IT BEGAN
TRACKING IT SEVERAL DAYS AGO. THE NAM HAS BEEN THE MOST NLY SINCE
THE STORM TIMING MOVED WITHIN 84HRS. THE MOST CONSISTENT MODEL
RUN-TO-RUN HAS BEEN THE CANADIAN GEM. THIS HAS CONSISTENTLY
ADVERTISED A TRACK THRU THE MIDDLE OF THE FA AND WILL BE LARGELY
WHAT THIS FORECAST IS BASED OFF OF...AT LEAST WITH THE SYNOPTIC
SCALE FEATURES. AND IT ISN/T MUCH SURPRISE THAT THE MOST CONSISTENT
MODEL NOW LIES SQUARELY IN THE MIDDLE OF THE 12Z MODEL SPREAD.
THE BIG QUESTIONS TO BE ANSWERED ARE...HOW QUICKLY WILL THE COLD AIR
SPILL IN TO CHANGE THE P-TYPE TO SNOW AND WHAT TO MAKE OF PCPN
DEVELOPMENT AT THE MESOSCALE LEVEL MUCH FARTHER TO THE NW AT THE
ONSET OF THE STORM.
AFTER THE MODELS WERE A BIT HESITANT TO BUILD IN THE COLD AIR A FEW
DAYS AGO...THEY HAVE ALL COME ON BOARD WITH THE IDEA THAT THE CDFNT
WILL MOVE THRU WEDNESDAY MORNING AND ERY AFTN AND WILL BE FOLLOWED
BY STRONG CAA AT THE BDRY LYR AND COOLING THRU THE COLUMN DUE TO
EVAPORATIONAL COOLING. THESE TWO FACTORS HAVE LED TO AN INCREASING
MODEL CERTAINTY ON AN EARLIER TRANSITION FROM RAIN TO SNOW...FOR
KUIN-KCOU DURING ERY WED AFTN...FOR KSTL-KFAM DURING LATE WED
AFTN...AND FOR KSLO DURING ERY WED EVNG. THE STRONG DYNAMICS OF THIS
STORM ALSO LEAD ME TO BELIEVE IT WILL BE ABLE TO FORCE SOME LEVEL OF
DYNAMICAL COOLING...THRU DEEP AND STRONG LIFT FROM WAA...
FRONTOGENESIS AND H300 JET COUPLING ON WEDNESDAY AND THEN ITS
EVENTUAL WINDING UP AS THE TWO STREAMS MERGE WEDNESDAY EVENING WITH
COMPLETED DEVELOPMENT OF A DEFORMATION ZONE AND THE BEST CHC FOR CSI
CONVECTIVE ELEMENTS FOR PARTS OF SWRN IL.
HAVE TAKEN A MORE AGGRESSIVE APPROACH ON POPS THAN THE PREVIOUS
FORECAST TO ACKNOWLEDGE THE MODEL SHIFT AND A BIT MORE INCREASED
CONFIDENCE ON TRACK...BUT EXACT PCPN CHANGEOVER TIMING IS TRICKY AND
SO SNOW FORECAST AMOUNTS ARE NOT A COMPLETE COMMITTMENT TO THE
CHANGEOVER TIMING UNTIL WE SEE SOME CONSISTENCY IN THIS AREA.
ANOTHER FEATURE THAT BEGAN TO APPEAR IN MODEL DATA YESTERDAY WAS
FRONTOGENETICAL BANDS IMMEDIATELY BEHIND THE CDFNT WEDNESDAY MORNING
THAT WILL LOSE THEIR UMPH HEADING LATE INTO THE DAY ON WEDNESDAY.
SHOULD THESE BANDS FULLY MATERIALIZE...WE COULD BE LOOKING AT
SEVERAL INCHES OF SNOW HERE...MUCH FARTHER NW AND SEPARATED FROM THE
MAIN DEFORMATION AXIS THAT IS EXPECTED TO FORM. CURRENT THINKING IS
MUCH OF THIS ENERGY WILL GO INTO SATURATION OF THE ATMOSPHERE AND
THEIR APPEARANCE IS EXPECTED TO BE RELATIVELY BRIEF...AND WILL AWAIT
BETTER MODEL HANDLING OF THIS FEATURE BEFORE ATTEMPTING TO COVER
BETTER.
NO HEADLINES ISSUED ATTM...BUT SPS HAS BEEN ISSUED TO HANDLE THIS
POSSIBLE INTERLUDE.
FOR THE EXTENDED PERIOD (THURSDAY NIGHT - MONDAY)...MODELS CONTINUE
TO CONVERGE ON WHAT TO DO FOR FRIDAY AND PERHAPS EVEN INTO SATURDAY
NOW...BY KEEPING A RATHER DEEP MID LEVEL TROF OVER THE GRTLKS REGION
AND MAINTAINING THE VERY COLD AIR THAT WILL BE IN PLACE AT THE
BEGINNING OF THIS PERIOD. WITH MORE CONFIDENCE WE CAN LOWER THE MAX
TEMPS EVEN MORE ON FRIDAY AND WITH UPPER LEVEL PIECES OF ENERGY
CONTINUING TO ROTATE AROUND WITH DEEP CYCLONIC FLOW...WILL EXTEND
THE CHANCE FOR FLURRIES THRU FRIDAY OVER MUCH OF THE FA. THIS
WEEKEND STILL LOOKS QUITE UNCERTAIN ALTHOUGH MORE OPERATIONAL MODEL
SOLNS ARE JUMPING ON THE GFS/DGEX BANDWAGON AND LEAVING THE ECMWF
ALONE ALTHOUGH THIS MAY NOT BE CORRECT ENTIRELY AS IT WAS THE ECMWF
THAT HAS BEEN CONSISTENTLY SHOWING THE SLOWER EXIT OF THE DEEP MID
LEVEL TROF WHICH SEEMS TO NOW BE THE WAY TO GO. SO WITH THIS IN
MIND...WILL CONTINUE TO TAKE A MIDDLE OF THE ROAD PATH WHICH WILL BE
MORE REMINISCENT OF THE ENSEMBLE MEAN FORECAST...SLOWING TRANSITIONS
OF PATTERNS DOWN AND EVENTUALLY WINDING UP WITH A ZONAL FLOW HEADING
INTO EARLY NEXT WEEK. SOME MODEL SOLNS ADVERTISING SMALL PCPN CHCS
ON SUNDAY WITH A FROPA BUT GIVEN THE HI SPREAD OF SOLNS...WILL GO
DRY FOR NOW WITH MODERATION IN TEMPS TOWARDS CLIMO...BUT WITH
OVERALL TROFING IN PLACE OVER THE CONUS...FINAL RESULT WILL STILL BE
BLO NORMAL.
TES
&&
.AVIATION...
/526 PM CST MON NOV 30 2009/
FOR THE 00Z TAFS...
VFR FLIGHT CONDITIONS AND LIGHT SOUTHWEST FLOW WILL PREVAIL
TONIGHT. THERE IS SOME POTENTIAL FOR RIVER FOG TONIGHT...BUT WITH
DEWPOINT TEMPERATURES DOWN IN THE MID TO UPPER 20S I THINK THE
AIR WILL BE DRY ENOUGH TO KEEP FOG OUT OF KSUS. WINDS WILL BECOME
MORE SOUTHERLY ON TUESDAY AFTERNOON...WITH VFR CONDITIONS
CONTINUING TO PREVAIL.
CARNEY
&&
.LSX WATCHES/WARNINGS/ADVISORIES...
MO...NONE.
IL...NONE.
&&
$$
WFO LSX
