FXUS63 KLSX 070544
AFDLSX

AREA FORECAST DISCUSSION...UPDATED AVIATION
NATIONAL WEATHER SERVICE ST LOUIS MO
1144 PM CST SUN DEC 6 2009

.DISCUSSION...
/344 PM CST SUN DEC 6 2009/

FORECAST CHALLENGE CONTINUES TO BE CHANCES OF WINTRY PRECIP TONIGHT 
AND HOW A MAJOR WINTER STORM WILL AFFECT THE AREA AT MIDWEEK.  
MODELS HAVE BEEN SHOWING AGREEMENT WITH BOTH SYSTEMS MOVING ACROSS 
THE REGION AND HAVE NOT CHANGED GOING FORECAST VERY MUCH.  HAVE 
ENOUGH CONFIDENCE AT THIS TIME TO ISSUE A WINTER STORM WATCH FROM 
TUESDAY INTO WEDNESDAY FOR NERN MO/W CNTRL IL.

LEAD SHORTWAVE CURRENTLY RACING EWD ACROSS THE CNTRL HIGH PLAINS 
WILL MOVE ACROSS MO/IL THIS EVENING.  TIME CROSS SECTIONS ARE STILL 
SHOWING A NARROW WINDOW OF ASCENT BEING PRODUCED BY THE SHORTWAVE 
WHICH IS ALIGNED WITH LOWER-MID LEVEL ATMOSPHERE MOISTURE...SO HIGH 
CHANCE/LIKELY POPS ARE STILL WARRANTED. LOW LEVEL MOISTURE WILL
STILL RESIDE IN THE WAKE OF THE TROUGH AS WEAK SUBSIDENCE SITS
IN...SO THIS IS A GOOD SET UP FOR PATCHY -FZDZ AND HAVE IT GOING
AREAWIDE AFTER 06Z AND OVER THE ERN PART OF THE CWA ALONG WITH
SOME FLURRIES IN THE ERN PART OF THE CWA ON MONDAY MORNING.

AFTER A BREAK LATE MONDAY THROUGH MONDAY NIGHT...PRECIPITATION WILL 
INCREASE ACROSS THE AREA ON TUESDAY AS LARGE SCALE ASCENT INCREASES 
AHEAD OF SECOND MORE ROBUST TROUGH EJECTING OUT OF THE WEST.
GOREE/YOUNKIN TECHNIQUE USING 500 MB VORT/850 MB LOW POSITIONS
FROM THE GFS/NAM/UKMET/ECMWF/CANADIAN GEM CONTINUE TO BE
CONSISTENT IN PLACING THE HEAVY SNOW AXIS FROM NERN KS/THRU NRN
MO/SERN IA INTO NRN IL. LOOKS LIKE THE FAR NRN CWA WILL BE ON THE
SRN EDGE OF THE HEAVY SNOW. THERE IS STILL SOME QUESTION OF
PRECIPITATION TYPE WITH THE NAM/GFS/LOCAL WRF FORECAST SOUNDINGS
ALL SHOWING LOW LEVEL PROFILES AT OR JUST SLIGHTLY ABOVE FREEZING
LATE TUESDAY INTO TUESDAY EVENING AS FAR NORTH AS QUINCY MEANING
THAT PRECIP MAY MIX WITH OR CHANGEOVER TO RAIN OR SLEET. HOWEVER A
CHANGE IN A DEGREE OR TWO OF TEMP AND A SLIGHT SHIFT SWD WITH THE
HEAVY SNOW AXIS COULD MEAN MORE SNOW THAN IS CURRENTLY FORECAST IN
THE WATCH AREA. MODELS ARE STILL SHOWING SIGNIFICANT DEEPENING OF
THE SFC LOW...SO HAVE KEPT THE WINDY CONDITIONS GOING AND ALSO
KEPT MENTION OF BLOWING SNOW LATE TUESDAY AND WEDNESDAY IN THE
WATCH AREA. ELSEWHERE HAVE MAINLY LIQUID PRECIPITATION WITH LITTLE
OR NO SNOW ACCUMULATION BECAUSE OF THE SHARP SRN GRADIENT IN
EXPECTED SNOWFALL AND THE EXTENT OF WARM AIR THAT WILL HAVE
WORKED INTO THE SYSTEM.

MUCH UNCERTAINTY AS WE GO INTO LATE WEEK AND NEXT WEEKEND AS FLOW 
BECOMES ZONAL ONCE MIDWEEK SYSTEM LIFTS QUICKLY EWD AND ELONGATED 
POLAR VORTEX SETS UP OVER CANADA.  TRYING TO TIME WEAK SHORTWAVES IN 
ZONAL FLOW IS TENUOUS THIS FAR OUT...SO DECIDED TO USE PERSISTENCE 
AND KEPT SLIGHT CHANCES FOR -RN/-SN FRIDAY/FRIDAY NIGHT WHICH HAS 
SUPPORT FROM THE GFS/GEFS WHICH ACTUALLY HAS BETTER 24 CONTINUITY 
TODAY THAN THE ECWMF.  MY THURSDAY MORNING LOWS IN THE UPPER SINGLE 
DIGITS TO AROUND 10F MAY NEED TO BE ADJUSTED DOWNWARD IN LATER 
FORECASTS...ESPECIALLY IF THERE IS A SIGNIFICANT NEW SNOW COVER WITH 
PROGGED 850MB TEMPS AROUND -14C.  REST OF TEMPS ARE BASED ON A BLEND 
OF GOING TEMPS AND HPC GUIDANCE.

BRITT

&&

.AVIATION...
/1142 PM CST SUN DEC 6 2009/

FOR THE 06Z TAFS...LIGHT SNOW WILL CONTINUE TO SLIDE NORTHEAST.
VISIBILITES WILL GRADUALLY INCH UP AS SNOW LIGHTENS. GIVEN ALL THE
CLOUDS ACROSS THE MIDWEST AND TO THE SOUTHWEST...HAVE FOLLOWED THE
MET GUIDANCE. IT SEEMS TO BE DOING A MUCH BETTER JOB WITH THE
CIGS. THUS MVFR CAN BE EXPECTED THROUGH 24 - 30 HOURS. 

JPK

&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...WINTER STORM WATCH FROM TUESDAY MORNING THROUGH WEDNESDAY 
     MORNING FOR KNOX-LEWIS-MARION-SHELBY.

IL...WINTER STORM WATCH FROM TUESDAY MORNING THROUGH WEDNESDAY 
     MORNING FOR ADAMS IL-BROWN IL.

&&

$$

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