259 
FXUS63 KLSX 232105
AFDLSX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ST LOUIS MO
405 PM CDT SAT MAR 23 2013

.SHORT TERM...(THROUGH LATE TONIGHT)
ISSUED AT 403 PM CDT SAT MAR 23 2013

(TONIGHT THROUGH SUNDAY NIGHT)

MOST MODEL GUIDANCE CAME IN THIS MORNING LOOKING REMARKABLY SIMILAR 
TO PREVIOUS RUNS.  THE ONE OUTLIER WAS THE GFS WHICH TOOK THE 
POSITIONS OF THE LOW AT VARIOUS LEVELS AND PUSHED THEM ALL FURTHER 
SOUTH.  THE REST OF THE GUIDANCE WAS PRETTY STEADY...TAKING THE 
850MB LOW JUST SOUTH OF ST. LOUIS SUNDAY UP INTO THE OHIO VALLEY BY 
SUNDAY NIGHT.  THE THERMODYNAMICS OF THIS SYSTEM REMAIN BASICALLY 
UNCHANGED.  THE FIRST WAVE OF PRECIP IS ALREADY STARTING OVER 
WESTERN MISSOURI.  THIS WILL BE THE WARM ADVECTION WING AS THE 
SYSTEM GATHERS STRENGTH OVER THE EASTERN PLAINS.  STILL SOME 
QUESTION AS TO HOW QUICKLY/HOW FAR SOUTH THE PRECIPITATION WILL 
CHANGE FROM RAIN TO SNOW AS THERE APPEARS TO BE A WEDGE OF VERY DRY 
LOW LEVEL AIR.  DEWPOINTS MIXED OUT INTO THE 20S THIS AFTERNOON 
ACROSS MOST OF THE AREA.  GOING TO HAVE TO WATCH THE RAIN/SNOW LINE 
VERY CLOSELY...EVEN AS FAR SOUTHEAST AS ST. LOUIS.  JUST SAW AN 
ACARS SOUNDING FROM KSTL WHICH SHOWS A VERY DRY BOUNDARY LAYER...AS 
WELL AS ANOTHER VERY DRY LAYER FROM 800-700MB.  IF THIS DRY LAYER 
CAN PERSIS UNTIL THE PRECIP STARTS...THE PRECIP WILL CHANGE TO SNOW 
CONSIDERABLY MORE QUICKLY THAN FORECAST.

REGARDLESS...THE BULK OF THE SNOW IS LIKELY TO BE WITH THE 
DEFORMATION ZONE ON SUNDAY.  ALL INDICATORS POINT TO HEAVY SNOW WITH 
STRONG VERTICAL MOTION AHEAD OF THE 500MB LOW...ABUNDANT MOISTURE 
AND EYE-POPPING LOW-MID LEVEL FRONTOGENESIS WRAPPING AROUND THE LOW 
LEVEL CIRCULATION.  ICING ON THE HEAVY SNOW CAKE ON SUNDAY CONTINUES 
TO BE EPV VALUES BETWEEN -0.5 AND +0.25 INDICATING CSI/CI IN THE 
COLUMN...AND OF COURSE 50-100J/KG OF CAPE WRAPPING INTO THE SYSTEM.  
ALL OF THE MODELS SLOW THE SYSTEM DOWN A BIT FROM EARLIER 
FORECASTS....SO HAVE CONVERTED THE REST OF THE WATCH TO A 
WARNING...AND KEPT THE WARNING GOING IN THE ST. LOUIS METRO AREA 
EAST AND NORTHEAST THROUGH MIDNIGHT SUNDAY NIGHT TO ACCOUNT FOR THE 
SLOWER MOVEMENT OF THE DEF-ZONE.  STORM TOTAL SNOWFALL AMOUNTS ARE A 
LITTLE HIGHER THAN PREVIOUS FORECASTS WITH THE SLOWER DEF-ZONE...BUT 
STILL EXPECT THE HEAVIEST BAND TO BE ALONG AND NORTH OF THE I-70 
CORRIDOR.

CARNEY

&&

.LONG TERM...(SUNDAY THROUGH NEXT SATURDAY)
ISSUED AT 403 PM CDT SAT MAR 23 2013
(MONDAY-TUESDAY)

RENMANTS OF NORTHERN ROCKIES LOW FORECAST TO ROTATE SE AROUND THE 
BACK SIDE OF THE EXITING SYSTEM...AND INTO MISSOURI...ON MONDAY.  
WITH THE UNSEASONABLY COLD AIRMASS...SCATTERED SNOW SHOWERS SHOULD 
BE FAIRLY COMMON ACROSS MOST OF THE CWA. THIS PRECIP THREAT SHOULD 
WIND DOWN TO FLURRIES ON MONDAY EVENING AS REMNANTS OF THE UPPER LOW 
DRIFTS INTO THE OHIO VALLEY.  

TUESDAY SHOULD BE A DRY DAY AS N-S AXIS OF CANADIAN HIGH APPROACHES 
FROM THE WEST.  DIRTY NW FLOW AND LOW LEVEL CYCLONIC CIRCULATION 
SHOULD MEAN A FAIR AMOUNT OF CLOUDINESS...BUT HAVE KEPT THE FORECAST 
DRY FOR NOW. 

WITH 850MB TEMPS AOB -8 THROUGH MUCH OF THE EARLY WEEK AND 
CONSIDERABLE CLOUDINESS...DAYTIME HIGHS ARE GOING TO CONTINUE THEIR 
STRUGGLE TO EVEN COME CLOSE TO NEAR AVERAGE LEVELS.  ATTM...HAVE 
KEPT HIGHS PRETTY MUCH IN THE 30S...WITH OVERNIGHT LOWS IN THE 20S. 

(WEDNESDAY-SATURDAY)

BROAD MEAN TROF OVER THE EASTERN CONUS IS EXPECTED TO HOLD TOUGH 
INTO THE LATTER HALF OF NEXT WEEK.  TRANQUIL WEATHER SHOULD CONTINUE 
ON WEDNESDAY AS THE RIDGE WORKS ACROSS THE AREA....BUT AFTER MIDWEEK 
MEDIUM RANGE SOLUTIONS ARE HINTING AT SEVERAL DISTURBANCES RIPPLING 
DOWN THE BACK SIDE OF THE UPPER TROF...SO HAVE A SMATTERING OF 
SLIGHT CHANCE/LOW CHANCE POPS IN THE THURSDAY-FRIDAY TIME FRAME.
HOWEVER...DID TIE A HIGHER COVERAGE OF CHANCE POPS TO THE FRIDAY 
NIGHT/SATURDAY TIME FRAME.  UPPER LOW IS FORECAST TO PUSH INTO THE 
GREAT LAKES DURING THIS TIME...WITH A TROF EXTENDING SW OF THE LOW 
SWEEPING ACROSS THE AREA. 

TEMPERATURES WILL GRADUALLY MODERATE DURING THIS TIME...BUT IN SPITE 
OF THAT IT CERTAINLY APPEARS THAT READINGS WILL REMAIN AT OR BELOW 
NORMAL.

TRUETT

&&

.AVIATION...(FOR THE 18Z TAFS THROUGH 18Z SUNDAY AFTERNOON)
ISSUED AT 1045 AM CDT SAT MAR 23 2013

LOWERING AND THICKENING CEILINGS EXPECTED AS WELL ADVERTISED STORM
SYSTEM MOVES TOWARD THE REGION. SHOWERS SHOULD INCREASE FROM WEST
TO EAST LATE THIS AFTERNOON AND THIS EVENING...WITH PRECIPITATION
MIXING AND CHANGING OVER TO SNOW ALONG AND NORTH OF INTERSTATE 70
LATE THIS EVENING OR OVERNIGHT. REMAINDER OF THE AREA SHOULD THEN
SEE RAIN TRANSITION TO SNOW ON SUNDAY...WITH IT BECOMING HEAVY
DURING THE LATE MORNING AND EARLY AFTERNOON HOURS. IFR CONDITIONS
ARE ALMOST CERTAIN TO OCCUR BY TOMORROW MORNING. WIND WILL
INITIALLY BE FROM THE EAST/NORTHEAST AND THEN BACK TO THE
NORTHWEST AS LOW PRESSURE MOVES SOUTH AND EAST OF THE REGION ON
SUNDAY.

SPECIFICS FOR KSTL...
LOWERING AND THICKENING CEILINGS EXPECTED WITH SHOWERS DEVELOPING
DURING THE EVENING. SOME UNCERTAINTY REGARDING THE EXACT TIME OF
CHANGE OVER...WITH SOME OF THE MOST RECENT GUIDANCE SUGGESTING AN
EARLY CHANGEOVER THAN WHAT WAS BELIEVED EARLIER THIS MORNING. FOR
NOW HAVE BEEN CONSERVATIVE WITH A CHANGEOVER AROUND 12Z TO SNOW.
SNOW...HEAVY AT TIMES...WILL IMPACT THE AIRPORT LATE SUNDAY
MORNING AND INTO THE AFTERNOON WITH AIRPORT MINIMUMS LIKELY BEING
REACHED. SIGNIFICANT SNOW ACCUMULATION IS EXPECTED WITH THIS STORM
SYSTEM. INITIALLY WIND DIRECTION WILL BE FROM THE NORTHEAST...BUT
WILL TURN TO THE NORTHWEST AND BECOME BLUSTERY SUNDAY AFTERNOON AS
THE LOW PRESSURE CENTER PASSES TO THE SOUTHEAST.

CVKING

&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...WINTER WEATHER ADVISORY FROM 7 AM TO 7 PM CDT SUNDAY FOR 
     CRAWFORD MO-IRON MO-MADISON MO-REYNOLDS MO-ST. FRANCOIS MO-
     STE. GENEVIEVE MO-WASHINGTON MO.

     WINTER STORM WARNING FROM MIDNIGHT TONIGHT TO 7 PM CDT SUNDAY 
     FOR AUDRAIN MO-BOONE MO-CALLAWAY MO-COLE MO-FRANKLIN MO-
     GASCONADE MO-JEFFERSON MO-KNOX MO-LEWIS MO-LINCOLN MO-
     MARION MO-MONITEAU MO-MONROE MO-MONTGOMERY MO-OSAGE MO-PIKE 
     MO-RALLS MO-SHELBY MO-WARREN MO.

     WINTER STORM WARNING FROM MIDNIGHT TONIGHT TO MIDNIGHT CDT 
     SUNDAY NIGHT FOR ST. CHARLES MO-ST. LOUIS CITY MO-ST. LOUIS 
     MO.

IL...WINTER WEATHER ADVISORY FROM 7 AM TO 7 PM CDT SUNDAY FOR 
     RANDOLPH IL-WASHINGTON IL.

     WINTER STORM WARNING FROM MIDNIGHT TONIGHT TO 7 PM CDT SUNDAY 
     FOR ADAMS IL-BROWN IL-MONROE IL-PIKE IL.

     WINTER STORM WARNING FROM MIDNIGHT TONIGHT TO MIDNIGHT CDT 
     SUNDAY NIGHT FOR BOND IL-CALHOUN IL-CLINTON IL-FAYETTE IL-
     GREENE IL-JERSEY IL-MACOUPIN IL-MADISON IL-MARION IL-
     MONTGOMERY IL-ST. CLAIR IL.

&&

$$

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