FXUS63 KLSX 100459
AFDLSX

AREA FORECAST DISCUSSION...UPDATED AVIATION
NATIONAL WEATHER SERVICE ST LOUIS MO
1059 PM CST WED DEC 9 2009

.DISCUSSION...
/447 PM CST WED DEC 9 2009/

PRIMARY CONCERNS THIS FCST PACKAGE ARE ONGOING WINDS AND OVERNIGHT 
LOWS/WIND CHILLS.

PRES GRADIENT BETWEEN 975MB SFC LOW OVER MI AND SFC HIGH OVER THE 
SRN PLAINS WILL CONTINUE WEAKENING TONIGHT AS THE LOW MOVES 
NE...LEADING TO A DECREASE IN SUSTAINED WIND SPEEDS. WINDS HAVE 
ALREADY FALLEN BELOW ADVISORY CRITERIA...THUS HAVE CANCELLED THE 
WIND ADVISORY.

ATTN THEN TURNS TO OVERNIGHT LOWS. COMPARISON OF MOS GUIDANCE WITH 
LOCAL AND UPSTREAM AFTN TEMPERATURES SUGGESTS MOS GUIDANCE IS TOO 
WARM...ESPECIALLY GIVEN PERSISTENT CLOUDS THAT LIMITED DAYTIME 
HEATING. LOCAL CIG/WIND CLIMATOLOGY SUGGESTS TEMPS DROP APPROX 15-17 
DEG IN DEC WITH STG WINDS...NO CIGS...AND W OR N WINDS. IF THE 
CLOUDS DO NOT CLR AS FAST AS EXPECTED...A DROP OF ONLY 10 DEG IS 
MORE LIKELY. DESPITE CLRG SKIES AND APCHG HIGH PRES CENTER...THERE 
ARE TWO FACTORS WORKING AGAINST IDEAL RADIATIONAL COOLING 
CONDITIONS. FIRST...WINDS WILL BE STRONG ENOUGH TO PRODUCE MIXING 
AND INHIBIT THE FORMATION OF A SFC-BASED NOCTURNAL INVERSION. 
SECOND...MOST OF THE LSX CWA DOES NOT HAVE A FRESH SNOW PACK. THAT 
SAID...THIS IS VERY COLD AIR AND WIDESPREAD LOWS IN THE SINGLE 
DIGITS ABOVE ZERO ARE EXPECTED. GIVEN OVERNIGHT WINDS...WIND CHILL 
VALUES WILL DROP BELOW ZERO FOR SEVERAL HOURS WITH THE LOWEST WIND 
CHILL READINGS ACROSS THE NORTHWESTERN COUNTIES. ATTM WIND CHILL 
ADVISORY CRITERIA IS NOT EXPECTED TO BE MET. AFTER ANOTHER DAY OF
BELOW NORMAL TEMPERATURES ON THU...INCREASING H85 TEMPERATURES
AND SLY WINDS AROUND THE RETREATING SFC HIGH WILL LEAD TO A WARM
UP ON FRI.

SAT THROUGH WED...EXTENDED PD BEGINS WITH SLY FLOW AROUND THE BACK
SIDE OF SFC HIGH CENTERED OVER THE SERN US UNDER NEARLY ZONAL FLOW
ALOFT. DESPITE DIFFERENCES BETWEEN ECMWF AND GFS/GEM...EACH OF
THOSE MODELS IS HINTING THAT ONE OR TWO SHORTWAVE TROFS WILL MOVE
ACROSS THE CENTRAL US ON SAT/SUN. GIVEN LACK OF MOISTURE AND FAST
MOVEMENT...OPTED NOT TO BROADBRUSH SCHC POPS FOR THESE TWO WEAK
SYSTEMS AND INSTEAD FOCUSED SCHC POPS ON STRONGER FEATURE FCST TO
IMPACT THE AREA ON SUN NIGHT/MON. ALTHOUGH MODEL DIFFERENCES
REMAIN...CDFNT IS EXPECTED TO MOVE THROUGH THE CWA ON MON
NIGHT...THUS DECREASED OVERNIGHT LOWS GIVEN ARCTIC ORIGIN OF THE
AIR MASS.

KANOFSKY

&&

.AVIATION...
/1029 PM CST WED DEC 9 2009/

FOR THE 06Z TAFS...W-NWLY SFC WNDS WILL CONTINUE TO GRADUALLY
DECREASE IN SPEED THRU THE LATE NIGHT AS THE SFC PRESSURE GRADIENT
ACROSS THE CWA RELAXES DUE TO THE DEEP SFC LOW JUST NE OF MI
MOVING FURTHER NE OF THE AREA AND THE SFC RDG ACROSS THE PLAINS
BUILDING SEWD INTO MO. THE SFC WNDS MAY PICK UP SLIGHTLY IN SPEED
AGAIN BY LATE MRNG BUT WILL NOT BE NEARLY AS STRONG AS THEY WERE
TDA...AROUND 27014G24KT. THE SFC WNDS SHOULD FINALLY DIMINISH THU
NGT AND BACK AROUND TO A SWLY DIRECTION WITH THE SFC HI CENTERED
JUST S OF THE CWA AT 00Z FRI. THE LOW LEVEL CLOUDINESS...2500 FT
SHOULD ADVECT E OF STL/SUS BY 06Z THU...WITH A CLEAR SKY EXPECTED
AT THE TAF SITES THROUGH THE FORECAST PERIOD.

GKS

&&

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

$$

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