239 
FXUS63 KLSX 210424
AFDLSX

AREA FORECAST DISCUSSION...UPDATED AVIATION
NATIONAL WEATHER SERVICE ST LOUIS MO
1124 PM CDT THU SEP 20 2012

.UPDATE...
ISSUED AT 947 PM CDT THU SEP 20 2012

IT APPEARS THAT THE ELEVATED CONVECTION TGT WILL BE A LITTLE
SLOWER TO DEVELOP AND SPREAD NEWD INTO OUR AREA. CONVECTION SHOULD
DEVELOP ACROSS EXTREME SWRN MO LATER THIS EVNG ON NOSE OF
STRENGTHENING SWLY LOW LEVEL JET...THEN SHIFT E-NEWD AS THE LOW
LEVEL JET TRANSLATES E-NEWD INTO SWRN MO. CONVECTION SHOULD SPREAD
INTO PORTIONS OF CNTRL AND SERN MO BY 12Z FRI PER THE LATEST NAM
MODEL AND HRRR. THERE MAY BE LIGHTER SCATTERED SHOWERS OR AT
LEAST SPRINKLES FROM MID LEVEL CLOUDS FURTHER N ACROSS NERN MO
INTO W CNTRL IL LATE TGT. PUT THE HIGHEST POPS ACROSS SERN MO LATE
TGT/EARLY FRI MRNG WHERE THERE WILL BE THE BEST LOW LEVEL MOISTURE
AND ELEVATED INSTABILITY.

GKS

&&

.SHORT TERM...
ISSUED AT 323 PM CDT THU SEP 20 2012
(TONIGHT)

MAIN CONCERN CONTINUES TO BE THE CHANCE OF THUNDERSTORMS OVERNIGHT.

NOT MUCH CHANGE TO PREVIOUS THINKING THAT THIS EVENING WILL REMAIN 
DRY BEFORE LARGE SCALE ASCENT AND LOW LEVEL CONVERGENCE INCREASES 
BETWEEN 06-12Z.  THINK THUNDERSTORMS WILL INITIATE OVER CENTRAL AND 
SOUTH CENTRAL MISSOURI DURING THIS PERIOD AND THEN MOVE QUICKLY EAST 
TOWARD THE MISSISSIPPI RIVER BY 12Z.  FORECAST SOUNDINGS ARE SHOWING 
UPWARDS OF 500 J/KG OF MUCAPE OVER CNTRL MO LATE TONIGHT...SO WITH 
FORECAST DEEP LAYER SHEAR VALUES OVER 50KTS THERE MAY STILL BE SOME 
THREAT OF MAINLY SUB-SEVERE HAIL.

GERNERALLY FOLLOWED MOS TEMPERATURES WHICH ARE IN GOOD AGREEMENT.

BRITT

&&

.LONG TERM...
ISSUED AT 323 PM CDT THU SEP 20 2012
(TOMORROW THROUGH SUNDAY)

TOMORROW PRESENTS AN INTERESTING FCST PROBLEM BECAUSE THE MODEL 
GUIDANCE FALLS INTO TWO SEPARATE YET EQUALLY SIGNIFICANT GROUPS...AT 
LEAST IN TERMS OF SENSIBLE WX FOR THE LSX CWA. THE MAIN FEATURES OF 
INTEREST ARE A STALLED FNTL BDRY EXTENDING ACROSS SRN MO/NRN AR...A 
DVLPG SFC LOW ALONG THE BDRY OVER THE TX/OK PANHANDLE REGION...AND 
AN APPROACHING UPPER LEVEL VORT MAX DROPPING SEWD OUT OF CANADA AND 
ROTATING AROUND THE DEEPENING GRTLKS TROF. 

ONE SET OF MODEL GUIDANCE DEPICTS THE UPPER WAVE INDUCING A SFC LOW 
OVER THE DAKOTAS/MN OVERNIGHT AND THEN DROPPING SEWD WITH 
TIME...REACHING CENTRAL MO ON FRI AROUND 18-21Z. THE OTHER SET OF 
MODEL GUIDANCE DEPICTS THE SFC LOW OVER OK/TX LIFTING NEWD ALONG THE 
BDRY OVERNIGHT ALONG WITH A STRONG H85 LLJ...EVENTUALLY REACHING 
SWRN MO BY 12Z FRI AND LIFTING THE BDRY NWD /ALMOST TO KSTL/ AS A 
WMFNT BEFORE THE ARRIVAL OF THE UPPER LEVEL VORT MAX PUSHES THE SFC 
LOW SEWD WHICH ALLOWS THE BDRY TO SETTLE SWD AGAIN AS A CDFNT. A 
FEW MODEL RUNS DO DEPICT A HYBRID SOLUTION WITH THE INDUCED SFC LOW 
OVER THE PLAINS MERGING WITH THE LIFTING OK/TX LOW LEADING TO AN 
ELONGATED SFC TROF RATHER THAN SEPARATE LOW PRESSURE CENTERS. ALL OF 
THESE SCENARIOS ARE EQUALLY PLAUSIBLE AND THERE ISN\'T ANYTHING 
READILY APPARENT IN THE SFC OR UPPER AIR OBS TO JUSTIFY FOLLOWING 
ONE SOLUTION OVER ANOTHER. FOR THIS REASON THERE IS A FAIR AMOUNT OF 
UNCERTAINTY REGARDING THE TEMP AND PCPN FCST FOR TOMORROW. IT STILL 
LOOKS LIKE THE MOST LIKELY AREA FOR PCPN IS THE FAR SRN CWA DURING 
THE MORNING /REMNANTS FM OVERNIGHT CONVECTION WITH THE LLJ/ AND THE 
FAR SERN CWA DURING THE AFTN /ASSOCIATED WITH THE UPPER LEVEL VORT 
MAX AND A SFC BDRY OR SFC LOW/. 

A MUCH STRONGER UPPER LEVEL VORT MAX WILL DRIVE A STRONGER CDFNT 
THROUGH THE AREA DURING THE DAY ON SATURDAY. DESPITE STRONG UPPER 
LEVEL FORCING...IT LOOKS LIKE MOISTURE WILL BE LIMITED AND AT BEST 
ONLY SOME MID AND HIGH CLOUDS ARE EXPECTED WITH FROPA. CANNOT RULE 
OUT A SPRINKLE OR TWO ACROSS THE EXTREME NERN CWA BUT THE MOST 
FAVORABLE AREA FOR SHRA WILL BE LOCATED FARTHER NORTH AND EAST. HIGH 
PRESSURE BUILDS IN TO THE REGION ON SAT NIGHT BUT THE RIDGE AXIS 
WILL REMAIN WEST OF THE CWA THEREFORE DID NOT DROP TEMPERATURES VERY 
FAR AND REMAINED CLOSE TO THE PREVIOUS FCST. SUNDAY LOOKS QUITE COOL 
WITH THE SFC HIGH MOVING ACROSS THE CWA FOLLOWED BY ANOTHER COOL 
NIGHT ON SUNDAY NIGHT. THERE ARE SOME MODEL DIFFERENCES REGARDING 
THE POSITION OF THE SFC HIGH AT 12Z MON. IF THE SFC HIGH IS DIRECTLY 
OVERHEAD THEN OVERNIGHT TEMPERATURES MAY BE A FEW DEGREES TOO WARM 
DUE TO RADIATIONAL COOLING. 

MODELS AGREE ON THE OVERALL PICTURE OF A SFC RIDGE AXIS BENEATH 
CYCLONIC FLOW ALOFT BUT DISAGREE ON HOW QUICKLY THE HIGH SHIFTS EWD 
AND ON HOW QUICKLY THE GRTLKS TROF LIFTS. THERE ARE SOME INDICATIONS 
THAT A WMFNT MAY DVLP AND MOVE THROUGH THE AREA ON TUESDAY WITH PCPN 
CHCS INCREASING FOR TUE-THU. THERE IS ENOUGH UNCERTAINTY ON THESE 
POINTS TO STICK CLOSE TO THE RECOMMENDED INITIALIZATION PER CRH 
POLICY FOR THE END OF THE EXTENDED FCST PD.

KANOFSKY

&&

.AVIATION...
ISSUED AT 1114 PM CDT THU SEP 20 2012

WEAK HIGH PRESSURE MOVING ACROSS THE FORECAST AREA LATE THIS
EVENING...WITH A LIGHT AND VARIABLE WIND. STATIONARY FRONT TO THE
SOUTH WILL SERVE AS A FOCUS FOR THUNDERSTORM DEVELOPMENT ACROSS
SOUTHERN MISSOURI OVERNIGHT AND INTO FRIDAY MORNING.
OTHERWISE...LIGHT RAIN SHOWERS ARE POSSIBLE FURTHER NORTH AHEAD OF
THE SHORTWAVE AND WEAK LOW PRESSURE. COVERAGE OF RAIN SHOWERS ARE
STILL UNCERTAIN AND DRY LOWER LEVELS OF THE ATMOSPHERE DUE TO
AFOREMENTIONED HIGH PRESSURE COULD RESULT IN MAINLY VIRGA...SO
HAVE INCLUDED SOME VCSH AT TERMINALS UNTIL WE SEE IT ON RADAR.

WIND WILL INCREASE FROM THE SOUTH/SOUTHEAST AHEAD OF THE TROF AND
SWING AROUND TO THE WEST BEHIND IT. A REINFORCING COLD FRONT WILL
DROP SOUTH ACROSS THE AREA FRIDAY EVENING/NIGHT WITH WINDS
BECOMING NORTHERLY.

SPECIFICS FOR KSTL...
VFR CONDITIONS EXPECTED WITH A CHANCE OF A FEW SHOWERS AHEAD OF
THE TROF THAT SHOULD PASS ACROSS THE TERMINAL LATE FRIDAY
AFTERNOON. REINFORCING COLD FRONT WILL BRING WINDS AROUND TO THE
NORTH AFTER 06Z SATURDAY.

CVKING

&&

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

$$

WFO LSX