547 FXUS63 KLSX 280815 AFDLSX AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE ST LOUIS MO 315 AM CDT FRI SEP 28 2012 .SHORT TERM... ISSUED AT 313 AM CDT FRI SEP 28 2012 (TODAY) THE PRIMARY FCST ISSUE FOR THIS FCST PD IS DETERMINING WHICH PARTS OF THE CWA ARE MOST LIKELY TO RECEIVE PCPN THROUGH 00Z. 07Z METARS AND MSAS SATURATED EQUIVALENT POTENTIAL TEMPERATURE FIELDS DEPICTED AN E-W ORIENTED BDRY BISECTING THE CWA. ALOFT...A SHORTWAVE TROF WAS LOCATED OVER CENTRAL AND WRN MO PER WATER VAPOR SATELLITE IMAGERY ALONG WITH AN ANTICYCLONICALLY CURVED JET STREAK AT H25 OVER CENTRAL IL NOTED ON THE LATEST RUC ANALYSIS AND SUPPORTED BY AREA VWP/PROFILER OBS. LARGE SCALE LIFT AHEAD OF THIS SHORTWAVE AND RER DYNAMICS ASSOCIATED WITH THE CURVED JET STREAK WERE SUPPORTING ISO-SCT SHRA ACROSS THE SRN HALF OF THE CWA...MAINLY ALONG AND S OF THE BDRY. THESE FEATURES ARE EXPECTED TO MOVE E OF THE CWA BY DAYBREAK WITH PCPN DIMINISHING DUE TO SUBSIDENCE IN THE WAKE OF THE DEPARTING SHORTWAVE AND DRIER AIR FILTERING IN TO THE REGION AS A SFC HIGH SINKS SWD. HOWEVER...THE BDRY WILL STILL BE PRESENT /ALBEIT SLOWLY SINKING SWD/ ACROSS PART OF THE AREA THROUGH THE AFTN HOURS...AND ANOTHER SHORTWAVE WAS LOCATED UPSTREAM AT 07Z NEAR THE KS/CO BORDER. DESPITE THE DRIER AIR FILTERING IN FROM THE NORTH...MODELS DO SHOW SOME 300-250MB DIVERGENCE ACROSS THE SRN CWA DURING THE DAY AND WHEN COMBINED WITH THE CO/KS SHORTWAVE THERE MAY BE ENOUGH SUPPORT FOR ISO-SCT SHRA THIS AFTN INVOF THE BDRY. KANOFSKY && .LONG TERM... ISSUED AT 300 AM CDT FRI SEP 28 2012 (TONIGHT - TUESDAY) ZONAL FLOW ALOFT THAT HAS GIVEN US A PROLONGED PERIOD OF UNSETTLED WX THIS PAST WEEK WILL TRANSITION TO NW TO N FLOW THAT SHOULD GIVE US A QUIETER PERIOD FOR A BIT. THIS CHANGE IN FLOW WILL INITIALLY BE HELPED OUT BY A POLAR VORTEX DIGGING INTO THE GRTLKS REGION THIS WEEKEND AND LATER BY A TROF THAT DIGS INTO THE CNTRL AND S CNTRL CONUS EARLY NEXT WEEK...WITH BOTH OF THESE FEATURES EXPECTED TO BE FAR ENOUGH AWAY TO HAVE LITTLE IMPACT. THE SFC IS EXPECTED TO HAVE A WEAK RIDGE PREVAILING FOR MUCH OF THIS PERIOD...ALTHOUGH A TROF WILL TAKE A SWIPE MAINLY AT THE IL SIDE ON SATURDAY NIGHT AND SUNDAY. BELOW NORMAL PWATS FOR THIS WEEKEND...A STABLE ATMOSPHERE...AND MOST IMPORTANTLY...LIFTING MECHANISMS TOO WEAK...SHOULD PRECLUDE ANY PCPN DEVELOPMENT THIS WEEKEND. NEAR AVERAGE TEMPS FOR LATE SEPTEMBER ARE EXPECTED...WHICH FAVOR THE MAV MOS NUMBERS WHICH IN MOST CASES ARE RUNNING A BIT LOWER THAN THE MET MOS. UP UNTIL THIS POINT THERE IS GOOD AGREEMENT BETWEEN THE MODELS. WE START TO SEE SOME DISAGREEMENT BEGINNING MONDAY WHERE THE ECMWF DRAGS A CDFNT THRU WHEREAS THE GFS KEEPS IT WEST OF THE FA WHERE IT THEN DISSIPATES IT AND RE-ESTABLISHES THE RIDGE. IT APPEARS WITH RESPECT TO THE BOTTOM LINE...THIS IS A MINOR QUIBBLE AS THE ECMWF STRUGGLES TO BRING ANY MOISTURE UP IN TIME FOR WHEN IT BRINGS THE CDFNT THRU...AND IF THIS DOES HAPPEN...SHOULD GO THRU DRY AS A SYSTEM FURTHER TO THE SE WILL STEAL THE SHOW AND HOG THE SENSIBLE WX. WILL NEED TO WATCH EXACTLY WHERE THIS SERN CONUS SYSTEM GOES AS IT COULD EDGE SOME PARTS OF SERN MO AND SRN IL ON MONDAY. TEMPS COULD BE A BIGGER SOURCE OF ERROR HERE...NAMELY IF THE FRONT ACTUALLY GOES THRU OR NOT...BUT TEMPS AT OR A BIT ABOVE AVERAGE SEEM THE BETTER BET. (WEDNESDAY - THURSDAY) GFS AND ECMWF CONTINUE THEIR STANDOFF INTO THE MID AND LATER PARTS OF NEXT WEEK. THE ECMWF FEATURES ZONAL FLOW RETURNING BUT MORE IN THE FORM OF A PLATEAUED OUT BUILDING RIDGE FROM THE SW WHILE THE GFS MAINTAINS A TROF OVER THE LOWER MS VALLEY WHICH THEN LIFTS NEWD INTO THE LOWER OH VALLEY AS IT GETS ABSORBED. EITHER WAY...BOTH ARE TAKING ANOTHER...STRONGER...CDFNT THRU THE AREA DURING THIS TIME BUT THE DIFFS IN FLOW ALOFT WILL MAKE FOR A TIMING QUANDARY WITH THE FRONT BY AS MUCH AS 24HRS. THE GFS TAKES THE CDFNT THRU WEDNESDAY WHILE THE ECMWF DOESN/T UNTIL THURSDAY. BUT ONCE AGAIN...MOISTURE RETURN FROM THE GULF IS EFFECTIVELY BLOCKED AND WILL MAKE FOR LIMITED AMOUNTS TO WORK WITH. VERY QUESTIONABLE WHETHER OR NOT FRONT CAN GENERATE PCPN HERE BUT IF STRENGTH OF COLD AIR BEHIND IS SUFFUCIENT...IT MAY. ABOVE AVERAGE TEMPS TO CONTINUE AHEAD OF FRONT. TES && .AVIATION... ISSUED AT 1152 PM CDT THU SEP 27 2012 HAVE ADDED VCSH AT KCOU AND THE ST. LOUIS METRO TAFS OVER THE NEXT 6 HOURS AS SCATTERED -RA HAS DEVELOPED OVER WRN MO AND WILL MOVE EAST OVERNIGHT. OTHERWISE...NOT CERTAIN IF CIGS WILL FALL INTO THE MVFR RANGE THROUGH 14Z...SO HAVE CIGS IN THE BKN035-040 RANGE. AFTER 15Z...EXPECT DRY AND VFR CONDITIONS THE REST OF THE PERIOD. SPECIFICS FOR KSTL...HAVE ADDED VCSH FROM 08-11Z TO REFLECT RECENT DEVELOPMENT OF -RA OVER CNTRL/WRN MO. THERE IS STILL SOME POSSIBILITY THAT CIGS WILL FALL INTO THE MVFR RANGE OVERNIGHT...BUT WITH PERSISTENT 10-15KT WINDS BETWEEN 1-3K FT NOTED ON THE KLSX AND TSTL WIND PROFILES THAT WILL IS BRINGING DRY AIR INTO THE REGION...DO NOT FEEL CONFIDENT ENOUGH AT THIS POINT TO INCLUDE IN A PREDOMINANT GROUP IN THE TAF. OTHERWISE...EXPECT DRY AND VFR CONDITIONS TO RETURN TO THE AREA BY MID-MORNING FRIDAY. BRITT && .PRELIMINARY POINT TEMPS/POPS... ST LOUIS 75 52 75 56 / 10 0 0 0 QUINCY 71 47 76 50 / 5 0 0 0 COLUMBIA 73 48 75 51 / 10 0 0 0 JEFFERSON CITY 73 49 74 51 / 20 5 0 0 SALEM 72 48 72 50 / 10 0 0 5 FARMINGTON 72 50 73 50 / 20 5 0 0 && .LSX WATCHES/WARNINGS/ADVISORIES... MO...NONE. IL...NONE. && $$ WFO LSX