Issued by NWS Sioux Falls, SD
000 FXUS63 KFSD 220937 AFDFSD AREA FORECAST DISCUSSION FOR SOUTHEAST SD/SOUTHWEST MN/NORTHWEST IA NATIONAL WEATHER SERVICE SIOUX FALLS SD 340 AM CDT MON SEP 22 2008 .DISCUSSION... ATTM...ANOTHER ROUND OF TS HAS RECENTLY FIRED UP IN A NARROW LINE FROM EAST CENTRAL SD...EXTENDING SWD TO ALONG THE YANKTON/BON HOMME CO LINE. THIS ACTIVITY IS ALONG THE BACK EDGE OF THE DEPARTING S/W. IT IS NOT MOVG EWD VERY FAST...AND IT WL LIKELY WEAKEN SOMEWHAT THRU THE EARLY MORNING HRS. BUT SOME OF IT COULD VERY WELL HOLD TOGETHER THRU THE ERN HALF OF THE FA SO SCT TO LIKELY POPS ARE WARRANTED VERY EARLY TODAY. BOTH THE 00Z AND 06Z NAM40 PICKED UP FAIRLY STRONG MID LVL PV NEAR THE BACK EDGE OF THE S/W WHICH IS LIKELY AIDING THE CURRENT CONVECTION...ALONG WITH A VERY STOUT LLJ AND PERSISTENT WAA. WHAT IS BOTHERSOME...IS THE LLJ DOES NOT SLACKEN OFF THIS MORNING WHILE THIS PV EXITS INTO PARTS OF SW MN AND NW IA. SO AM CONCERNED THAT ANY LITTLE BUBBLE OF WAA WHICH DECIDES TO GENERATE WL ENHANCE THE BROAD ASCENT AND SPARK SOME TS IN OUR ERN FA POSSIBLY RIGHT INTO MIDDAY. THUS LINGERED POPS ALONG AND E OF I 29 FM 15Z-18Z. HAVING AN UNSTABLE SW FLOW ALOFT CERTAINLY DOES NOT HELP MATTERS EITHER WITH THIS SCENARIO. WL ALSO HAVE TO KEEP A CLOSE EYE ON OUR WRN FA EARLY THIS MORNING AND WATCH TO SEE IF ANY ISOLD ACCAS WL SPARK A TS OR TWO IN THOSE AREAS. THERE IS CURRENTLY ACCAS IN CENTRAL SD MOVG EWD. AS FAR AS MAX TEMPS...BASICALLY STAYED THE COURSE. THERE SHOULD BE SOME SUNSHINE EVERYWHERE IN OUR FA AT LEAST BY AFTERNOON...WITH THE OVERALL CLEAREST AREAS IN OUR WEST. THEREFORE WITH THE STOUT SSE WINDS...SEE NO REASON WHY TEMPS WL NOT WARM UP ONCE AGAIN. OUR ATTENTION THEN TURNS TO TONIGHT AND TUESDAY WITH THE NEXT VIGOROUS S/W PASSAGE. BOTH THE NAM AND GFS HAVE GREATLY SLOWED DOWN THE CRUX OF THE ENERGY WITH THIS NEXT WAVE...AND THE ATMOSPHERE IS QUITE CAPPED OFF AHEAD OF IT. THEREFORE DECIDED TO SLOW DOWN OUR POPS...WHICH WL MEAN THAT THEY LINGER INTO TUE MORNING THRU MUCH OF OUR FA MINUS OUR FAR W. EVEN HAD TO LINGER POPS THRU OUR ERN HALF TUE AFTERNOON AS STRONG SW FLOW ALOFT STILL EXISTS COUPLED WITH A WEAK SFC BOUNDARY WHICH MOVES TOWARD THE I 29 CORRIDOR BEFORE WEAKENING AND STALLING. THEREFORE WOULD NOT BE SURPRISED TO SEE SOME TS DEVELOP OFF AND ON ALL TUE AFTERNOON ALONG AND E OF I 29 WITH PERSISTENT WAA AND BROAD ASCENT. DUE TO ABUNDANT INSTABILITY...THERE COULD BE A THREAT FOR SVR WX IN OUR FAR WEST MON EVENING. ALSO... ELEVATED INSTABILITY PER THE NAM H85-H8 LI FIELDS AND H925-H85 MU CAPES REVEAL MODERATE INSTABILTY ALL NIGHT AND TUE MORNING WITH THE MAIN UPPER WAVE PASSAGE. THE HIGHEST POPS ARE WARRANTED MONDAY NIGHT IN OUR WEST WHO ARE ALSO IN A FAVORED ENTRANCE REGION OF A STRONG JET STREAK WHICH IS ELONGATED SOUTH TO NORTH JUST TO THE NW OF THIS AREA. AS THIS JET STREAK MOVES EWD LATE MON NIGHT AND TUE...THE REST OF THE CWFA WL ALSO BE IN A SUBTLE ENTRANCE REGION WHICH MAY ENHANCE LIFT. FOR TEMPERATURES...ALTERED OUR LOWS AND MAX TEMPS ON TUE VERY LITTLE AS MODEL LLV TEMPS HAVE NOT CHANGED AS WELL AS FCST WINDS. TUE NIGHT AND WED...A LLV BOUNDARY LOOKS TO HANG UP JUST FAR ENUF SOUTH OF OUR FA AS TO NOT IMPACT THIS AREA. REALLY DID NOT ALTER TEMPS HARDLY AT ALL AS THE NAM AND GFS CONT TO BE IN PRETTY GOOD AGREEMENT WITH RECENT RUNS IN THEIR H925-H85 TEMP FCSTS. HOWEVER... THERE ARE LARGE DISCREPANCIES BETWEEN THE TWO MODELS IN HOW THEY HANDLE THE SFC WINDS TUE NIGHT AND WED IN THIS AREA. THE GFS IS QUITE BULLISH WITH SFC HIGH PRESSURE IN THIS AREA WHEREAS THE NAM BRINGS AN ELY FETCH IN A LOT SOONER. IT IS SO DIFFICULT AT THIS POINT TO TELL WHICH SOLN MAY BE BETTER. SO BLENDED THE TWO INTO OUR CURRENT FCSTS WHICH GIVES A REASONABLE WIND ESTIMATE. AT ANY RATE...THE WINDS SHOULD BE PRETTY LIGHT ANYWAY. THEN THE NEXT INTERESTING FEATURE IS WED NIGHT AND THURSDAY. THE LAST COUPLE OF RUNS OF THE GFS/GEM AND ECMWF SHOW A SUBTLE MID LVL S/W MOVG THRU THIS AREA WITH A MODERATELY STRONG LLJ RETURNING TO THIS AREA. THIS IS ALWAYS GOOD FOR SOME ACCAS TS AS THE WAA IS PRETTY STRONG. && .AVIATION... ONGOING CONVECTION VICINITY OF KFSD AND KSUX SHOULD DIE OFF MID TO LATE MORNING LEAVING VFR BUT WINDY CONDITIONS ACROSS THE FA FOR THE AFTERNOON...AS MODERATE SOUTHERLY FLOW PERSISTS IN ADVANCE OF AN APPROACHING COLD FRONT. INCREASING INSTABILITY AS THE FRONT GETS CLOSER WILL RAISE THE RISK OF THUNDERSTORMS AT KHON BY EVENING...BUT WARM AIR CAPPING WILL LIMIT THE THUNDERSTORM THREAT AT KFSD AND KSUX TIL AFTER 23/09Z. SOME ISOLATED MVFR CIGS IN THE HEAVIER CONVECTIVE SHOWERS THIS MORNING...MORE WIDESPREAD MVFR CIGS EARLY TUESDAY MORNING AS THE FRONT MOVES THROUGH THE AREA. && .FSD WATCHES/WARNINGS/ADVISORIES... SD...NONE. MN...NONE. IA...NONE. NE...NONE. && $$ MJF/HARMON
