000 FXUS63 KFSD 291712 AFDFSD AREA FORECAST DISCUSSION FOR SOUTHEAST SD/SOUTHWEST MN/NORTHWEST IA NATIONAL WEATHER SERVICE SIOUX FALLS SD 1111 AM CST THU NOV 29 2007 .DISCUSSION... TEMPS AGAIN STRUGGLING TO RECOVER TODAY...AND INCREASING HIGH CLOUD COVER AND LIGHT WINDS WITHIN SURFACE RIDGE NOT HELPING MATTERS ANY. SIMILAR TO PAST TWO DAYS...RUC SEEMS TO HAVE BEST HANDLE ON COOLER AIR SO WILL TREND IN THAT DIRECTION COOLING HIGHS FOR TODAY...ESP IN THE NORTHERN CWA. STILL LOOK ON TRACK FOR A QUICK DUSTING OF SNOW MOVING THROUGH THE AREA TONIGHT...THOUGH STILL HAVE A LOT OF DRY AIR TO OVERCOME BEFORE PRECIP WILL BEGIN REACHING THE GROUND. WILL LIKELY BE DOING SOME ADDITIONAL FINE TUNING TO PLACEMENT OF BETTER LIGHT SNOW CHANCES TONIGHT AFTER LOOKING AT MODELS MORE CLOSELY EARLY THIS AFTERNOON. UPDATED GRIDS/PFM HAVE BEEN SENT...AND WILL GET A NEW ZFP OUT SHORTLY. && .AVIATION... HIGH CLOUDS WILL SPILL IN FROM THE WEST THROUGH THE MORNING HOURS WITH STEADILY FALLING CEILINGS INTO THE AFTERNOON. VFR EXPECTED THROUGH EARLY AFTERNOON BUT BY LATE IN THE DAY...COULD SEE CIGS DROP INTO HIGH MVFR RANGE AS NEXT COLD FRONT APPROACHES. ALSO...COULD SEE SOME SCATTERED LIGHT SNOW SHOWERS OR FLURRIES THURSDAY EVENING...BUT IS NOT LIKELY TO RESTRICT VISIBILITIES. $$ .PREV DISCUSSION... CHALLENGING FCST THIS GO AROUND...WITH MODEL DISCREPANCY LARGE WHICH DOES NOT HELP MATTERS ANY. FORGOED HEADLINES FOR NOW...BUT WE THOUGHT SERIOUSLY ABOUT THEM. MORE ON THAT LATER. IN THE NEAR TERM TODAY...NARROW BAND OF LOWER CLOUDS HANGING TOUGH JUST N OF A SUBTLE WIND SHIFT LINE IN OUR NRN FA ATTM. MORE IMPORTANTLY...THICKER HIGH CLOUDS ARE MOVG RAPIDLY EWD. THESE CLOUDS WL LIKELY TEMPER HIGH TEMPERATURES ON THE COOL SIDE TODAY. BASICALLY I TOOK THE H925 TEMPS AND RAISED THEM ABOUT 4 TO 5 DEGS...AS THERE SHOULD NOT BE A LOT OF MIXING TODAY DUE TO THE LIGHT LOW LEVEL WINDS. BUT ALL IN ALL...DUE TO LIGHT WINDS...NOT A BAD DAY IN STORE WX WISE. FOR TONIGHT...SUBTLE UPPER AIR DISTURBANCE MOVES THRU THIS AREA WITH A SFC CDFNT PASSAGE. STILL A BAND OF MID LVL FRONTOGENESIS EXISTS WITH THIS DISTURBANCE...SO KEPT SOME CHCS FOR -SN GOING RIGHT THRU THE HEART OF THE FSD FA FM W TO E. DYNAMICS RAPIDLY EXITS OUR ERN FA BETWEEN 06Z TO 09Z...SO ANY PCPN SHOULD COME TO AN END BY THEN. RH TIME SECTIONS ARE SHOWING SOME RAPIDLY DEPARTING CLOUD COVER AFT 09Z...SO LOW TEMPS COULD FALL PRETTY GOOD WITH THE MODERATELY STRONG CAA. NW FLOW MIXING POTENTIAL SHOULD HELP KEEP THE LOW TEMPS FM TOTALLY PLUMMETING HOWEVER. FRIDAY WL BE A BREEZY DAY OVR MUCH OF OUR FA WITH THE W TO NW SFC FLOW LINGERING. DEPENDING ON THE DEGREE OF MIXING...IF WE MIX ABOVE H9...MY HIGH TEMPS WL BE TOO COOL. FRIDAY NIGHT...STRONG UPPER AIR DISTURBANCE BEGINS TO DIG IN THE INTERMOUNTAIN WEST TURNING THE SFC FLOW AROUND TO THE SE. MID TO HIGH CLOUDS WL BEGIN TO RAPIDLY INCREASE FM THE SW. WITH THE INCREASING SFC WINDS AND CLOUD COVER... DECIDED TO GO WELL ABOVE GUIDANCE READINGS MOST LOCATIONS. ONE WONDERS IF EVEN THESE LOW TEMP READINGS WL BE TOO COOL IN TIME. AS FAR AS PCPN...GFS IS EXTREMELY BULLISH IN BRINGING LIGHT QPF INTO MUCH OF THE FSD FA. THE NAM HAS NONE. DECIDED TO KEEP QPF IN OUR FAR SRN FA WHERE SOME SEMBLANCE OF MSTR EXISTS BLO H7. EVEN THE RH TIME SECTIONS ON THE GFS ARE FAIRLY DRY BLO ABOUT H75 BETWEEN 06Z-12Z SAT MORNING IN OUR FAR SRN FA. THEREFORE BELIEVE THAT THE GFS IS UP TO ITS TRICKS OF OVEREXPANDING QPF AND NOT ALLOWING FOR INITIAL EVAPORATION. BUT ON SATURDAY...BOTH THE NAM AND GFS SHOW A LOT OF SATURATION BEGINNING OVR THE FA BY 12Z. THE NAM KEEPS THE SFC LOW DOWN IN THE SWRN PLAINS AND WAS DISCOUNTED. THE GEM BRINGS THE SFC LOW UP TO SIOUX CITY...WHILE THE GFS IS A BIT FURTHER SOUTH. FOR SFC WINDS... DECIDED TO GO MOSTLY WITH THE GFS BUT IN REALITY...I LIKE THE ECMWF SFC PATTERN THE BEST WITH THE SFC LOW ELONGATED FM NRN KS TO W CENTRAL IA BY 00Z SUNDAY. THIS MATCHES UP WELL WITH THE VARIOUS MODELS UPPER AIR PATTERN. VERY STRONG PV IS NOTED IN THE MID LVLS WITH THIS SYSTEM LIFTING NWD IN AN W TO E BAND ACROSS THE FA...LINGERING IN OUR ERN FA SAT NIGHT. SO THE TROP FOLD WL HAVE TO BE WATCHED IN TIME FOR SFC LOW INTENSIFICATION. AT ANY RATE... BECAUSE OF THE SRN SOLN OFFERED BY THE NAM...THE NAM H85-H7 THERMAL PROFILE IS LIKELY TOO COLD. THEREFORE FOLLOWED THE ECMWF AND GFS THERMAL PROFILE ABOVE THE SFC WHICH IS MUCH WARMER. WAA AND BROAD ASCENT IS EXTREMELY STRONG ON SATURDAY...AND AM DEFINITELY WORRIED ABOUT MIXED PCPN WORKING ITS WAY INTO NW IA AND IMMDEDIATELY ADJACENT AREAS AT LEAST BY MIDDAY SATURDAY. INITIALLY...THE THERMAL PROFILES START OFF COLD AT 12Z SATURDAY. AND WITH PLENTY OF DEEP SATURATION...ICE SHOULD BE NO PROBLEM AT THE TOP OF THE CLOUD LYR EARLY SATURDAY. BUT AS THE WARMER AIR INTRUDES IN FM H85-H7 IN OUR SERN FA BY LATE MORNING SATURDAY...WX TYPE COULD GET INTERESTING. THEREFORE OPTED TO PUT IN A WINTERY MIX THROUGH OUR SE QUARTER INTO SAT EVENING. DECIDED TO FORGO HEADLINES ATTM...BECAUSE IF THE SFC LOW PLACEMENT OFF OF THE GEM HAPPENS TO PAN OUT...IT IS NOT OUT OF THE REALM OF POSSIBILITIES THAT NW IA COULD CHANGE TO PURE RAIN BY MIDDAY SATURDAY. THIS HAS HAPPENED BEFORE. EVEN THE GFS SFC LOW PLACEMENT COULD BE CLOSE TO PURE RAIN. BUT THE ECMWF SFC LOW PLACEMENT WOULD BRING IN A LOT OF LOW LEVEL COLD AIR UNDER THE WARM AIR ALOFT...AND THIS SOLN WAS BOUGHT ATTM. DECIDED TO KEEP DIURNAL TEMP SPREADS TO A MINIMUM ON SATURDAY FOR THE OBVIOUS REASONS. SYSTEM SHOULD THEN BEGIN TO PULL OUT RATHER RAPIDLY AFTER MIDNIGHT SATURDAY NIGHT. ENDED PCPN ACCORDINGLY FM W TO E ON SUNDAY. && .FSD WATCHES/WARNINGS/ADVISORIES... SD...NONE. MN...NONE. IA...NONE. NE...NONE. && $$ JH