000 FXUS63 KFSD 260633 AFDFSD AREA FORECAST DISCUSSION FOR SOUTHEAST SD/SOUTHWEST MN/NORTHWEST IA NATIONAL WEATHER SERVICE SIOUX FALLS SD 1230 AM CST WED DEC 26 2007 .DISCUSSION... FRONTOGENETICAL BAND OF SNOW PERSISTING ACROSS THE SOUTHEAST HALF OF THE FA TONIGHT AND HAVE MADE ADDITIONAL ADJUSTMENTS TO ADVISORIES... PRIMARILY TO EXTEND BULK OF NORTHWEST IOWA UNTIL 12Z. SNOW INTENSITY APPEARS TO BE WANING A BIT FROM EARLIER...BUT SURFACE OB VSBYS STILL INDICATING DECENT QUARTER TO HALF INCH PER HOUR RATES POSSIBLE WITH EMBEDDED HIGHER RATES. && .AVIATION... MVFR TO IFR CONDS ACRS SERN CWA THRU 12Z WED WITH VFR BECOMING MVFR TO THE N. && .PREV DISCUSSION... MID LVL FNTLGEN BAND OF SNOW CONTS TO SIT ACRS SE SD INTO SW MN. THIS SHUD SLOWLY WEAKEN THIS EVE AS ENERGY ASOCD WITH THIS BAND LIFTS NEWD. SO WUD EXPECT THIS BAND TO PRODUCE AROUND AN ADDITIONAL INCH OF SN. MODELS SHOWING SECOND H65 VERY STRONG FNTLGEN BAND OF SNOW DEVLPNG AFTER 00Z ACRS NW IA...JUXTAPOSITIONED IN THE RR QUAD OF A JET STREAK TO OUR N AND VERY STRONG MID LVL PV. SAT IMAGES SHOW THIS BEGINNING TO OCCUR OVR SWRN NEB AS SNOW IS INCREASING THERE. THIS IS SHIFTING EWD AND CUD GET 2-4 INCHES OF SNOW ACRS NW IA TONIGHT. SO WL KEEP ADVY GOING THERE THRU 09Z AND END ACRS REST OF ADVY BY 02Z. ON WED...LINGERING MID LVL FRONTOGENESIS AND UPPER -DIV Q...COUPLED WITH THE RR QUAD OF THE JET AND MID LVL PV REMAINS OVR NW IA AND A SMALL PORTION OF SW MN AROUND WINDOM AND JACKSON. THEREFORE KEPT SOME LIKELY POPS FOR SN GOING ALONG OUR ERN BORDER WITH DMX. THERE COULD BE A LINGERING FLURRY AROUND SPENCER AND STORM LAKE IA EARLY AFTERNOON ON WED...AS THE DYNAMICS IS HUGGING THAT AREA FM 18Z-21Z... BUT OPTED TO SHUT THE SNOW OFF AT 18Z FOR NOW. RH TIME SECTIONS SHOW STUBBORNLY HIGH RH IN THE LOW LEVELS THRU OUR ENTIRE FA ON WED... EVEN IN OUR WEST. I CAN PICTURE THIS ACROSS OUR EAST...BUT ONE WONDERS IF THE HIGH LOW LEVEL RH IS AN ARTIFACT OF THE COLD AIR IN OUR WEST. WITH THE SUBSIDENCE INVOLVED...OPTED TO DECREASE THE CLOUDS OVR MUCH OF THE FA...HOLDING ONTO SOME MOSTLY CLOUDY SKIES IN OUR FAR E. WENT WITH THE COOLER SIDE OF GUIDANCE TEMPS WITH THE FRESH SNOW COVER. HOWEVER IT WL NOT BE REAL COLD AS MODEL H925 TEMPS ARE STILL AROUND -3 TO -6C. THEREFORE BELIEVE WIDESPREAD 20S ARE WARRANTED. FOR WED NIGHT...S/W RIDGING PASSES OVR THE AREA GIVING DRY CONDITIONS. HOWEVER THERE WL BE A LOT OF MID AND HIGH CLOUDS MOVG IN FM THE SW...TRAVELING SW TO NE. THEREFORE LOWS WED NIGHT ARE VERY TRICKY. IT COULD BE ONE OF THESE DEALS THAT OUR SWRN FA MAY NOT FALL OFF THAT MUCH WED EVENING AND OVERNIGHT...CONSIDERING THE CLOUDS AND RETURN SFC FLOW. BUT IN OUR ERN THIRD...HIGH CLOUDS WL ARRIVE BUT MAY NOT BE THAT THICK. THEREFORE WENT PRETTY CHILLY FOR LOWS IN OUR E AND MILD IN OUR SW. ALSO...CONTINUED THE TREND OF GOING A BIT NON DIURNAL...WITH SOME SLIGHTLY RISING OR STEADY TEMPS IN OUR E AFTER MIDNIGHT. BUT DID NOT WANT TO GO TOO NUTS WITH THE NON DIURNAL TREND AS THE WINDS ARE STILL NOT REAL FAVORABLE IN OUR ERN HALF FOR A STEADY RISE OF TEMPS. THEN ON THU...ANOTHER WELL ADVERTISED LARGE UPPER TROF MOVES INTO THE SWRN PLAINS...KICKING UP INTO THE CENTRAL PLAINS THU NIGHT. DUE TO THE COLD SFC HIGH PRES HANGING OUT TO OUR NE...DRY AIR IN THE LOW LEVELS CONTINUALLY ADVECTS INTO MUCH OF FA ON THU. THEREFORE ELIMINATED THE POPS IN OUR W THU MORNING...AND SCALED THEM BACK TO OUR SW THU AFTERNOON. PER THE NAM AND GFS...ISENTROPIC LIFT BEGINS TO GET PRETTY STRONG IN THE 305-310K LYR BY 18Z THU IN THOSE AREAS AND IS HARD TO IGNORE. BUT THE LOW LEVEL DRY AIR WL BE HARD TO OVERCOME. WITH THE COOL DRY AIR COMING IN FM MN...WENT WL BLO GUIDANCE HIGHS IN OUR ERN HALF...ESP IN SW MN WHERE AN ENE SFC FLOW NEVER DOES THEM ANY FAVORS...ESP WITH FRESH SNOW COVER. BUT THEN ON THU NIGHT...THE PROFILE SATURATES UP FINALLY...AND THE ASCENT IS STRONG IN OUR SE QUARTER...INCLUDING NW IA AND IMMEDIATE ADJACENT AREAS. SO KEPT SOME LIKELY POPS GOING IN OUR SE CORNER WITH AREAS OF SNOW ELSEWHERE. THIS SYSTEM BEARS WATCHING...SINCE THE MODELS HAVE NOT BEEN HANDLING QPF WELL LATELY...ESP THE NAM OUTPUT. AND THE ISENTROPIC LIFT IS VERY STRONG WITH THIS SYSTEM IN OUR E. GENERALLY WENT ABOVE GUIDANCE FOR LOWS...ESP IN OUR ERN HALF WITH THE CLOUD COVER. DYNAMICS LINGERS IN OUR ERN HALF ON FRIDAY BUT THEN DEPARTS RAPIDLY. IN THE EXTENDED SAT THRU TUE...CONFIDENCE IS AGAIN NOT REAL STRONG WITH THE FAST FLOW ALOFT. USED A BLEND OF THE 00Z ECMWF AND 00Z GFS FOR THIS PKG. FOR DETAILS...THE 06Z GFS STARTED TO LOOK FUNNY BY SUNDAY IN BRINGING UPPER TROFFING INTO WRN CALIFORNIA...THEN DIGGING IN FURTHER IN OUR SW CONUS MON AND TUE. THIS SOLN WAS NOT BACKED UP BY ANY OTHER MEDR MODELS...INCLUDING THE 12Z GFS. IN ADDITION...THE SMALL S/W ENERGY PASSING THRU THIS AREA SUNDAY AND SUNDAY NIGHT PER THE 00Z GFS WAS NOT COLLABORATED BY OTHER MODELS...SO ITS QPF OUTPUT WAS IGNORED FOR NOW. THE GFS AND ECMWF ARE HOWEVER CONTINUING TO SHOW A COLD AIR SURGE COMING DOWN MON AND MON NIGHT. PRETTY GOOD FRONTAL BOUNDARY WITH THIS FEATURE IS NOTED...WITH HIGH LEVELS OF RH BEHIND THE SYSTEM. THEREFORE BELIEVE SOME SMALL CHCS FOR -SN ARE WARRANTED ON MONDAY WITH VERY COLD AIR ALOFT...PROGGED AT -20C TO -22C AT H7. DID NOT TANK LOWS TOO BAD NEXT TUE MORNING HOWEVER DUE TO THE ANTICIPATED SFC WINDS BEING PRETTY STRONG. AND WENT BLO MOST GUIDANCE READINGS NEXT TUE WITH VERY COLD AIR AROUND. ALSO WENT A FEW DEGS BLO MANY GUIDANCE VALUES ON SATURDAY WITH THE FRESH SNOW COVER IN PLACE. && .FSD WATCHES/WARNINGS/ADVISORIES... SD...SNOW ADVISORY TO 3 AM CST WEDNESDAY FOR SDZ067-070. SNOW ADVISORY TO 6 AM CST WEDNESDAY FOR SDZ071. MN...SNOW ADVISORY TO 3 AM CST WEDNESDAY FOR MNZ098. SNOW ADVISORY TO 6 AM CST WEDNESDAY FOR MNZ081-089-090. IA...SNOW ADVISORY TO 6 AM CST WEDNESDAY FOR IAZ001>003-012>014-020>022-031-032. NE...SNOW ADVISORY TO 6 AM CST WEDNESDAY FOR NEZ013-014. && $$ JH