000 FXUS63 KFSD 300201 AFDFSD AREA FORECAST DISCUSSION FOR SOUTHEAST SD/SOUTHWEST MN/NORTHWEST IA NATIONAL WEATHER SERVICE SIOUX FALLS SD 800 PM CST FRI DEC 29 2006 .DISCUSSION... UPDATED FCST FOR A BAND OF SNOW FM JUST W OF YKN TO MML. SNOWFALL AMOUNTS OF 2 TO 3 INCHES WILL OCCUR WITH THIS BAND THRU 06Z. THIS IS ASSOCD WITH H80 FNTGEN AND PROGGED WELL BY BOTH 18Z GFS AND NAM. THIS FEATURE BEGINS TO WEAKEN BY 06Z WITH FOCUS FOR HVYR SNOW SHIFTING TO THE W. JUST ENUF LIFT TO SWITCH THE R OVR TO SNOW WITH THIS FEATURE WITH LIGHTER PRECIP JUST SE OF THIS BAND IN THE FORM OF RAIN. HAVE CHANGED THE ZR ADVY TO A SNOW ADVY AS IT IS ALL SNOW RIGHT NOW..BUT CUD CHANGE TO ZR BY MORN. ALSO ADDED KINGSBURY AND MINER TO ADVY BECAUSE OF SNOW AND THEN PSBLTY OF ZR LATER TONIGHT. THE PRECIP MAY NOT CHANGE TO ZR- WHERE SNOW IS OCCURING DUE TO THE COOLING OF THE ATMOS BY THE SNOW. WL NEED TO WATCH THIS WHICH CUD CAUSE HVYR SNOW AMOUNTS THAN CURRENTLY FCST. .PREV DISCUSSION... SIGNIFICANT PRECIP EVENT UNDERWAY...WITH PRECIP TYPE ISSUES OVER MAINLY THE WESTERN CWA THE MAIN FORECAST CHALLENGE. AS OF 22Z... WEAK SURFACE REFLECTION OF INVERTED TROUGH LOCATED BETWEEN JAMES VALLEY AND I-29...WITH SLIGHTLY NNE WINDS ALONG AND EAST OF I-29 DRAWING ON SOMEWHAT MORE MOIST AIR...WHILE MORE NNW WINDS IN THE JAMES VALLEY AND WEST ARE PULLING IN SLIGHTLY DRIER/COOLER AIR INTO THE JAMES VALLEY. NAM SEEMED TO HAVE A LITTLE BETTER HANDLE ON WARM AIR ALOFT SO FAR TODAY...WITH KSUX AIRCRAFT SOUNDING MEASURING +4C AT 850MB AROUND 19Z. NAM ALSO SEEMS TO HAVE HAD BETTER HANDLE ON THE WARM LAYERS ALOFT ON THESE TYPE OF MIXED PRECIP SCENARIOS OVER THE LAST FEW EVENTS...AND THUS HAVE FAVORED WARMER SCENARIO OF NAM OVER THE NEXT 36-48 HOURS. AS THE MAIN LOW PRESSURE AREA TO OUR SOUTH CONTINUES TO EVOLVE OVER THE CENTRAL PLAINS...INVERTED TROUGH STRENGTHENS AND NAM/GFS SIMILAR IN SHIFTING WINDS THROUGH THE JAMES VALLEY BACK TO A MORE NNE DIRECTION BY SATURDAY MORNING. HOWEVER...STILL ENOUGH TIME FOR LOWER PORTIONS OF THE JAMES VALLEY TO ALSO SEE SURFACE TEMPS DROP BELOW FREEZING TONIGHT...AND HAVE ISSUED FREEZING RAIN ADVISORY FROM HANSON THROUGH BON HOMME COUNTIES FOR TONIGHT INTO SATURDAY MORNING. THESE AREAS WILL SEE SOME SNOW FOR A WHILE YET THIS EVENING...BUT THINK FREEZING RAIN WILL BE MAIN HAZARD HERE...THUS THE DIFFERENCE IN ADVISORY TYPES. WITH EXPECTED PRECIP AMOUNTS ON THE ORDER OF 1 TO 2 INCHES THROUGHOUT MOST OF THE CWA...PROJECTED SNOW AND ICE ACCUMULATIONS FOR THE WESTERN CWA STILL SOMEWHAT VARIABLE...DEPENDENT LARGELY ON WESTWARD PUSH OF SURFACE INVERTED TROUGH LATE TONIGHT THROUGH SATURDAY NIGHT. HOWEVER WITH A LITTLE MORE CONFIDENCE IN NAM SOLUTION...HAVE TRENDED TOWARD EVEN JAMES VALLEY WARMING ENOUGH TO SEE A SWITCH TO JUST LIQUID RAIN BY SATURDAY AFTERNOON...CONTINUING INTO SATURDAY NIGHT...BEFORE INVERTED TROUGH PUSHES BACK TO THE EAST AND COOLER AIR IS PULLED BACK INTO THE AREA. WITH THIS SWITCH TO RAIN LIMITING SNOW/ICE ACCUMULATION POTENTIAL...FEEL OK IN RESOLVING REMAINDER OF INHERITED WATCH TO A WINTER WEATHER ADVISORY. SOUTHWEST CORNER OF THE CWA MORE LIKELY TO REMAIN LOCKED INTO COLDER SURFACE AIR...ESPECIALLY GREGORY COUNTY WHERE ELEVATION WILL ALSO PLAY A FACTOR...SO GOING WINTER STORM WARNING LOOKS GOOD THERE. TRANSITION TO SNOW FROM NORTHWEST TO SOUTHEAST SUNDAY AND SUNDAY NIGHT STILL LOOKS REASONABLE. NAM KEEPS INVERTED TROUGH FARTHER WEST THROUGH THE DAY SUNDAY...WHICH DOES NOT SEEM UNREASONABLE GIVEN ANOTHER WEAK IMPULSE IN THE NORTHERN STREAM WHICH COULD HOLD NORTHERN END OF THE TROUGH FARTHER WEST ALONG THE CANADIAN BORDER. SNOWFALL ON THE ORDER OF 1 TO 3 INCHES STILL LOOKS POSSIBLE FOR AREAS EAST OF THE JAMES VALLEY THROUGH EARLY MONDAY. MON THRU FRI...WEATHER CONDITIONS TO BECOME MUCH MORE TRANQUIL ACROSS THE REGION. THE LAST VESTIGES OF STORM SYSTEM WILL LIKELY CONTINUE TO EFFECT THE AREA ON MONDAY AS UPPER LOW FINALLY EXITS...MOVING INTO THE ERN GREAT LAKES REGION. ONLY CONCERN ON MONDAY IS WITH THE RESIDUAL MOISTURE...SHOULD BE ENOUGH LIFT TO SQUEEZE OUT SOME FLURRIES...HOWEVER WITH LIMITED MIDLEVEL MOISTURE AND 700 MB TEMPERATURES ONLY AROUND -8 C...FREEZING DRIZZLE IS A POSSIBILITY. FROM MON NIGHT INTO TUE...GFS FINALLY SHOWING LOW LEVEL MOISTURE SLIDING EAST OF THE CWA...ALTHOUGH WOULD NOT BE SURPRISED IF IT TAKES A BIT LONGER TO BRING IN DRIER AIR...AS WESTERLY FLOW LOOKS TO BE FAIRLY WEAK. FROM TUE ON...MAIN CHALLENGE WILL BE TEMPERATURE FORECAST. TEMPERATURES WILL BE LARGELY DEPENDANT ON SNOW AND CLOUD COVER. GENERALLY WESTERLY FLOW WITH MILD AIR OVER THE REGION FOR THIS TIME OF YEAR. AREAS WHICH RECEIVE ACCUMULATING SNOW FROM CURRENT STORM SYSTEM COULD EASILY SEE LOW TEMPERATURES IN THE SINGLE DIGITS A FEW NIGHTS IF SKIES CLEAR. GENERALLY STAYED CLOSE TO GUIDANCE...THRU THE PERIOD...AS LITTLE CONFIDENCE WITH SNOW COVER...AND EVEN LESS CONFIDENCE IN CLOUD COVER. && .AVIATION... WITH PRECIPITATION SPREADING ACROSS THE REGION TONIGHT...IFR TO OCCASIONAL LIFR VISIBILITIES AND CEILINGS WILL PREVAIL AT ALL TAF SITES THROUGH 18Z ON SATURDAY. MIXED FROZEN PRECIPITATION WILL BE POSSIBLE...MAINLY WEST OF KMHE THROUGH 09Z...THEN TRANSITIONING TO MOSTLY RAIN WESTWARD TO NEAR KHON AND K9V9 BY 12Z ON SATURDAY MORNING. PLAIN RAIN IS EXPECTED OVER THE REMAINDER OF SOUTHEASTERN SD...NORTHWESTERN IA...AND SOUTHWESTERN MN THROUGH THE PERIOD. && IA...NONE. MN...NONE. NE...NONE. SD...WINTER STORM WARNING THIS EVENING TO 6 PM CST SUNDAY FOR SDZ050-057-063. WINTER WEATHER ADVISORY THIS EVENING TO 6 PM SUNDAY FOR SDZ038-052-053-058-059-064. WINTER WX ADVY TONIGHT THROUGH NOON SATURDAY FOR SDZ039-054060-065-068. $$ JH/SL/JM