440 FXUS63 KUNR 151737 AFDUNR AREA FORECAST DISCUSSION FOR WESTERN SD AND NORTHEASTERN WY NATIONAL WEATHER SERVICE RAPID CITY SD 1037 AM MST TUE JAN 15 2013 .AVIATION...18Z TAFS IFR/LIFR CIGS/VIS IN SNOW WILL DEVELOP OVER NERN WY INTO THE BLKHLS AND NWRN SD BY THIS AFTERNOON. OVER THE REMAINING SD PLAINS...CIGS ARE EXPECTED TO DROP TO MVFR EARLY THIS AFTERNOON OVER MOST LOCATIONS...WITH AREAS OF IFR VIS EXPECTED IN AREAS OF LIGHT SNOW. CONDITIONS WILL IMPROVE LATE TONIGHT FROM WEST TO EAST AS SNOW ENDS. && .PREV DISCUSSION... /ISSUED 856 AM MST TUE JAN 15 2013/ UPDATE...VIGOROUS LL WAA UNDER THE LEFT EXIT REGION OF AN ANTICYCLONICALLY CURVED UPPER JET CONTINUES TO SPREAD SE AND INTENSIFY OVER THE REGION. LL FORCING FOR ASCENT WILL CONTINUE TO INCREASE OVER THE REGION...MAXIMIZING LATE THIS AFTERNOON AND EARLY EVENING OVER NE WY. UPSLOPE ENHANCEMENT EXPECTED WITH A RELATIVELY DECENT SETUP FOR A STRONG RESPONSE. FROUDE NUMBERS SURPASS 1 THIS AFTERNOON IN THE NORTHERN BH...WITH RH IN THE FORCING LAYER OVER 90 PERCENT. ONLY LIMITING FACTOR IS STRONG THERMAL RIDGING IN THE LL/S WHICH WILL PUSH THE SNOW PRODUCTION ZONE OUT OF THE DGZ...IN ADDITION TO SUPPORTING MARGINAL LL LAPSE RATES. REGARDLESS...GIVEN A STRONG DYNAMICAL SYSTEM...FORECAST MODELS ARE SUPPORTING DECENT QPF AMOUNTS...AND GIVEN MEAN SNOW RATIOS 2 TO 5 INCHES ARE EXPECTED OVER THE NORTHERN BLACK HILLS...WITH LOCALLY HIGHER AMOUNTS LIKELY WHERE FORCING IS MAXIMIZED IN TIME. HAVE DECIDED TO ISSUE A WINTER WEATHER ADV IN THE NORTHERN BH FOR THESE CONCERNS. ALL ELSE ON TRACK FOR NOW. PREV DISCUSSION... /ISSUED 332 AM MST TUE JAN 15 2013/ DISCUSSION...10Z SFC ANALYSIS SHOWS HIGH PRESSURE WEST OF THE ROCKIES WITH A LEE SIDE TROUGH EAST OF THE ROCKIES. RESULTING IN A TIGHT SURFACE PRESSURE GRADIENT TO THE WEST OF THE FORECAST AREA. UPPER ANALYSIS SHOWS A RIDGE OVER THE WEST COAST...WITH A LARGE AMPLITUDE TROUGH EXTENDING FROM A DEEP LOW PRESSURE SYSTEM OVER HUDSON BAY...ACROSS THE DAKOTAS AND INTO EASTERN COLORADO AND NEW MEXICO. WAA ALOFT WILL INCREASE OVER THE AREA TODAY WITH ISENTROPIC LIFT DEVELOPING OVER THE COLDER AIR AT THE SURFACE...MAINLY OVER THE WESTERN HALF OF THE FORECAST AREA. THE ISENTROPIC LIFT AND INCREASING MOISTURE WILL LEAD TO SNOW DEVELOPING OVER NORTHEASTERN WYOMING LATER THIS MORNING AND THEN SPREADING EASTWARD INTO THE BLACK HILLS AND FAR NORTHWESTERN SOUTH DAKOTA THIS AFTERNOON. INCREASING WEST TO NORTHWEST WINDS WILL SUPPORT UPSLOPE SNOW OVER PARTS OF THE NORTHERN BLACK HILLS. NAM FROUDE PLOT SHOWS THE BEST PERIOD FOR UPSLOPE ENHANCEMENT WOULD OCCUR FROM 18Z TODAY UNTIL AFT 06Z TONIGHT. WITH MORE WESTERLY WIND COMPONENT...EXPECT TO SEE THE HIGHEST SNOW ACCUMULATIONS OVER THE WEST-NORTHWEST SIDE OF THE BLACK HILLS. LOOKS LIKE A WINDY DAY OVER NORTHWESTERN SOUTH DAKOTA. NAM IS SHOWING 40-45KT 850MB WINDS OVER NORTHWESTERN SOUTH DAKOTA THIS AFTERNOON. HOWEVER...WITH THE WAA OVERRUNNING THE COLDER AIR AT THE SURFACE EXPECT MOMENTUM TRANSFER TO THE SURFACE TO BE LIMITED. AVAILABLE LIFT AND MOISTURE WILL BE MORE LIMITED OVER THE REST OF WESTERN SOUTH DAKOTA SO JUST EXPECT MINOR SNOW ACCUMULATIONS THERE THIS AFT AND EVENING. WEDNESDAY HAS THE POTENTIAL TO BE VERY WINDY WITH NAM INDICATING STRONGER WINDS ALOFT AND WEAK LOW LEVEL CAA AIDING MIXING AND MOMENTUM TRANSFER TO THE SURFACE. EXTENDED...WEST TO NORTHWEST FLOW WILL CONTINUE OVER THE REGION THROUGH THE END OF THE WEEK...BRINGING MILD TEMPERATURES AND DRY CONDITIONS. CHANGES WILL COME THIS WEEKEND AS UPPER FLOW SHIFTS MORE NORTHERLY AS A STRONG COLD FRONT PUSHES INTO THE AREA. TEMPERATURES WILL COOL DRAMATICALLY BY SUNDAY AND MONDAY AS ARCTIC AIR SETTLES ACROSS THE NORTHERN PLAINS...THOUGH HOW COLD IT WILL GET IS STILL IN QUESTION AS MODELS DIFFER HOW FAR WEST THE COLDEST AIR WILL PUSH. && .UNR WATCHES/WARNINGS/ADVISORIES... SD...WINTER WEATHER ADVISORY UNTIL 11 PM MST THIS EVENING FOR CENTRAL BLACK HILLS-NORTHERN BLACK HILLS. WY...WINTER WEATHER ADVISORY UNTIL 11 PM MST THIS EVENING FOR WYOMING BLACK HILLS. && $$ SHORT TERM...JC LONG TERM....WFO UNR AVIATION...15