FXUS63 KUNR 011649
AFDUNR

AREA FORECAST DISCUSSION FOR WESTERN SD AND NORTHEASTERN WY
NATIONAL WEATHER SERVICE RAPID CITY SD
949 AM MST TUE DEC 1 2009

.DISCUSSION...UPDATED FORECAST THIS MORNING FOR SNOW SHOWERS DEVELOPING
ACROSS NORTHEAST WYOMING...THE BLACK HILLS...AND NORTHWEST SOUTH
DAKOTA. INCREASED POPS FOR THE REST OF THIS MORNING AND THIS
AFTERNOON. ALSO HAVE INCREASED SNOW AMOUNTS ACROSS THE NORTHERN
BLACK HILLS. EXPECT AROUND 1 INCH ACROSS THE NORTHERN BLACK HILLS
TODAY. THE HIGHER ELEVATIONS COULD SEE A LITTLE MORE. THE REST OF
THE FORECAST IS ON TRACK. 

&&

.PREV DISCUSSION... /ISSUED 254 AM MST TUE DEC 1 2009/ 

DISCUSSION...WATER VAPOR IMAGERY SHOWS A DEEPENING TROUGH CENTERED 
ACROSS THE NORTHERN ROCKIES WITH A STRONG WAVE DIVING DOWN FROM 
BRITISH COLUMBIA. SURFACE ANALYSIS SHOWS STRONG COLD FRONT JUST NOW 
ENTERING FAR NORTHWEST SOUTH DAKOTA AS LOW PRESSURE SLIDES ACROSS 
NORTH DAKOTA. STRONG PRESSURE RISES WILL BRING SOME VERY WINDY 
CONDITIONS TO THE SOUTH DAKOTA PLAINS TODAY. FORECAST SOUNDINGS 
SUGGEST WINDS OF 20 TO 35 MPH TRANSFERRING TO THE SURFACE...WITH 
GUSTS UP TO 50 MPH POSSIBLE. AS THE TROUGH SWINGS THROUGH AND LAPSE 
RATES STEEPEN...SNOW SHOWERS WILL DEVELOP MAINLY ACROSS NORTHEAST 
WYOMING AND FAR WESTERN SOUTH DAKOTA. SOME SNOW SHOWERS WILL 
CONTINUE THROUGH THE NIGHT TONIGHT...BUT WILL BE MORE LIMITED IN 
COVERAGE. TEMPERATURES TODAY WILL BE TRICKY GIVEN THE FRONTAL 
PASSAGE COOLING TEMPS OFF THROUGH THE DAY. HIGHS WILL BE IN THE 
LOWER TO MID 30S ACROSS NORTHEAST WYOMING AND NORTHWEST SOUTH 
DAKOTA...AND UPPER 30S TO MID 40S ACROSS THE REST OF THE SOUTH 
DAKOTA PLAINS. TEMPS WILL THEN QUICKLY DROP AS WINDS WEAKEN AROUND 
SUNSET...WITH LOWS TONIGHT IN THE TEENS ACROSS MOST LOCATIONS AND 
PERHAPS SINGLE DIGITS ACROSS THE BLACK HILLS.

A LARGE AND DEEP UPPER LOW WILL SINK SOUTHWARD ACROSS THE UPPER 
MIDWEST WEDNESDAY WITH A VERY DEEP COLD AIR MASS ACROSS THE NORTHERN 
PLAINS. TEMPS AT 700 MB ARE FORECAST TO DROP TO -22C. TEMPS AT 850 
MB ON WEDNESDAY WILL BE AT -10C...THEN DROPPING TO -16C BY THURSDAY 
AS EVEN COLDER AIR ADVECTS IN. FORECAST PROFILES CONTINUE TO SHOW A 
GOOD DEAL OF INSTABILITY AND MOISTURE ACROSS THE LOW TO MID LEVELS 
AS THE ARCTIC AIRMASS SLIDES OVER. THIS WILL BRING SNOW SHOWERS TO 
MUCH OF THE CWA WEDNESDAY...SLIGHTLY DECREASING THROUGH WEDNESDAY 
NIGHT. NORTHWEST WINDS ARE EXPECTED TO BRING UPSLOPE SHOWERS TO THE 
NORTHERN BLACK HILLS WEDNESDAY WITH PERHAPS A FEW INCHES OF 
ACCUMULATION POSSIBLE. THE SNOW SHOWERS WILL GRADUALLY DECREASE 
ACROSS THE FORECAST AREA THURSDAY AS INSTABILITY WEAKENS IN THE LOW 
LEVELS...AND FINALLY WANE BY THURSDAY NIGHT. TEMPERATURES WEDNESDAY 
WILL BE LIMITED TO THE 20S...AND EVEN COLDER THURSDAY AS COLD AIR 
ADVECTION PERSISTS...WITH HIGHS IN THE UPPER TEENS TO MID 20S. SOME 
PARTS OF THE BLACK HILLS MAY SEE HIGHS IN THE LOWER TO MID TEENS.

EXTENDED...GFS AND ECMWF ARE IN GENERALLY GOOD AGREEMENT WITH THE 
PLACEMENT OF THE SYNOPTIC SCALE FEATURES FOR FRIDAY AND SATURDAY BUT 
DIFFER ON THE TEMPERATURES...WITH THE GFS BEING THE COLDER OF THE 
TWO. THE GFS SHOWS A COUPLE OF WAVES OF COLDER AIR ARRIVING WITH 
SOME SNOW FLURRIES WHILE THE ECMWF KEEPS THE COLDEST AIR TO THE EAST 
UNTIL SATURDAY NIGHT WITH DRY WEATHER. FOR NOW HAVE SPLIT THE 
DIFFERENCE AND HAVE LOWERED TEMPERATURES FROM THE PREVIOUS FORECAST. 

MODELS CONTINUE TO DIFFER ON PLACEMENT/TIMING/STRENGTH OF NEXT UPPER 
TROF DIGGING INTO THE WESTERN CONUS. WILL KEEP THE LOW POPS FOR NOW 
AS POTENTIAL EXISTS FOR SOME LIGHT SNOW SUN AND MON.

&&

AVIATION...COLD FRONT WILL CROSS THE AREA THIS MORNING FOLLOWED BY 
STRONG AND GUSTY NORTHWEST WINDS AND SOME SNOW SHOWERS. MVFR 
CONDITIONS ARE EXPECTED OVER NORTHEASTERN WYOMING AND PARTS OF 
NORTHWESTERN SOUTH DAKOTA INTO EARLY AFTN WITH VFR CONDITIONS OVER 
THE REST OF WESTERN SOUTH DAKOTA. 

&&

.UNR WATCHES/WARNINGS/ADVISORIES...
SD...WIND ADVISORY UNTIL 5 PM MST /6 PM CST/ THIS AFTERNOON FOR 
     BENNETT-BUTTE-CUSTER CO PLAINS-HAAKON-HARDING-HERMOSA FOOT 
     HILLS-JACKSON-MELLETTE-NORTHERN FOOT HILLS-NORTHERN MEADE 
     CO PLAINS-PENNINGTON CO PLAINS-PERKINS-RAPID CITY-SHANNON-
     SOUTHERN MEADE CO PLAINS-STURGIS/PIEDMONT FOOT HILLS-TODD-
     TRIPP-ZIEBACH.

WY...NONE.
&&

$$

WETENKAMP