FXUS63 KUNR 010954
AFDUNR
AREA FORECAST DISCUSSION FOR WESTERN SD AND NORTHEASTERN WY
NATIONAL WEATHER SERVICE RAPID CITY SD
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 FROM 7 AM MST /8 AM CST/ THIS MORNING TO 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.
&&
$$
CALDERON/RICHARDS
