FXUS63 KUNR 090247
AFDUNR

AREA FORECAST DISCUSSION FOR WESTERN SD AND NORTHEASTERN WY
NATIONAL WEATHER SERVICE RAPID CITY SD
747 PM MST TUE DEC 8 2009

.DISCUSSION...02Z SFC ANALYSIS SHOWS THE BIG WINTER STORM SYSTEM
IS NOW CENTERED OVER NORTHERN MISSOURI WITH ARTIC HIGH PRESSURE
OVER EASTERN MONTANA...SAGGING SLOWLY SOUTHEAST. LIGHT SNOW OR
FLURRIES CONTINUE OVER MOST OF THE FORECAST AREA AT THIS TIME SO
HAVE RAISED POPS FOR THE REST OF THIS EVENING...WITH LITTLE OR NO
ADDITIONAL ACCUMULATION EXPECTED ACROSS MUCH OF THE AREA. HAVE
EXTENDED THE WINTER WEATHER ADVISORY FOR MELLETTE...TODD AND TRIPP
COUNTIES IN SOUTH CENTRAL SOUTH DAKOTA...WHERE THE WIND IS
STRONGER AND THE SNOW HEAVIER UNTIL 12Z. EXPECTING TO SEE LIGHT SNOW AND
FLURRIES TO VERY GRADUALLY END OVERNIGHT.

&&

.AVIATION...WIDESPREAD MVFR CIGS/IFR VSBY WILL VERY GRADUALLY IMPROVE
ACROSS NORTHEASTERN WYOMING AND WESTERN SOUTH DAKOTA TONIGHT.
AREAS OF LIGHT SNOW OR FLURRIES CAN BE EXPECTED OVERNIGHT.

&&

.PREV DISCUSSION... /ISSUED 206 PM MST TUE DEC 8 2009/ 

DISCUSSION...SATELLITE/WATER VAPOR IMAGERY SHOWS A DEEP TROUGH 
CROSSING THE WESTERN CONUS WITH A MAJOR WINTER STORM TREKKING ACROSS 
THE CENTRAL PLAINS. THIS IS CONTINUING TO BRING SNOW NORTH ACROSS 
THE CWA. SURFACE OBS SHOW LIGHT SNOW ACROSS THE AREA WITH WINDS 
SHIFTING MORE NORTHERLY AS THE SURFACE LOW CROSSES INTO KANSAS. 
TONIGHT...SNOW WILL END FROM WEST TO EAST THIS EVENING AS THE WINTER 
STORM CONTINUES TOWARD THE MIDWEST. WILL LET THE WINTER WEATHER 
ADVISORY CONTINUE FOR SOUTHWEST AND SOUTH CENTRAL SOUTH DAKOTA UNTIL 
03Z. EVEN COLDER AIR WILL ADVECT IN TONIGHT WHICH WILL DROP LOWS 
WELL BELOW ZERO...THE COLDEST SPOTS EXPECTED TO BE ACROSS NORTHEAST 
WYOMING WHERE TEMPS COULD REACH 20 BELOW ZERO. WINDS TONIGHT WILL 
GENERALLY BE IN THE 5 TO 10 MPH RANGE...AND EVEN STRONGER ACROSS 
SOUTH CENTRAL SOUTH DAKOTA. THIS WILL RESULT IN WIND CHILLS OF 25 TO 
35 BELOW TONIGHT AND CONTINUING THROUGH WEDNESDAY MORNING. WILL 
ISSUE A WIND CHILL ADVISORY FOR ALL AREAS TONIGHT...ENDING NOON MST 
WEDNESDAY. 

PARTLY CLOUDY SKIES AND DRIER WEATHER ARE EXPECTED WEDNESDAY WITH 
HIGHS AGAIN IN THE SINGLE DIGITS. SEVERAL WAVES WILL CROSS THE CWA 
WITHIN THE NORTHWESTERLY FLOW WEDNESDAY NIGHT AND THURSDAY AS THE 
UPPER TROUGH SLIDES TO THE EAST. THIS IS EXPECTED TO BRING FLURRIES 
TO NORTHEAST WYOMING THROUGH NORTHWEST SOUTH DAKOTA...AND EVEN INTO 
THE BLACK HILLS THURSDAY. RELATIVELY WARMER AIR WILL BEGIN ADVECTING 
INTO THE AREA EARLY THURSDAY...BRINGING HIGHS IN THE TEENS. WIND 
CHILLS EARLY THURSDAY MORNING COULD REACH NEAR WIND CHILL ADVISORY 
CRITERIA OF 25 BELOW ZERO EARLY THURSDAY MORNING...BUT WILL WAIT TO 
SEE WHAT MODELS SHOW IN THE NEXT FEW RUNS.

EXTENDED...THE TREND OF THE COLD AIR STAYING LONGER THAN WHAT HAS 
BEEN ANTICIPATED CONTINUES TODAY AS THE LARGE UPPER TROUGH SHOULD 
REMAIN OVER ALBERTA-SASKATCHEWAN-MANITOBA INTO THIS WEEKEND. EVEN 
THOUGH THE NORTHWEST FLOW ON FRIDAY WILL TRANSITION TO A ZONAL FLOW 
ON SATURDAY...THE SURFACE COLD AIR MASS WILL REMAIN ENTRENCHED OVER 
MOST OF THE CWA...AIDED BY SNOW COVER AND WEAK MIXING. HOWEVER...A 
BIT OF WARM AIR WILL OSCILLATE INTO NERN WY AND THE BLKHLS...SETTING 
UP A FAIRLY STRONG BAROCLINIC ZONE. CONCURRENT WITH THIS...LOW-LEVEL 
ISENTROPIC LIFT WILL OCCUR OVER THE 3-DIMENSIONAL BOUNDARY AND ALLOW 
FOR BOUTS OF LIGHT SNOW...WITH THE BEST CHANCES ON SUNDAY. BASED ON 
THE LATEST GFS ENSEMBLE OUTPUT...THE WARM AIR PROBABLY WILL NOT 
RETURN FOR EVERYONE UNTIL THE MIDDLE OF NEXT WEEK. ONE CAVEAT... 
HOWEVER...IS THE FAIRLY LOW PREDICTABILITY OF RIDING OVER THE WRN 
U.S. FROM THE RMOP (<50-60%).

&&

.UNR WATCHES/WARNINGS/ADVISORIES...
SD...WIND CHILL ADVISORY UNTIL NOON MST /1 PM CST/ WEDNESDAY FOR 
     BENNETT-BUTTE-CENTRAL BLACK HILLS-CUSTER CO PLAINS-FALL 
     RIVER-HAAKON-HARDING-HERMOSA FOOT HILLS-JACKSON-MELLETTE-
     NORTHERN BLACK HILLS-NORTHERN FOOT HILLS-NORTHERN MEADE CO 
     PLAINS-PENNINGTON CO PLAINS-PERKINS-RAPID CITY-SHANNON-
     SOUTHERN BLACK HILLS-SOUTHERN FOOT HILLS-SOUTHERN MEADE CO 
     PLAINS-STURGIS/PIEDMONT FOOT HILLS-TODD-TRIPP-ZIEBACH.

     WINTER WEATHER ADVISORY UNTIL 6 AM CST WEDNESDAY FOR MELLETTE-
     TODD-TRIPP.

WY...WIND CHILL ADVISORY UNTIL NOON MST WEDNESDAY FOR NORTHEASTERN 
     CROOK-NORTHERN CAMPBELL-SOUTHERN CAMPBELL-WESTERN CROOK-
     WESTON-WYOMING BLACK HILLS.

&&

$$