FXUS63 KUNR 220355
AFDUNR

AREA FORECAST DISCUSSION FOR WESTERN SD AND NORTHEASTERN WY
NATIONAL WEATHER SERVICE RAPID CITY SD
855 PM MST MON DEC 21 2009

.DISCUSSION...03Z SURFACE ANALYSIS HAD A HIGH OVER NORTHEASTERN ND
AND A STATIONARY FRONT FROM EASTERN MT INTO EASTERN WY THROUGH NE.
WEAK UPSLOPE FLOW COMBINED WITH SMALL T/TD SPREADS SUPPORTING FZFG
ON THE SD PLAINS...BECOMING DENSER AS THE NIGHT GOES ON. 00Z KUNR
SOUNDING HAD WELL DEFINED INVERSION. HAVE HOISTED DENSE FOG
ADVISORY OVER MOST OF THE SD PLAINS EAST OF THE BLACK HILLS. COULD
SEE SOME PATCHY FZDZ PER THE SOUNDING. TONIGHT/S FORECAST WILL BE
UPDATED APPROPRIATELY.

LOOKING AT UPCOMING STORM...00Z NAM KEEPS FAR WESTERN SD ON THE
FRINGES OF THE BEST ACTION...BUT EARLIER RUNS OF GFS WERE FURTHER
WEST WITH A MORE WELL-DEFINED INVERTED TROUGH. WILL KEEP GOING
FORECAST INTACT UNTIL FULL GUIDANCE CAN BE ASSESSED.

&&

.AVIATION...LIFR DUE TO FZFG WILL CONTINUE AT KRAP THROUGH EARLY
TUESDAY MORNING.

&&

.PREV DISCUSSION... /ISSUED 200 PM MST MON DEC 21 2009/ 

DISCUSSION...CURRENT SURFACE ANALYSIS SHOWS A LOW OVER THE PACIFIC 
NORTHWEST WITH A PRESSURE TROUGH EXTENDING SOUTHEAST THRU NE WY...SW 
SD THEN SOUTH INTO NE NEW MEXICO. S/W WILL PUSH THROUGH TONIGHT 
BRINGING IN SOME DRYER AIR BEHIND IT AS THE TROF OUT WEST BEGINS TO 
DIG SOUTH.

TONIGHT...EXPECT A CHANCE OF SNOW ASSOCIATED WITH THE S/W 
PASSAGE...A BREAK IN THE PRECIP AROUND 06Z ASSOCIATED WITH SOME 
DRIER AIR OVER NE WY BEHIND THE S/W AND AHEAD OF THE NEXT UPPER 
LEVEL TROF. PATCHY FOG WILL LINGER OVER PORTIONS OF THE NORTHERN AND 
EASTERN FOOTHILLS AND SEWD ACROSS THE SD PLAINS AS THE LOW CLOUDS 
OVER THE AREA HAVE PREVENTED ANY WARMING.

FOR TUESDAY...UPPER TROF DIVING INTO PACIFIC NW WILL BEGIN TO DIVE 
SOUTHEAST TOWARDS GREAT BASIN...PUSHING UPPER RIDGE EAST. SW FLOW
ALOFT WILL INCREASING MSTR ONCE AGAIN AND INCREASING THE CHANCE OF 
SNOW THROUGHT THE FA BY LATE AFTERNOON.

TUES NITE THRU WED NITE...BY 00Z WED UPPER LOW WL BE IN SRN NEVADA
THIS UPPER LOW WL MOVE INTO NWRN NEW MEXICO BY 12Z WED AND INTO 
OKLAHOMA BY 12Z THURS WITH LARGE INVERTED TROF DVLPNG ACRS NRN
PLAINS.

A VERY MOIST UPSLOPE FLOW WL DVLP ACRS WRN SD BY EARLY TUES EVENING
AND CONT ACRS REGION THRU 12Z THURS. 850MB/700MB FLOW WL SHIFT FROM
ESE AT 00Z WED TO ERLY WED AND NE WED NITE. COMBINATION OF BROAD
UPSLOPE FLOW/ISENTROPIC LIFT AND PLENTY OF MSTR SHOULD PRODUCE AN
EXTENDED PERIOD OF SNOW ACRS WRN SD/NERN WY FROM LATE TUES AFT/
EARLY TUE EVENING INTO THURSDAY AM.

AT THIS TIME APPEARS MOST AREAS SHOULD GET AT LEAST 3-6 INCHES OF
SNOW...WITH BEST ACCUMS LIKELY IN NRN/ERN FOOTHILLS AND NRN BLACK
HILLS WHERE USPLOPE WILL BE MAXIMIZED AND ACRS SOUTH CENTRAL SD
WHERE BEST CHC FOR SIGNIFICANT WRAP AROUND WILL BE AS UPPER LOW 
MOVES THRU SERN NEB INTO IOWA BY FRI AFT. THUS WILL ISSUE A WINTER
STORM WATCH FOR MUCH OF WRN SD AND BEAR LODGE IN NERN WY MAINLY
FROM 00Z WED THRU 00Z FRI.

EXTENDED...MEDIUM RANGE MODELS CONTINUE TO SHOW A DEEP TROF ACRS
THE NORTHERN PLAINS DURING THE EXTENDED. A STRONG SHORTWAVE WILL 
SWING OUT OF THE SOUTHERN ROCKIES INTO THE MIDWEST ON THURSDAY AND 
FRIDAY. THE FORECAST AREA WILL REMAIN ON THE FRINGES OF THIS SYSTEM 
WITH BEST CHANCES OF PRECIP ACROSS SOUTH CENTRAL SOUTH DAKOTA. 
COLDER AIR IS STILL EXPECTED TO KEEP MAX TEMPS BELOW SEASONAL 
AVERAGES THROUGH MUCH OF THE EXTENDED PERIOD.

AVIATION...IFR CONDITIONS WILL CONTINUE ACROSS MUCH OF THE SOUTH 
DAKOTA PLAINS WITH LOW CIGS. DENSE FOG ACROSS SOME AREAS OF THE 
SOUTH DAKOTA PLAINS INCLUDING KRAP IS EXPECTED TO CONTINUE TONIGHT 
INTO TUESDAY MORNING. SNOW WILL SLIDE MAINLY ACROSS THE NORTHERN 
HALF OF THE CWA ALSO REDUCING VSBY. VFR CONDITIONS ARE EXPECTED 
ACROSS MUCH OF NORTHEAST WYOMING.

&&

.UNR WATCHES/WARNINGS/ADVISORIES...
SD...WINTER STORM WATCH FROM TUESDAY EVENING THROUGH THURSDAY 
     AFTERNOON FOR CENTRAL BLACK HILLS-NORTHERN BLACK HILLS-
     NORTHERN FOOT HILLS-RAPID CITY-STURGIS/PIEDMONT FOOT HILLS.

     DENSE FOG ADVISORY UNTIL 8 AM MST /9 AM CST/ TUESDAY FOR BENNETT-
     CUSTER CO PLAINS-HAAKON-HERMOSA FOOT HILLS-JACKSON-MELLETTE-
     NORTHERN MEADE CO PLAINS-PENNINGTON CO PLAINS-PERKINS-RAPID 
     CITY-SHANNON-SOUTHERN MEADE CO PLAINS-STURGIS/PIEDMONT FOOT 
     HILLS-TODD-TRIPP-ZIEBACH.

     WINTER STORM WATCH FROM LATE TUESDAY NIGHT THROUGH THURSDAY 
     AFTERNOON FOR BENNETT-BUTTE-HAAKON-JACKSON-MELLETTE-
     NORTHERN MEADE CO PLAINS-PENNINGTON CO PLAINS-SOUTHERN 
     MEADE CO PLAINS-TODD-TRIPP-ZIEBACH.

     WINTER STORM WATCH FROM TUESDAY AFTERNOON THROUGH LATE WEDNESDAY 
     NIGHT FOR CUSTER CO PLAINS-FALL RIVER-HERMOSA FOOT HILLS-
     SOUTHERN BLACK HILLS-SOUTHERN FOOT HILLS.

     WINTER STORM WATCH FROM TUESDAY AFTERNOON THROUGH THURSDAY 
     AFTERNOON FOR SHANNON.

WY...WINTER STORM WATCH FROM TUESDAY EVENING THROUGH THURSDAY 
     AFTERNOON FOR WYOMING BLACK HILLS.

&&

$$

HELGESON