FXUS63 KUNR 250257
AFDUNR

AREA FORECAST DISCUSSION FOR WESTERN SD AND NORTHEASTERN WY
NATIONAL WEATHER SERVICE RAPID CITY SD
800 PM MST THU DEC 24 2009

.UPDATE...QPF AND SNOWFALL GRIDS WERE UPDATED TO INCREASE 
ACCUMULATIONS A COUPLE INCHES OVER THE NEXT 48 HOURS. ABUNDANT 
MOISTURE ADVECTION AND HIGHER CONFIDENCE IN MODEL SOLUTIONS 
WARRANTED THE UPDATES.  AS MENTIONED IN THE PREVIOUS DISCUSSION THE 
MODELS EXTEND A WELL DEFINED TROWAL ACROSS WESTERN SOUTH DAKOTA FOR 
12 HOURS OR MORE FROM FRIDAY AFTERNOON ONWARD. NO CHANGES WERE MADE 
TO THE BLIZZARD AND WINTER STORM WARNING VALID TIMES. 

&&

.AVIATION...SOME WEAK CONVECTIVE SNOW BANDS PRODUCED BRIEF MVFR 
CONDITIONS FOR BOTH THE KRAP AND KGCC TERMINALS EARLIER IN THE 
EVENING AND SOME OF THAT ACTIVITY REMAINS IN THE VICINITY.  THE 
PRIMARY LONGER LASTING LOWERING OF CONDITIONS IS STILL EXPECTED TO 
BEGIN SOMETIME AFTER 06Z WITH AREA WIDE IFR CONDITIONS AFTER 12Z. 

&&

.PREVIOUS DISCUSSION...SOUTHERN STORM SYSTEM LIFTING FROM TX INTO SW 
ARK THIS AFTERNOON...WITH PRECIP SPREADING NORTHWARD UP THE 
MISSISSIPPI VALLEY. NORTHERN LOW IS DROPPING SOUTHWARD OVER THE 
BLACK HILLS WITH SOME SNOW OVER CENTRAL ND-CAN. SURFACE HIGH PUSHING 
INTO THE REGION FROM SOUTHERN CAN-MT WITH GUSTY NW WINDS ACROSS 
CENTRAL SD. 

FORECAST AREA IS CURRENTLY IN A BREAK AS DRY SLOT PRECEDING THE 
NORTHERN LOW MOVES OVER THE AREA...AND BEFORE THE STORM APPROACHES 
LATE TONIGHT. THE EXCEPTION IS THE NORTHERN BLACK HILLS...WHERE 
RADAR IS INDICATING SOME UPSLOPE SNOW AROUND ONEIL PASS.

THE DIFFICULTY OF THE FORECAST IS THE POSITION AND MERGER OF THE 
LOWS...ESPECIALLY AS THEY START TO ROTATE OVER IA-WI. DRIER AIR WILL 
REMAIN OVER THE FAR WESTERN SD PLAINS EARLY TONIGHT...THEN SNOW WILL 
INCREASE FROM EAST TO WEST AS PRECIP STARTS TO WRAP AROUND THE LOW. 
MODELS SHOW PERSISTENT TROWAL AROUND THE LOW THROUGH EARLY 
SATURDAY...AS WELL AS DEEP MOISTURE THROUGH THE DENDRITIC ZONE THAT 
WILL INCREASE SNOWFALL. FROUDE NUMBER WILL REMAIN ABOVE 1 OVER THE 
NORTHERN BLACK HILLS UNTIL AROUND MIDNIGHT FRIDAY...SO SNOWFALL OF 1 
TO 2 FEET IS LIKELY...AND MAY BE EVEN HIGHER. GUSTY NORTHWEST WINDS 
WILL INCREASE AS SURFACE LOW MOVES CLOSER...WITH GUSTS REACHING 50 
MPH. THE SNOW AND STRONG WINDS WILL PRODUCE BLOWING SNOW...SO HAVE 
UPGRADED THE WINTER STORM WARNING TO A BLIZZARD WARNING FOR THE 
PLAINS OF WRN SD. SNOW WILL DECREASE LATE SAT...BUT AGAIN WITH 
UNCERTAINTY OF THE MOVEMENT OF THE LOW...HAVE LEFT POPS IN THROUGH 
SAT NIGHT.

EXTENDED...UPPER RIDGE WILL BUILD INTO THE NORTHERN PLAINS LATE 
SUNDAY INTO MONDAY...WITH TEMPERATURES SLOWLY WARMING UP. WARMEST 
TEMPERATURES OVER NORTHEAST WYOMING AND BLACK HILLS...WHILE STRONG 
INVERSION LIMITS WARM UP ON THE PLAINS. RIDGE BREAKS DOWN 
MID-WEEK...WITH SLIGHT CHANCE OF SNOW LATE WEDNESDAY INTO THURSDAY. 
&&

.AVIATION...LIGHT SNOW/FLURRIES ACROSS THE BLACK HILLS THROUGH THIS 
EVENING WITH MVFR CIGS. SNOW AND STRONG WINDS WILL SPREAD ACROSS THE 
AREA LATE TONIGHT AND PERSIST THROUGH SATURDAY MORNING WITH 
WIDESPREAD IFR/LIFR CIGS AND VSBY.

&&

.UNR WATCHES/WARNINGS/ADVISORIES...
SD...BLIZZARD WARNING FROM 5 AM MST / 6 AM CST FRIDAY UNTIL 11 AM
     MST /NOON CST SATURDAY FOR THE PLAINS OF WESTERN SOUTH DAKOTA
     AND FOOTHILLS.
     WINTER STORM WARNING TO 11 AM MST SATURDAY FOR THE BLACK
     HILLS.
     WINTER WEATHER ADVISORY FROM 5 AM FRIDAY UNTIL 11 AM MST
     SATURDAY FOR SOUTHERN BLACK HILLS AND EXTREME SOUTHWESTERN SD.
WY...BLIZZARD WARNING FROM 5 AM FRIDAY TO 11 AM MST SATURDAY FOR
     NORTHEASTERN CROOK.
     WINTER STORM WARNING UNTIL 11 AM MST SATURDAY FOR WYOMING
     BLACK HILLS.
     WINTER WEATHER ADVISORY FROM 5 AM FRIDAY UNTIL 11 AM MST
     SATURDAY FOR THE PLAINS OF NORTHEASTERN WY.