FXUS63 KUNR 251659
AFDUNR

AREA FORECAST DISCUSSION FOR WESTERN SD AND NORTHEASTERN WY
NATIONAL WEATHER SERVICE RAPID CITY SD
1000 AM MST FRI DEC 25 2009

.DISCUSSION...THE BLIZZARD HAS BEGUN ACROSS MUCH OF THE REGION. 
CURRENT SURFACE ANALYSIS SHOWS INTENSE LOW PRESSURE OVER CENTRAL 
IA...WITH ARCTIC HIGH PRESSURE OVER ALBERTA. UPPER LEVEL ANALYSIS 
HAS ONE LOW OVER EASTERN IA AND ANOTHER OVER NORTH CENTRAL KS.  
THESE LOWS ARE BEGINNING TO CONSOLIDATE INTO ONE LOW ACROSS THE 
UPPER MIDWEST. UPPER LEVEL ENERGY IS ROTATING WESTWARD AROUND THE 
LOWS ACROSS THE DAKOTAS AND INTO EASTERN WY AND MT. WINDS ARE 
STRONGEST ACROSS THE SD PLAINS...WHERE GUSTS TO AROUND 60 MPH ARE 
BEING RECORDED ACROSS PARTS OF NORTHWEST AND WEST CENTRAL SD. TEMPS 
ARE MOSTLY IN THE SINGLE DIGITS...WITH WIND CHILLS OF 15 BELOW TO 30 
BELOW ZERO. SNOWFALL AMOUNTS OF UP TO A COUPLE OF INCHES HAVE FALLEN 
SO FAR...WITH VSBYS AT OR BELOW A QUARTER MILE ACROSS MUCH OF THE SD 
PLAINS.  

SNOW IS NOW COVERING THE AREA...MOSTLY ON THE LIGHTER SIDE AT THIS 
POINT. HOWEVER...SOME HEAVIER BANDS ARE BEGINNING TO DEVELOP ACROSS 
NORTHERN AND EASTERN PARTS OF THE AREA...MOVING EAST-SOUTHEAST...AS 
STRONGER UPPER LEVEL ENERGY...LIFT...AND WAA IS WRAPPING IN FROM THE 
EAST. THE HEAVIEST SNOWS ARE STILL EXPECTED LATER TODAY INTO 
SATURDAY MORNING. SIGNIFICANT UPSLOPE SNOWFALL IS STILL EXPECTED 
ACROSS NORTHERN HILLS...WHERE SOME AREAS WILL LIKELY SEE CLOSE TO 2 
FEET OF SNOW OR MORE BY LATE SATURDAY. 

HAVE UPDATED FORECAST PRIMARILY TO RAISE POPS OVER FAR SOUTHERN AND 
WESTERN PORTIONS OF THE CWA TO NEAR 100 PERCENT AS AT LEAST SOME 
LIGHT SNOW NOW COVERS MOST OF THE AREA. OTHERWISE...FORECAST IS IN 
GOOD SHAPE FOR THE TIME BEING.        

PREVIOUS DISCUSSION...THE CHRISTMAS BLIZZARD IS STARTING TO TAKE 
SHAPE OVER WRN SD AS A MAJOR TROWAL WAS PUSHING BACK INTO THE AREA. 
MODELS AGREE PRETTY WELL WITH THE TWO MAIN PV ANOMALIES ROTATING 
AROUND EACH OTHER AND EVENTUALLY MERGING NEAR ERN IA. THE GIST OF 
THE FORECAST HAS NOT CHANGED MUCH...HOWEVER...WILL BE EXTENDING THE 
WRNGS/ADVYS UNTIL 24Z SATURDAY BASED ON TRENDS FOR A SLOWER EXIT OF 
THE SYSTEM. HAVE ALSO UPGRADED TO A WINT STM WRNG FOR CROOK COUNTY 
BASED ON HIGHER SNOW TOTALS. USING A SNOW RATIO OF 15-1 COMBINED 
WITH THE CURRENT QPF GIVES SNOWFALL OF TWO FEET IN THE NRN BLKHLS 
...8-12 INCHES OVER MOST OF THE WRN SD PLAINS...3-8 INCHES OVER NERN 
WY...AND ONLY 2-4 INCHES OVER THE SRN BLKHLS AND FOOTHILLS WHERE THE 
DOWNSLOPE HOLE WILL BE CONSPICUOUS. HAVE A LITTLE CONCERN THE SNOW 
AMOUNTS COULD BE TOO LOW OVER THE NRN BLKHLS WHERE THE DENDRITIC 
ZONE IS RATHER DEEP (BUT THE LAPSE RATES ARE NEAR ISOTHERMAL)... 
WHICH RESULTS IN A SHARP CROSS-HAIR SIGNATURE TONIGHT AND SATURDAY. 
ISENTROPIC LIFT ALSO WILL BE QUITE STRONG THROUGH SATURDAY OVER MUCH 
OF THE AREA...MANIFEST MOSTLY AS 700-MB WAA TODAY AND TONIGHT...THEN 
850-MB WAA SATURDAY AND SATURDAY NIGHT. BASED ON THIS WOULD NOT BE 
SURPRISED TO SEE WRNGS/ADVYS EXTENDED INTO SATURDAY EVNG AT SOME 
POINT. THE END OF THE SNOW WILL NOT COME UNTIL LATE SUNDAY...THOUGH 
SNOWFALL AMOUNTS WILL BE CONSIDERABLY LESS ON SUNDAY...RELATIVE TO 
THE NEXT TWO DAYS.

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 TUESDAY... 
WITH SLIGHT CHANCE OF SNOW LATE WEDNESDAY INTO THURSDAY.

&&

.AVIATION...SNOW AND STRONG WINDS WILL BE FOUND ACROSS THE AREA  
TODAY...AND PERSIST INTO SATURDAY. WIDESPREAD IFR/LIFR CIGS AND VSBY 
ARE EXPECTED DURING THIS TIME FRAME.

&&

.UNR WATCHES/WARNINGS/ADVISORIES...
SD...BLIZZARD WARNING UNTIL 5 PM MST/6 PM CST SATURDAY FOR THE
     PLAINS OF WESTERN SOUTH DAKOTA AND NRN/ERN FOOTHILLS.
     WINTER STORM WARNING UNTIL 5 PM MST SATURDAY FOR THE NRN
     AND CNTRL BLKHLS.
     WINTER WEATHER ADVISORY UNTIL 5 PM MST SATURDAY FOR SRN
     BLKHLS AND EXTREME SWRN SD.
WY...BLIZZARD WARNING UNTIL 5 PM MST SATURDAY FOR NERN CROOK
     COUNTY.
     WINTER STORM WARNING UNTIL 5 PM MST SATURDAY FOR WYOMING
     BLKHLS AND CROOK COUNTY PLAINS.
     WINTER WEATHER ADVISORY UNTIL 5 PM MST SATURDAY FOR 
     CAMPBELL AND WESTON COUNTIES.

&&

$$