FXUS63 KUNR 261724
AFDUNR

AREA FORECAST DISCUSSION FOR WESTERN SD AND NORTHEASTERN WY
NATIONAL WEATHER SERVICE RAPID CITY SD
1024 AM MST SAT DEC 26 2009

.DISCUSSION...CURRENT SURFACE ANALYSIS SHOWS DEEP LOW PRESSURE OVER 
EASTERN IA...WITH ARCTIC HIGH PRESSURE OVER SOUTH CENTRAL CANADA. 
UPPER LEVEL ANALYSIS HAS LOW OVER WI...DRIFTING SLOWLY TO THE NORTH. 
SERIES OF DISTURBANCES ROTATING AROUND LOW ARE MOVING INTO THE AREA 
FROM THE NORTHEAST WITH PLENTY OF MID AND UPPER LEVEL MOISTURE 
ASSOCIATED WITH THEM. HEAVY SNOW STILL FALLING ACROSS THE NORTHERN 
HILLS AREA...WHERE UP TO 3 FEET HAS BEEN REPORTED SO FAR. STRONG 
WINDS CONTINUE ACROSS MUCH OF THE AREA WITH TIGHT PRESSURE GRADIENT 
IN PLACE. SIGNIFICANT BLOWING AND DRIFTING SNOW IS BEING REPORTED 
ACROSS THE SD PLAINS...WITH VSBYS STILL BELOW A QUARTER MILE IN MANY 
SPOTS. SOME LOCATIONS ACROSS THE WESTERN SD PLAINS HAVE RECEIVED 
OVER A FOOT OF SNOW...WITH DRIFTS 5 TO 10 FEET HIGH. CURRENT RADAR 
SHOWING SNOW ACROSS A GOOD PART OF NORTHEASTERN WY AND WESTERN 
SD...WITH SOME BREAKS IN SNOW SHOWING UP OVER THE NORTHEASTERN CWA. 
HOWEVER...MORE SNOW IS LOCATED ACROSS CENTRAL ND...MOVING TO THE 
SOUTHEAST...SO PERIODS OF SNOW SHOULD CONTINUE INTO THE 
AFTERNOON. 

NO CHANGES TO HEADLINES ARE PLANNED UNTIL THE AFTERNOON. ADDITIONAL 
ACCUMULATIONS OF UP TO A FOOT ARE EXPECTED ACROSS THE NORTHERN HILLS 
THROUGH THE AFTERNOON...WITH GENERALLY ANOTHER 1 TO 3 INCHES 
EXPECTED ACROSS THE SD PLAINS. SNOW SHOULD BEGIN TO TAPER OFF IN 
INTENSITY AND COVERAGE ACROSS THE PLAINS LATER THIS AFTERNOON AND 
TONIGHT. WINDS WILL ALSO BEGIN TO SLOWLY DIMINISH BY THIS EVENING. 
LOOKS LIKE SNOW WILL CONTINUE HEAVY AT TIMES THROUGH THE AFTERNOON 
ACROSS THE NORTHERN HILLS AREA...THEN GRADUALLY DECREASE 
TONIGHT...ESPECIALLY AFTER LATE EVENING. WILL PROBABLY BE ABLE TO 
CANCEL SOME HEADLINES ACROSS SOUTHERN AND WESTERN PORTIONS OF THE 
CWA THIS AFTERNOON...BUT WILL LIKELY KEEP SOME VARIATION OF 
HEADLINES GOING INTO TONIGHT FOR MUCH OF WESTERN SD AND THE BLACK 
HILLS WITH SOME LIGHTER SNOW AND AREAS OF BLOWING SNOW PERSISTING. 
WILL DETERMINE THIS LATER ON.  

CHANGES TO FORECAST WERE TO BUMP TEMPS UP A LITTLE BIT TODAY FOR 
MANY AREAS AND MAKE SOME ADJUSTMENTS TO WX...POPS...AND WIND FOR 
THIS AFTERNOON AND TONIGHT. 

PREVIOUS DISCUSSION...AT 08Z BLIZZARD CONDITIONS CONTINUE TO POUND 
WRN SD. SFC MAP SHOWS DEEP LOW IN WRN IOWA WITH A STG NNW FLOW ACRS 
WRN SD/NERN WY. ALOFT...DEEP UPPER LOW IS IN ERN IOWA WITH A MOIST 
NE FLOW WRAPPING COPIOUS AMTS OF MOISTURE ACRS REGION. AS FOR 
SNOWFALL TOTALS...UPSLOPE MACHINE IN HIGH GEAR WITH 20-30 IN NRN BLK 
HLS...WITH 4-8 ON SD PLAINS AND 2-4 ACRS NERN WY PLAINS.

MOISTURE BEING DRAWN INTO STORM IS MOST IMPRESSIVE...ESP FOR LATE 
DECEMBER...STORM IS SO BIG MOISTURE IS BEING DRAWN OFF PACIFIC
OCEAN ACRS MEXICO/GULF OF MEXICO AND THEN UP EAST COAST BEFORE 
WRAPPING BACK ACRS SRN CANADA AND THEN INTO NRN PLAINS. WILL LIKELY
BE A LONG TIME BEFORE WE SEE ANOTHER DECEMBER STORM LIKE THIS. 

TODAY/TONIGHT...DEEP UPPER LOW IN ERN IOWA WILL MV NE INTO SRN
WISCONSIN BY 12Z SUN. THIS WILL KEEP A MOIST NE UPSLOPE FLOW ALOFT
OVR REGION. BROAD ISENTROPIC LIFT WILL CONT THIS MORNING AND SLOWLY
WEAKEN BY THIS AFT. IDEAL UPSLOPE CONDS WILL CONT ACRS NRN BLK HLS/
FOOTHILLS THRU ABOUT 06Z SUNDAY AND THEN FINALLY WEAKEN. THUS...
EXPECT PCPN TO CONT ACRS REGION THRU TODAY WITH PCPN TAPPERING OFF
TONIGHT. WL CONT ALL WARNINGS/ADVISORIES THRU 00Z. EXPECT ANOTHER
2-4 INCHES ON SD PLAINS TODAY AND THIS EVENING WITH 1-2 ON NERN WY 
PLAINS AND ANOTHER 8-16 INCHES OVER NRN BLACK HILLS WHERE A FEW 
PLACES MAY PUSH 50 INCHES TOTAL.
 
SUN/SUN NIGHT...UPPER LOW WILL FINALLY MV EAST INTO GT. LAKES.
THERE MAY BE A SNOW SHOWER OR TWO...ESP IN NRN BLACK HILLS...
OTHERWISE WE SHOULD FINALLY DRY OUT. UPPER RIDGE WILL BEGIN TO
BUILD INTO NRN PLAINS SUNDAY NIGHT. TEMPS SHOULD WARM INTO THE
20S.

MON/MON NITE...UPPER RIDGE WILL DOMINATE BRINGING DRY WX AND TEMPS
MODERATING...ESP IN NERN WY/BLK HILLS WHERE HIGHS WILL PUSH INTO
30S.

TUES/TUES NITE...A WEAK UPPER TROF WILL MOVE ACRS THE AREA. TEMPS 
WILL AGAIN BE MAINLY IN 20S WITH DRY WX CONTINUING.

EXTENDED...BELOW AVERAGE TEMPERATURES WITH CYCLONIC/NORTHWESTERLY 
FLOW WILL PREVAIL TUESDAY THROUGH FRIDAY...ALTHOUGH THE RELATIVE 
MEASURE OF PREDICTABILITY OF THE 500-MB HEIGHTS IS FAIRLY LOW. HAVE 
GONE WITH FAIRLY MINIMAL/LOW POPS GIVEN THE GFS ENSEMBLES...WITH 
THE RELATIVELY BEST CHANCE OF SNOW LATE WEDNESDAY INTO THURSDAY AS
A CLIPPER-TYPE SYSTEM PASSES BY THE AREA. SOME MODEST WARMING IS 
EXPECTED AHEAD OF THIS SYSTEM ON TUESDAY AND WEDNESDAY...ESPECIALLY 
OVER NERN WY AND THE BLKHLS...BUT SNOW COVER WILL HELP MAINTAIN AN 
INVERSION OVER MOST OF THE SD PLAINS. THIS SHOULD BE FOLLOWED BY 
ANOTHER DECENT SURGE OF ARCTIC AIR FOR THE END OF THE YEAR... 
CONSISTENT WITH HOW DECEMBER BEGAN.

&&

.AVIATION...WIDESPREAD IFR/LIFR VSBYS EXPECTED THROUGH THE FORENOON 
HOURS DUE TO SNOW AND BLOWING SNOW. GRADUAL IMPROVEMENT IS EXPECTED 
AT KGCC...WITH MVFR CONDITIONS BY MIDDAY AND VFR CONDITIONS BY EVNG. 
HOWEVER...KRAP SHOULD EXPERIENCE IFR VSBYS THROUGH THEY DAY...WITH 
MVFR CONDITIONS THIS EVNG WITH LIGHT SNOW AND SOME BLOWING SNOW 
PERSISTING.

&&

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

&&
$$