FXUS63 KUNR 241006
AFDUNR

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

.DISCUSSION...CURRENT SURFACE ANALYSIS PLACES HIGH OVER NORTHWEST 
MONTANA WITH LOW OVER NORTHEAST TEXAS...WHILE ARCTIC FRONT STRETCHES 
FROM THE EASTERN DAKOTAS INTO EASTERN NEBRASKA. UPPER LEVEL ANALYSIS 
SHOWS DOUBLE LOW SETUP...WITH THE FIRST LOW PUSHING THROUGH WEST 
TEXAS...AND SECOND LOW DIVING SOUTH THROUGH MONTANA. AREA RADARS 
INDICATING BROAD AREA OF SNOW OVER THE CENTRAL AND EASTERN DAKOTAS 
SLOWLY SLIDING OFF TO THE EAST. FORECAST CHALLENGE WILL BE HANDLING 
THE MERGER OF THE LOWS...AND THE WRAP AROUND PRECIP AND WINDS 
DEVELOPING FROM THIS SYSTEM.

FOR TODAY...SOUTHERN LOW SHIFTS NORTH TOWARDS MISSOURI WHILE 
NORTHERN LOW SLIDES SOUTH TOWARDS NEBRASKA BEFORE STARTING TO 
MERGE...WHILE SURFACE LOW STARTS HEADING TOWARDS IOWA. A LULL IN THE 
SNOWFALL IS EXPECTED AS SOME DRIER AIR WORKS INTO THE 
SYSTEM...PARTICULARLY OVER THE WESTERN CWFA. SNOW WILL BE ON THE 
INCREASE FROM EAST TO WEST TONIGHT AS PRECIP STARTS TO WRAP OVER THE 
MERGING LOW. GUSTY NORTHWEST WINDS WILL BE ON THE INCREASE AS 
SURFACE LOW TIGHTENS UP. 

ON FRIDAY...CENTER OF MERGED STACKED UP LOW LOITERS OVER IOWA...WITH 
WRAP AROUND SNOW CONTINUING OVER THE AREA AS WELL DEFINED TROWAL 
PUSHES IN...PARTICULARLY OVER THE EASTERN CWFA. BROAD AREA OF 
MID-LEVEL LIFT NOTED OVER WRN/S CNTRL SODAK...WITH FORECAST 
SOUNDINGS PLACING SNOW GROWTH IN THE DENDRITIC ZONE THROUGH MUCH OF 
THE DAY...SO WILL HAVE TO KEEP AN EYE ON ANY HEAVY BANDS THAT SETUP. 
FOR UPSLOPE SNOW OVER THE NRN BLKHLS...FROUDE NUMBER REMAINS ABOVE 1 
THROUGH THE PERIOD WITH PLENTY OF MOISTURE TO WORK WITH...SO EXPECT 
THAT TO CONTINUE. THE MAIN PROBLEM OVER THE PLAINS WRN SODAK WILL BE 
WINDS. WITH TIGHT PRESSURE GRADIENT DUE TO STACKED LOW...MODELS ARE 
NOW BRINGING 40-50 KT 850 MB WINDS OVER THE AREA...WITH 30+ KT AT 
THE SURFACE BY FRIDAY EVENING. WITH THE FRESH SNOW...EXPECT THAT 
SOME NEAR BLIZZARD CONDITIONS MAY DEVELOP SO HAVE UPPED MOST OF THE 
ADVISORY AREA TO A WINTER STORM WARNING. WILL HAVE DAY SHIFT GIVE 
THIS ANOTHER LOOK WITH THE 12Z RUN AS MORE UPGRADES MAY BE NEEDED.

FOR SATURDAY...UPPER LOW TRIES TO SLOWLY SHIFT EAST (THOUGH NOW THE 
06Z NAM IS TRYING TO LOOP IT BACK TOWARDS SOUTHEAST MINNESOTA). 
EXPECT SNOW AND WIND TO CONTINUE THROUGH THE MORNING BEFORE 
DISSIPATING LATER IN THE DAY...THOUGH WILL HAVE TO SEE HOW MODELS 
SHIFT IT OUT ON THE NEXT RUN. HAVE EXTENDED ALL HEADLINES THROUGH 
SATURDAY MORNING.

EXTENDED...UPPER LEVEL LOW SLOWLY SHIFTS EAST ON SUNDAY. UPPER RIDGE 
WILL BUILD INTO THE NORTHERN PLAINS LATE ON SUNDAY INTO MONDAY. 
TEMPERATURES WILL SLOWLY WARM TO NEAR AVERAGE ACROSS NORTHEAST 
WYOMING INTO THE BLACK HILLS EARLY NEXT WEEK...BUT REMAIN BELOW 
AVERAGE ACROSS THE SOUTH DAKOTA PLAINS DUE TO SNOW COVER. DRY 
CONDITIONS ARE GENERALLY EXPECTED FROM SUNDAY INTO EARLY NEXT WEEK.

&&

.AVIATION...LIGHT SNOW WILL TAPER OFF ACROSS THE AREA THIS MORNING 
...THEN REDEVELOP THIS EVENING AND CONTINUE THROUGH CHRISTMAS DAY. 
VFR CONDITIONS SHOULD PREVAIL UNTIL SNOW REDEVELOPS THIS EVENING. 
IFR/LIFR VISIBILITIES WITH SNOW WILL BECOME WIDESPREAD AFTER 12Z 
FRIDAY MORNING. 

&&

.UNR WATCHES/WARNINGS/ADVISORIES...
SD...WINTER STORM WARNING UNTIL 11 AM MST /NOON CST/ SATURDAY FOR 
     BENNETT-CENTRAL BLACK HILLS-HAAKON-JACKSON-MELLETTE-
     NORTHERN BLACK HILLS-TODD-TRIPP-ZIEBACH.

     WINTER WEATHER ADVISORY UNTIL 11 AM MST SATURDAY FOR FALL RIVER-
     SOUTHERN BLACK HILLS-SOUTHERN FOOT HILLS.

     WINTER WEATHER ADVISORY UNTIL 5 AM MST FRIDAY FOR BUTTE-CUSTER 
     CO PLAINS-HARDING-HERMOSA FOOT HILLS-NORTHERN FOOT HILLS-
     NORTHERN MEADE CO PLAINS-PENNINGTON CO PLAINS-PERKINS-RAPID 
     CITY-SHANNON-SOUTHERN MEADE CO PLAINS-STURGIS/PIEDMONT FOOT 
     HILLS.

     WINTER STORM WARNING FROM 5 AM FRIDAY TO 11 AM MST SATURDAY FOR 
     BUTTE-CUSTER CO PLAINS-HARDING-HERMOSA FOOT HILLS-NORTHERN 
     FOOT HILLS-NORTHERN MEADE CO PLAINS-PENNINGTON CO PLAINS-
     PERKINS-RAPID CITY-SHANNON-SOUTHERN MEADE CO PLAINS-
     STURGIS/PIEDMONT FOOT HILLS.

WY...WINTER STORM WARNING UNTIL 11 AM MST SATURDAY FOR WYOMING BLACK 
     HILLS.

     WINTER WEATHER ADVISORY UNTIL 5 AM MST FRIDAY FOR NORTHEASTERN 
     CROOK.

     WINTER STORM WARNING FROM 5 AM FRIDAY TO 11 AM MST SATURDAY FOR 
     NORTHEASTERN CROOK.