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FXUS63 KUNR 202140
AFDUNR
AREA FORECAST DISCUSSION FOR WESTERN SD AND NORTHEASTERN WY
NATIONAL WEATHER SERVICE RAPID CITY SD
240 PM MST FRI NOV 20 2009
DISCUSSION
21Z SFC ANALYSIS SHOWS LEE SIDE TROUGH EXTENDING
FROM A LOW PRESSURE CENTER OVER ALBERTA INTO EASTERN WYOMING AND
EASTERN COLORADO. UPPER ANALYSIS SHOWS RIDGING OVER THE NORTHERN
PLAINS WITH A TROUGH MOVING ONTO THE WEST COAST.
THE PRESSURE GRADIENT WILL TIGHTEN TONIGHT AS THE SFC TROUGH
MOVES SLOWLY EAST INTO WESTERN SOUTH DAKOTA. GUSTY SOUTHWESTERLY
WINDS WILL DEVELOP WEST OF THE TROUGH AXIS OVER NORTHEASTERN
WYOMING THIS EVENING. PRESSURE GRADIENT ANALYSIS ALSO SHOWS THE
POTENTIAL FOR GUSTY SOUTHWESTERLY DOWNSLOPE WINDS TO DEVELOP OVER
THE NORTHERN BLACK HILLS AND THE ADJACENT FOOTHILLS TONIGHT.
FORECAST WINDS ALOFT SHOW THAT GUSTS AS HIGH AS 50 MPH WILL BE
POSSIBLE IN THOSES AREAS OVERNIGHT SO HAVE ISSUED A WIND ADVISORY
FOR THE NORTHERN AND NORTHEASTERN FOOTHILLS OF THE BLACK HILLS
TONIGHT.
ON SATURDAY...THE UPPER TROUGH APPROACHES FROM THE WEST...PUSHING
A COLD FRONT INTO NORTHEASTERN WYOMING AND NORTHWESTERN SOUTH
DAKOTA BY MIDDAY...THEN CROSSING THE REST OF THE AREA SATURDAY
AFTERNOON AND EVENING. THE FRONT WILL USHER IN COOLER AIR WITH
LIMITED MOISTURE. WILL CONTINUE LOW POPS IN THE WEST NEAREST BEST
FORCING.
ON SUNDAY...THE SURFACE HIGH SHIFTS EAST WITH RETURN FLOW IN THE
WEST. NEXT SHORTWAVE SWINGS INTO THE REGION FOR LOW POPS SUNDAY
NIGHT. HIGH TEMPERATURES WILL BE NOTICEABLY COLDER SUNDAY...A RETURN
TO MORE SEASONAL CONDITIONS.
EXTENDED...GFS ENSEMBLE MEMBERS SHOWING GOOD AGREEMENT ON SWINGING
TROUGH THROUGH THE CWA ON MONDAY. ECMWF AND GEM OPERATIONAL RUNS
ALSO CONSISTENT WITH THESE SOLUTIONS. THE ECMWF AND GEM RUNS DIVERGE
FROM GFS GOING INTO MONDAY NIGHT...WRAPPING UP AN UPPER LOW ALONG
THE NEBRASKA AND KANSAS BORDER. RMOP INDICATING LOWER PREDICTABILITY
WITH THIS SYSTEM. WILL KEEP CHANCE POPS ACROSS SOUTH CENTRAL
PORTIONS OF THE CWA ATTM MONDAY INTO TUESDAY. HAVE BUMPED TEMPS DOWN
ACROSS THE CWA UNDER COOL NORTHWEST FLOW.
THE GFS SHOWS ANOTHER WAVE SWEEPING ACROSS THE CWA TUESDAY NIGHT
INTO WEDNESDAY. NEITHER GEM NOR ECMWF AGREE WITH THIS SCENARIO.
SEVERAL GFS ENSEMBLE MEMBERS POINTING TO THIS SOLUTION...SO WILL
KEEP SLIGHT CHANCE POPS ACROSS THE CWA FOR THIS TIMEFRAME. MODELS
CONSISTENT WITH BUILDING RIDGE ACROSS THE WESTERN CONUS THURSDAY
INTO FRIDAY AHEAD OF NEXT TROUGH. ATTM IT APPEARS THAT THE CWA WILL
BE DRY GOING INTO LATE NEXT WEEK.
AVIATION
SOUTHWEST WINDS OF 20 TO 30 KTS WITH GUSTS TO 40 KTS
WILL DEVELOP ACROSS NORTHEAST WYOMING...THE NORTHERN BLACK
HILLS...AND NORTHERN FOOTHILLS TONIGHT. THE WINDS WILL SUBSIDE
SATURDAY MORNING. VFR CONDITIONS CAN BE EXPECTED THROUGH THE
FORECAST PERIOD.
UNR WATCHES/WARNINGS/ADVISORIES
SD...WIND ADVISORY FROM 9 PM THIS EVENING TO 6 AM MST SATURDAY FOR
NORTHERN FOOT HILLS-STURGIS/PIEDMONT FOOT HILLS.
WY...NONE.
GR/JW
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