Area Forecast Discussion
Issued by NWS Rapid City, SD

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000
FXUS63 KUNR 220506
AFDUNR

AREA FORECAST DISCUSSION FOR WESTERN SD AND NORTHEASTERN WY
NATIONAL WEATHER SERVICE RAPID CITY SD
1106 PM MDT MON APR 21 2014

.SHORT TERM...THIS EVENING THROUGH TUESDAY NIGHT
ISSUED AT 201 PM MDT MON APR 21 2014

AT 18Z A N/S SURFACE RIDGE WAS DRIFTING THROUGH THE
WESTERN DAKOTAS AND WESTERN NEBRASKA...AND THE UPPER LEVEL RIDGE WAS
CROSSING THE CONTDVD. TEMPERATURES HAD WARMED INTO THE MID 60S IN
THE LOWER ELEVATIONS AND WINDS WERE LIGHT AND VARIABLE. A MAJOR
PACIFIC STORM SYSTEM WAS MOVING RAPIDLY ONSHORE AND SPREADING A VERY
LARGE SHIELD OF PRECIPITATION ACROSS THE NORTHWESTERN U.S.

TONIGHT...THE LARGE SCALE PATTERN IS FORECAST TO PROGRESS RATHER
QUICKLY WITH THE RIDGE MOVING INTO THE UPPER MISS VALLEY AND THE
PACIFIC SYSTEM PUSHING INTO THE INTERMOUNTAIN REGION BY EARLY
TUESDAY.  THE SURFACE FLOW OVER WESTERN SOUTH DAKOTA AND NORTHEAST
WYOMING WILL SHIFT TO EAST AND THEN SOUTHEAST AND INCREASE.

TUESDAY...THE PACIFIC SYSTEM IS FORECAST TO PUSH INTO THE NORTHERN
ROCKIES BY LATE TUESDAY INCREASING THE SOUTHEAST FLOW AT THE
SURFACE. DEWPOINTS WILL RISE ACCORDINGLY INTO THE MID AND THEN UPPER
30S BY MIDDAY TUESDAY SETTING UP A GENERAL DESTABILIZATION OF THE
AIRMASS BY LATE INT HE DAY.  MOISTURE WILL STILL BE SOMEWHAT LIMITED
AND MID LEVEL SUPPORT FOR UVV STILL A LITTLE WEAK EVEN BY SUNSET.
TEMPERATURES WILL WARM ANOTHER 10 TO 15 DEGREES OVER TODAY BY THE
AFTERNOON...BUT INCREASING CLOUD COVER WILL PUT A LIMIT ON SURFACE
HEATING. A FEW LIGHT THUNDERSHOWERS ARE POSSIBLE OVER SOUTHWESTERN
SOUTH DAKOTA AND PARTS OF NORTHEAST WYOMING LATE IN THE DAY.

TUESDAY NIGHT... THE MAIN PACIFIC FRONT IS FORECAST TO MOVE INTO
NORTHEAST WYOMING DURING THE EARLY MORNING HOURS...BUT WIDELY
SCATTERED PREFRONTAL CONVECTION IS EXPECTED IN THE UNSTABLE AIR MASS
TUESDAY EVENING. HOWEVER WITHOUT SURFACE HEATING THE INSTABILITY IS
DEPICTED ON MODEL SOUNDINGS AS BEING MOSTLY ALOFT AND RATHER WEAK.
MUCAPE VALUES AROUND 200 TO 250 ARE POSSIBLE BRIEFLY LATE TUESDAY
EVENING....MOSTLY OVER THE WESTERN HALF OF THE FORECAST AREA. WIDELY
SCATTERED SHOWERS WILL CONTINUE TO DEVELOP AND SPREAD ACROSS MOST OF
THE FORECAST AREA BUT QPF VALUES ARE GENERALLY LIGHT.  BREEZY
SOUTHEAST WINDS WILL ENHANCE THE FEELING OF SPRING TIME CONDITIONS
WITH VERY MILD OVERNIGHT MINIMUMS.


&&

.LONG TERM...WEDNESDAY THROUGH MONDAY
ISSUED AT 201 PM MDT MON APR 21 2014

MODELS REMAIN IN REASONABLE AGREEMENT FOR THE WEDNESDAY
THROUGH FRIDAY TIME PERIOD. THE UPPER TROUGH WILL MOVE FROM THE
NORTHERN ROCKIES INTO THE EASTERN DAKOTAS/MINNESOTA BY THURSDAY
AFTERNOON. THE SURFACE LOW OVER SOUTHEASTERN MONTANA WEDNESDAY
MORNING WILL MOVE INTO SOUTHEASTERN SOUTH DAKOTA BY WEDNESDAY
EVENING DRAGGING A COLD FRONT ACROSS THE FORECAST AREA ON WEDNESDAY.
GUSTY SOUTHERLY WINDS AHEAD OF THE FRONT WILL SHIFT TO THE
WEST/NORTHWEST AND BECOME VERY WINDY BEHIND THE FRONT. MOISTURE WILL
BE LIMITED WITH THE SYSTEM WITH MAIN SYNOPTIC LIFT WELL TO THE
NORTH OF THE CWA...BUT A NARROW SLIVER OF INSTABILITY WILL DEVELOP
ALONG THE COLD FRONT BRINGING A CHANCE OF -SHRA/-TSRA. HIGH
TEMPERATURES WILL BE TRICKY WEDNESDAY DUE TO THE TIMING OF THE
FRONTAL PASSAGE. TEMPERATURES HAVE THE POTENTIAL TO APPROACH 80
DEGREES TO THE SOUTH OF THE FRONT OVER SOUTH CENTRAL SOUTH DAKOTA
WITH HIGHS AROUND 60 DEGREES BEHIND THE FRONT OVER PARTS OF
NORTHEASTERN WYOMING AND NORTHWESTERN SOUTH DAKOTA. A FEW SHOWERS
MAY WRAP AROUND THE UPPER LOW AND BACK INTO THE NORTHERN PARTS OF
THE FORECAST AREA ON WEDNESDAY NIGHT.

A TIGHT SURFACE PRESSURE GRADIENT COMBINED WITH LL COLD ADVECTION
WILL CREATE A VERY WINDY DAY AGAIN ON THURSDAY. CYCLONIC FLOW ALOFT
WILL CREATE A SLIGHT CHANCE OF SHOWERS...MAINLY OVER NORTHEASTERN
WYOMING...THE NORTHERN BLACK HILLS AND NORTHWESTERN SOUTH DAKOTA.

A LULL IN THE WEATHER IS STILL EXPECTED ON FRIDAY AS AN UPPER LEVEL
RIDGE CROSSES THE AREA.

FRIDAY NIGHT INTO SUNDAY...NEXT UPPER TROUGH MOVES FROM THE WEST
COAST INTO THE PLAINS STATES. MODELS ARE NOT IN VERY GOOD AGREEMENT
WITH PARTICULARS OF THIS SYSTEM. THEY GENERALLY AGREE ON THE
LOCATION OF UPPER AIR FEATURES BUT DIFFER GREATLY ON LOW LEVEL
THERMAL PROFILES. LATEST GFS/ECMWF RUNS CONTINUE TO SHOW 10C-15C
DIFFERENCE IN 850MB TEMPERATURES BY SUNDAY...WHICH WOULD MAKE
DIFFERENCE FOR PRECIPITATION TYPE. WILL CONTINUE TO BLEND THE
SOLUTIONS FOR NOW WITH CHANCE POPS AND A MIX OF RASN AT NIGHT.

&&

.AVIATION...(FOR THE 06Z TAFS THROUGH 06Z TUESDAY NIGHT)
ISSUED AT 1105 PM MDT MON APR 21 2014

VFR CONDITIONS EXPECTED THROUGH TUESDAY AFTERNOON. ISOLATED
THUNDERSTORMS ARE POSSIBLE LATE TUESDAY AFTERNOON/TUESDAY EVENING OVER
NORTHEAST WYOMING AND WESTERN SOUTH DAKOTA.

&&

.UNR WATCHES/WARNINGS/ADVISORIES...
SD...NONE.
WY...NONE.
&&

$$

SHORT TERM...CARPENTER
LONG TERM...10
AVIATION...7





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