Area Forecast Discussion
Issued by NWS Rapid City, SD

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000
FXUS63 KUNR 212321
AFDUNR

AREA FORECAST DISCUSSION FOR WESTERN SD AND NORTHEASTERN WY
NATIONAL WEATHER SERVICE RAPID CITY SD
521 PM MDT THU AUG 21 2014

.SHORT TERM...THIS EVENING THROUGH FRIDAY NIGHT
ISSUED AT 222 PM MDT THU AUG 21 2014

DEEP WESTERN NOAM TROUGH WILL CONTINUE TO SUPPORT ACTIVE
SOUTHWEST FLOW OVER THE REGION THROUGH THE PERIOD...WITH TWO
STAUNCH IMPULSES AFFECTING THE REGION...AFTER THIS EVENING/S
ADVANCING WAVE. A WAVERING FRONTAL BOUNDARY WILL PROVIDE THE FOCUS
FOR LL CYCLOGENESIS AND ASSOCIATED LL LIFT...SUPPORTING GOOD
CHANCES FOR RAIN AND THUNDERSTORMS ACROSS THE AREA...ESP FRIDAY
NIGHT. AS OF 20Z A SFC TROUGH WAS ENTERING THE WESTERN FA WITH A
WEAK IMPULSE EJECTING NE OUT OF WY. THIS WAVE WILL CONTINUE TO
SUPPORT INCREASING CHANCES FOR SHOWERS AND THUNDERSTORMS OVER THE
NW HALF INTO THIS EVENING. A FEW CELLS HAVE DEVELOPED OVER THE
BLACK HILLS GIVEN WEAKER INHIBITION...WITH MORE ACTIVITY EXPECTED
OFF THE BIG HORNS AND HIGH TERRAIN OF WY. HIRES RUNS OF THE
NMM/ARW/HRRR INDICATE THAT ISOLD/SCT THUNDERSTORMS MAY COALESCENCE
INTO A BIGGER CLUSTER OF TS AND PROGRESS NE ACROSS THE NW ZONES
THIS EVENING AS THE WAVES PUSHES NE. BEST FORCING FOR ASCENT WILL
REMAIN NW OF THE FA. PREVIOUS POPS WORK WELL WITH LIKELIES ACROSS
THE FAR NW...WITH MORE ISOLATED CHANCES TO THE SE ALONG THE
ASSOCIATED SFC TROUGH PUSH WHERE STRONGER CAPPING AND WEAKER LIFT
WILL BE PRESENT.

FRIDAY...A FEW LINGERING SHRA MAY BE PRESENT OVER THE EASTERN ZONES
FRI MORNING AS THE TROUGH PUSHES SE. THIS FEATURE WILL STALL IN
RESPONSE TO THE NEXT IMPULSE...THE EJECTING SW CONUS UPPER TROUGH.
WATER VAPOR IMAGERY CLEARLY SHOWS AMPLE DEEP LAYER MOISTURE BEING
INGESTED INTO THE SYSTEM PER A FLOW CONNECTION TO KARINA. HENCE
AMPLE DEEP LAYER MOISTURE WILL BE AVAILABLE WHEN THIS SYSTEM CROSSES
THE PLAINS AND A LL GOMEX CONNECTION IS ESTABLISHED. HAVE PRECIP
CHANCES INCREASING THROUGH THE DAY FRIDAY. HOWEVER...MOST SHRA/TS
MAY NOT ARRIVE INTO THE FA UNTIL FRIDAY NIGHT GIVEN LINGERING
SUBSIDENCE BEHIND TODAY/S WAVE. FORECAST MODEL CONSENSUS/AND
RUN-RUN CONSISTENCY PER THE TRACK/EVOLUTION/AND RAIN TOTALS CONTINUE
TO WAVER. HOWEVER...THERE IS A SIGNAL FOR HEAVY RAIN WITH THIS WAVE
GIVEN THE DEEP MOISTURE AVAILABLE. VERY HIGH PW VALUES IN EXCESS OF
ONE INCH IN MANY LOCATIONS...WITH SOME AREAS APPROACHING TWO WILL
LIKELY SUPPORT AN AREA OF HEAVY RAIN WITH THIS SYSTEM...ESP WHERE
H8-H7 POS THETA-E ADV IS STRONGEST. WHERE REMAINS THE QUESTION.
LATEST GUIDANCE SUGGEST THE SE FA. HOWEVER...GIVEN THE STAUNCH UPPER
JET ASSOCIATED WITH THE PAC NW UPPER TROUGH...THIS AREA MAY SHIFT NW
WITH FUTURE MODEL RUNS. AT THIS POINT...FEEL A POP RAISE WAS
WARRANTED WITH A LOCALLY HEAVY RAIN MENTION IN THE GRIDS FRIDAY
NIGHT. AS FOR TEMPS...E/NE FLOW WITH EXPECTED INCREASED CLOUD COVER
WILL SUPPORT A COOLER DAY NW AREAS. DID ADJUST TEMPS DOWN THERE.

SAT...LINGERING SHRA/TS POSSIBLE WITH A DRY PERIOD EXPECTED SW HALF
IN THE AFT. MAIN NW CONUS UPPER TROUGH WILL EJECT NE SAT/SAT
NIGHT...SUPPORTING RENEWED SFC PRESSURE FALLS AND REFLECTED LL SE
FLOW. POS THETA-E ADV REGIME SHOULD SUPPORT SHRA/TS ACROSS PORTIONS
OF THE FA...ESP THE NW ZONES SAT NIGHT. HAVE RETAINED AT LEAST A MID
POP VALUE ALL PLACES...WITH LIKELIES NW...ESP WITH AMPLE/SUFFICIENT
DEEP LAYER MOISTURE PRESENT PER H7 THETA-E PROGS. DID LOWER TEMPS
SAT GIVEN CLOUDS AND MOIST EASTERLY LL FLOW...ESP NW ZONES.

&&

.LONG TERM...SATURDAY THROUGH THURSDAY
ISSUED AT 222 PM MDT THU AUG 21 2014

UPPER LOW LIFTS NORTHEAST INTO MONTANA SUNDAY...WITH DRY SLOT
PUNCHING INTO WESTERN/CENTRAL SOUTH DAKOTA. BEST SUPPORT FOR
PRECIPITATION WILL BE NORTH OF THE FORECAST AREA. POST FRONTAL
STRATUS WILL LINGER THROUGH MUCH OF THE DAY SUNDAY...ESPECIALLY
ACROSS NORTHEAST WYOMING/NORTHWESTERN SOUTH DAKOTA. UPPER LOW
CONTINUES TO LIFT NORTHEAST INTO CANADA MONDAY...LEAVING SLOW
MOVING UPPER TROF ACROSS THE NORTHERN PLAINS. BELOW AVERAGE
TEMPERATURES WILL CONTINUE THROUGH MID WEEK...ALONG WITH A DAILY
CHANCE OF SHOWERS/THUNDERSTORMS.

&&

.AVIATION...(FOR THE 00Z TAFS THROUGH 00Z FRIDAY EVENING)
ISSUED AT 519 PM MDT THU AUG 21 2014

SHRA/TSRA EXPECTED OVER MUCH OF THE AREA THROUGH TONIGHT WITH
LOCAL MVFR CIGS/IFR VSBYS. ST WILL DEVELOP LATE TONIGHT OVER
NORTHEAST WY/NORTHWEST SD WITH AREAS IFR CIGS. CONDITIONS WILL
IMPROVE SOME FRIDAY MORNING...BUT ADDITIONAL SHRA/TSRA EXPECTED TO
DEVELOP FRIDAY AFTERNOON.

&&

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

$$

SHORT TERM...JC
LONG TERM...7
AVIATION...HELGESON




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