Area Forecast Discussion
Issued by NWS Rapid City, SD

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000
FXUS63 KUNR 250321
AFDUNR

AREA FORECAST DISCUSSION FOR WESTERN SD AND NORTHEASTERN WY
NATIONAL WEATHER SERVICE RAPID CITY SD
821 PM MST WED DEC 24 2014

.UPDATE...
ISSUED AT 816 PM MST WED DEC 24 2014

03Z SURFACE ANALYSIS HAD COLD FRONT FROM NORTHEAST ND INTO
SOUTHEAST MT THEN WEST INTO SOUTHWEST MT. LOW PRESSURE DEVELOPING
OVER SOUTHEAST MT ALONG FRONT. LATEST WATER VAPOUR LOOP SHOWED
NICE LOOKING SHORTWAVE/UPPER LOW OVER NORTHERN CENTRAL NV DRIVEN
BY 110KT JET. 100KT JET STREAK OVER SOUT H CENTRAL CANADA. AS
SYSTEM FROM THE WEST APPROACHES...MODEST COUPLING OF JETS WILL
OCCUR WITH SYNOPTIC LIFT OVER 850-700MB BAROCLINIC ZONE...WHICH
WILL SINK SOUTH WITH COLD FRONT OVERNIGHT INTO CHRISTMAS DAY. 6-12
HOUR PERIOD OF MODEST ISENTROPIC LIFT EXPECTED WITH A BAND OF 3-6
INCHES OF SNOW LIKELY...PER ONGOING FORECAST.

00Z KUNR SOUNDING AS WELL AS 21Z SREF/00Z NAM SUGGEST A
NARROW/BRIEF PERIOD OF -FZRA POSSIBLE FROM BUTTE COUNTY TO PHILIP
OVERNIGHT...SO WILL ADD A SLIGHT CHANCE TO FORECAST.

&&

.SHORT TERM...THIS EVENING THROUGH THURSDAY NIGHT
ISSUED AT 133 PM MST WED DEC 24 2014

UPPER RIDGING COVERS THE WESTERN CONUS...WITH A TROUGH DIGGING
INTO THE WEST COAST. STRATUS CONTINUES TO STREAM IN OVER THE
NORTHERN PLAINS. AXIS OF THE SURFACE TROUGH IS CROSSING THE
WESTERN DAKOTAS...AND WINDS ARE LIGHT AND SOUTHWESTERLY. TEMPS ARE
IN THE 30S...WITH SOME 40S ALONG THE EASTERN FOOTHILLS.

UPPER TROUGH WILL DEEPEN AS IT SHIFTS EASTWARD OVER THE WESTERN
STATES TONIGHT...WHILE NORTHERN STREAM CANADIAN LOW BEGINS SWEEPING
A COLD FRONT THROUGH THE NORTHERN PLAINS. AIDED BY JET
COUPLING...SNOW WILL SPREAD FROM MT INTO NORTHEAST WY AND WESTERN SD
OVERNIGHT. NAM HAS A GOOD AMOUNT OF MID-LEVEL FRONTOGENESIS AND
Q-VECTOR CONVERGENCE OVER THE CENTRAL CWA...WHILE THE GFS KEEPS THE
BEST FORCING FARTHER SOUTH. THE NAM/GFS/ECMWF/GEM AGREE THAT SNOW
WILL STRETCH FROM WY THROUGH NORTHEAST SD...BUT THE GFS IS THE
OUTLIER WITH GIVING OUR CWA LESSER AMOUNTS THAN AREAS FARTHER SOUTH.
TRENDED MORE TOWARDS OTHER MODELS WITH POPS...HAVING SNOW SPREADING
FROM NORTHWEST TO SOUTHEAST ACROSS OUR AREA THROUGHOUT CHRISTMAS AND
INTO CHRISTMAS NIGHT. SNOW AMOUNTS HAVE INCREASED SLIGHTLY...GENERALLY
3-6 INCHES...WITH 1-3 INCHES OVER FAR NORTHWEST SD. LOCALLY HIGHER
AMOUNTS ARE POSSIBLE...ESPECIALLY ACROSS CAMPBELL COUNTY...NEAR
THE PINE RIDGE...AND ACROSS THE NORTHERN HILLS. FROUDE NUMBER IS
NOT IMPRESSIVE...THEREFORE DO NOT EXPECT COPIOUS AMOUNTS OF
UPSLOPE SNOW OVER THE NORTHERN HILLS WITH THIS SYSTEM.

NORTHERLY WINDS WILL BE BREEZY ON CHRISTMAS...AT ABOUT 15-25
KTS...AND DIMINISH GRADUALLY OVERNIGHT. THIS WILL BE ENOUGH TO
CREATE SOME PATCHY BLOWING SNOW ACROSS THE AREA...BUT DO NOT EXPECT
AS MUCH BLOWING SNOW AS WITH THE LAST SYSTEM.

ALTHOUGH SNOWFALL MIGHT ONLY BE MARGINALLY ADVISORY CRITERIA FOR
SOME PLACES...FEEL THAT PATCHY BLOWING SNOW AND HOLIDAY TRAVEL
WARRANT A WINTER WEATHER ADVISORY FOR MOST OF THE CWA CHRISTMAS DAY
THROUGH CHRISTMAS NIGHT. WILL LEAVE THE FAR NORTHWEST OUT FOR
NOW...BUT LATER SHIFTS CAN REASSESS IF EXPANSION IS NECESSARY.

LOWS TONIGHT WILL BE IN THE 20S. COLD AIR ADVECTION THROUGHOUT THE
DAY WILL PREVENT TEMPS FROM RISING MUCH ON CHRISTMAS. HIGHS WILL BE
IN THE 20S TO LOWER 30S. CHRISTMAS NIGHT WILL BE FRIGID...WITH LOWS
RANGING FROM THE SINGLE DIGITS ACROSS THE WESTERN CWA TO THE MID
TEENS FARTHER EAST.

.LONG TERM...FRIDAY THROUGH WEDNESDAY
ISSUED AT 133 PM MST WED DEC 24 2014

LIGHT SNOW SHOULD COME TO AN END ACROSS SOUTH CENTRAL
SOUTH DAKOTA FRIDAY MORNING AS SHORTWAVE EJECTS NORTHEASTWARD AND
UPPER TROF AXIS SLIDES ACROSS THE AREA. TEMPERATURES WILL CONTINUE
TO TUMBLE AS ARCTIC HIGH PRESSURE SETTLES INTO THE AREA. DRY AND
COLD CONDITIONS WILL PERSIST INTO SUNDAY MORNING. THE NEXT UPPER
SHORTWAVE WILL CROSS THE NORTHERN PLAINS SUNDAY AFTERNOON INTO
MONDAY...BRINGING ADDITIONAL SNOW TO THE AREA. MODEL SOLUTIONS
VARY WITH THIS WAVE...WITH CONSENSUS POSSIBLY ROUTING THE BEST
SNOWFALL ALONG THE WESTERN AND SOUTHERN PORTIONS OF THE FORECAST
AREA. TEMPERATURES REMAIN BELOW AVERAGE THROUGH THE EXTENDED
PERIOD.

&&

.AVIATION...(FOR THE 00Z TAFS THROUGH 00Z THURSDAY EVENING)
ISSUED AT 411 PM MST WED DEC 24 2014

VFR CONDITIONS WILL PREVAIL THROUGH THIS EVENING WITH A SOLID
MID/HIGH CLOUD DECK. CONDITIONS WILL DETERIORATE FROM NORTH TO
SOUTH LATER TONIGHT INTO THURSDAY MORNING WITH WIDESPREAD IFR
CIGS/VSBYS DUE TO STRATUS/-SN. AREAS LIFR VSBYS/CIGS WILL OCCUR
WITH THE HEAVIER BANDS OF SNOW.

&&

.UNR WATCHES/WARNINGS/ADVISORIES...
SD...WINTER WEATHER ADVISORY FROM 5 AM TO 11 PM MST THURSDAY FOR
     SDZ012-013-024>026-028-072-073.

     WINTER WEATHER ADVISORY FROM 11 AM MST /NOON CST/ THURSDAY TO 5
     AM MST /6 AM CST/ FRIDAY FOR SDZ027-029>032-041>044-046-047-
     049-074.

WY...WINTER WEATHER ADVISORY FROM 5 AM TO 11 PM MST THURSDAY FOR
     WYZ057-058-071.

     WINTER WEATHER ADVISORY FROM MIDNIGHT TONIGHT TO 11 PM MST
     THURSDAY FOR WYZ054>056.

&&

$$

UPDATE...HELGESON
SHORT TERM...POJORLIE
LONG TERM...15
AVIATION...HELGESON





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