Area Forecast Discussion
Issued by NWS Riverton, WY

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000
FXUS65 KRIW 120957
AFDRIW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RIVERTON WY
257 AM MST FRI DEC 12 2014

.SHORT TERM...TODAY THROUGH SUNDAY NIGHT

WV IMAGERY SHOWING OMEGA PATTERN ACROSS THE CONUS...WITH AMPLIFIED
RIDGE OVR THE HIGH/CNTRL/NRN PLAINS. BOOKEND TROFS OVR EACH COAST.
JET AND ACCOMPANYING VORT MAX JUST ROUNDING THE HORN/BASE OF THE
AMPLIFYING TROF...BEGINNING TO TAKE ON A SLIGHTLY NEGATIVE TILT. SFC
SHOWS HIGH P TO THE SOUTH OVR THE FOUR CORNERS...WITH LEE TROF STILL
EXTENDING FROM NERN TO S CNTRL WY. SOME MAINLY UPR LVL CLOUDINESS
OVR MOST OF THE FA...WITH MOSTLY CLEAR SKIES ACROSS THE NW FA.

PATCHY FOG WILL BE POSSIBLE ONCE AGAIN THIS MORNING OVR PORTIONS OF
WRN GREEN RIVER BASIN AND THE WESTERN MOUNTAIN VALLEYS.
OTHERWISE...SFC LEE TROFFING CONTINUES ACROSS THE ERN ZONES. WARM
AGAIN WITH SOME MODERATE DOWNSLOPING HELPING THE CAUSE. NEAR
RECORD HIGHS EXPECTED AGAIN AROUND THE CWA TODAY...PARTICULARLY
OVR THE ERN FCST ZONES. OTHER AREAS THAT WILL BE CLOSE TO RECORD
INCLUDE PORTIONS OF THE WIND RIVER BASIN...EASTERN GREEN RIVER
BASIN...AND SWEETWATER COUNTY INCLUDING BOTH ROCK SPRINGS AND
POSSIBLY BIG PINEY.

BIG CHANGES ON THE WAY THIS WEEKEND...BUT WITH A RELATIVELY BENIGN
BEGINNING AS MID/UPR MOISTURE PUSHES INTO THE WEST LATER TONIGHT.
MODEST BLOCKING ACTION OF THE WRN TROF BY THE RIDGE OVR THE CNTRL
CONUS CONTINUES TO CAUSE NOTICEABLE CHANGES THROUGH THE EARLY FCST
PERIOD...WITH MUCH LARGER DIFFERENCES APPEARING THIS TIME AROUND AS
OPPOSED TO THE PREVIOUS 48 OR SO HRS. WHILE THE STRONGER PORTION OF
INCREASINGLY NEGATIVE TILTED TROF (BY SAT NIGHT) IS STILL WELL TO
THE SOUTH AND OVER SRN CO/NRN NM BY SUN MORNING...SPEED AND
SEPARATION FROM THE NRN PORTION OF THE TROF WILL ALLOW THE NRN
PORTION TO DEVELOP SLOWER AND SOMEWHAT SEPARATELY. THIS ALSO DELAYS
SLIGHTLY AND ALLOWS A SECONDARY MOISTURE PUSH TO COME ACROSS THE
CNTRL CWA A BIT FURTHER TO THE WEST THAN THE PREVIOUS MORE EASTERLY
FCST. WHILE THE WRN MOUNTAINS LOSE SOME PRECIP PROMISED EARLIER INT
HE WEEK...FRONTOGENESIS ACROSS WY NOW LOOKS EVEN BETTER ACROSS THE
REST OF THE FA. THE FRONT WILL DEVELOP AND MOVE BRISKLY SWD ACROSS
THE CWA SAT NIGHT WHILE A RELATIVELY DEEP AREA OF MOISTURE FROM THE
EPAC...NOW JUST  OFFSHORE OF CNTRL/MID CALIFORNIA...RIDES FROM S TO
N OVR THE FRONT SATURDAY NIGHT. AS TRAJECTORY OF MOISTURE CONTINUES
TO TREND SOMEWHAT WEST...ADJUSTED FCST SLIGHTLY WITH THIS IN
MIND.

GENERALLY LIGHT TO MODERATE SNOWS WILL BEGIN TONIGHT ACROSS THE WRN
FA...CONTINUING THRU THE SAT MORNING BEFORE TAPERING OFF. THEN
BEGINS A TRANSITION WHERE PRECIP WILL SHIFT ACROSS THE SOUTH AND
THEN UP AND ACROSS THE REGION EAST OF THE DIVIDE AS THE PREVIOUSLY
MENTIONED STRONG FRONTOGENESIS BEGINS. THE NEW TRAJECTORY OF THE
MOIST FLOW OUT OF THE SOUTH WILL THEN HAVE A DOUBLE BONUS OF BEING
LIFTED BY TERRAIN AND A RATHER STEEP FRONT. MUCH OF THE AREA E OF
THE DIVIDE ON SATURDAY...IF PRECIP FALLS BEFORE DARK...WILL FIRST
SEE LIGHT LIQUID PRECIP BEFORE MIXING WITH AND TURNING TO ALL SNOW
BY SAT EVENING. THERE MAY ALSO BE SOME WEAKLY CONVECTIVE ELEMENTS AT
THIS TIME WITH SOME ISOLATED HEAVIER SNOW SHOWERS
EARLY...PARTICULARLY ACROSS SWEETWATER COUNTY. ADDITIONALLY...AN
AREA CONVERGENCE LOOKS TO SET UP FROM THE FARSON AREA...TO SOUTH
PASS/BEAVER RIM...EAST INTO NATRONA COUNTY COUNTY BY SUNDAY
MORNING...WITH ADDITIONAL THE POTENTIAL FOR HEAVIER SNOWFALL ACROSS
THE AREA THROUGH THE DAY ON SUN. MOST LOCATIONS LOOK TO REMAIN BELOW
WARNING CRITERIA...EXCEPT FOR A FEW FAVORED LOCALIZED AREAS.
HOWEVER...WINTER ADVISORY LEVELS LOOK TO BE FAIRLY WIDESPREAD ACROSS
THE CWA...ESPECIALLY AT  LOWER ELEVATIONS EAST OF THE DIVIDE IN
ADDITION TO EASTERN SUBLETTE AND MOST OF SWEETWATER COUNTIES
INCLUDING THE I80 AND I25 CORRIDORS. FURTHER DETAILS ARE PRESENT IN
CURRENT SPS.

OTHERWISE...BREEZY TO LOW END WINDY CONDITIONS CONTINUE ACROSS THE
SRN/SERN WIND CORRIDOR TODAY THROUGH SATURDAY AS LEE TROF CONTINUES
TO STRENGTHENS ALONG WITH P GRAD AND MID/UPR LVL WINDS. 20G45 KTS WILL
BE FOUND OVR MANY OF THE HILLS AND MOUNTAINS IN THIS
AREA...INCLUDING CASPER.

.LONG TERM...MONDAY THROUGH FRIDAY

SYNOPSIS...A RIDGE OF HIGH PRESSURE WILL BUILD INTO THE AREA MONDAY
WITH SNOW SHOWERS DIMINISHING ACROSS CENTRAL AND EASTERN WYOMING
MONDAY MORNING.  THE NEXT PACIFIC SYSTEM WILL MOVE INTO THE SOUTHERN
AND CENTRAL ROCKIES LATE WEDNESDAY AND THURSDAY. MOST OF THE
PRECIPITATION WITH THIS SYSTEM IS EXPECTED TO STAY SOUTH OF
WYOMING.  TEMPERATURES THROUGH THIS PERIOD WILL BE NEAR OR SLIGHTLY
ABOVE NORMAL.

DISCUSSION...UPPER HIGH TO BUILD ACROSS WEST CENTRAL CANADA ON
MONDAY AND TUESDAY WITH REMNANTS OF THE WEEKEND`S UPPER LOW SLIDING
INTO THE PLAINS MONDAY MORNING.  GFS AND ECMWF DIFFER ON HANDLING OF
FLOW DOWNSTREAM OF CANADIAN UPPER HIGH WITH ENSEMBLE MEANS
SUPPORTING ECMWF SOLUTION OF UPPER LOW TAKING MORE OF AN EASTWARD
TRACK AFTER SLIDING DOWN INTO THE NORTHERN PLAINS MONDAY NIGHT
RATHER THAN PULLING IT BACK WESTWARD UNDER THE UPPER HIGH.   THIS
UPPER HIGH OVER CENTRAL CANADA WILL KEEP THE FLOW SPLIT ACROSS THE
WESTERN U.S. THROUGH THE PERIOD WITH NEXT STRONG ~175KT PUSHING EAST
OFF OF SOUTHERN JAPAN TRANSLATING THE PACIFIC AND SHARPENING THE
NEXT TROUGH OFF THE WEST COAST ON MONDAY.  THIS NEXT SYSTEM LOOKS TO
INITIALLY TAKE A FURTHER SOUTHERN ROUTE ACROSS THE SW U.S. THAN THIS
WEEKEND`S SYSTEM.  THIS TROUGH COMES INTO THE WEST RATHER FRAGMENTED
AND DETERMINISTIC RUNS HAVE DIFFERING SOLUTIONS ON THE SPEED AND
TRACK OF THESE SMALLER SCALE FEATURES.  HOWEVER...THERE IS GOOD
AGREEMENT KICKING OUT INITIAL SHORTWAVE NE ACROSS THE GREAT BASIN
MONDAY NIGHT BRINGING SOME SNOW SHOWERS BACK INTO THE WEST ON
TUESDAY.  COOL UPPER TROUGH PUSHES INTO THE AREA WEDNESDAY AND
THURSDAY WITH MAINLY ISOLATED TO SCATTERED SNOW SHOWERS ACROSS THE
WEST AND SOUTH.  UPPER RIDGE EXPECTED TO BUILD BACK IN FRIDAY INTO
EARLY NEXT WEEKEND WITH MAINLY DRY WEATHER AND A WARMING TREND.


&&

.AVIATION.../12Z ISSUANCE/

VFR CONDITIONS WILL PREVAIL TODAY AND TONIGHT. EXPECT BREEZY TO
WINDY CONDITIONS AT KCPR THROUGH THE FORECAST PERIOD. SCATTERED RAIN
SHOWERS WITH MOUNTAIN SNOW WILL DEVELOP OVER FAR WEST WY AFTER 06Z
SATURDAY. SOME MVFR CONDITIONS WILL DEVELOP BY 12Z SATURDAY OVER AND
NEAR THE FAR WEST MOUNTAINS.

&&

.FIRE WEATHER...

ABOVE NORMAL TEMPERATURES AND RELATIVELY LIGHT WIND EXPECTED AGAIN
TODAY AHEAD OF A RETURN TO WINTER OVER THE WEEKEND. RELATIVELY DRY
WITH SOME GUSTY WIND FOUND ALONG THE FOOTHILLS EAST OF THE DIVIDE
AS WELL AS FROM SOUTH PASS TO CASPER TODAY...WILL SOMEWHAT ENHANCE
FIRE DANGER THIS AFTERNOON. SMOKE DISPERSION WILL GENERALLY BE
MODERATE TO GOOD ON TODAY. SATURDAY AND SUNDAY...A NEW WINTER
STORM SYSTEM WILL WORK ITS WAY ACROSS THE FORECAST AREA FROM
SOUTHWEST TO NORTHEAST...OFFERING MEASURABLE SNOW...MUCH LOWER
TEMPERATURES AND HIGHER RELATIVE HUMIDITY AREA WIDE THROUGH THE
WEEKEND.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
NONE.
&&

$$

SHORT TERM...BRAUN
LONG TERM...AEM
AVIATION...AR
FIRE WEATHER...BRAUN






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