Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 131807
AFDRIW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RIVERTON WY
1107 AM MST SAT DEC 13 2014

.UPDATE...

SNOWFALL OVER WEST AND SOUTHWEST STRONGER AND MORE WIDESPREAD
THROUGH THE MORNING THAN PREVIOUSLY ANTICIPATED.  DEEP CONVECTION
DEVELOPING OVER NE UT HAS SPREAD HEAVY SNOW SHOWERS ACROSS
INTERSTATE 80 IN SWEETWATER COUNTY AND AT KRKS AIRPORT WHERE
VSBY DOWN TO 1/4 MILE WITH A 400 FOOT CEILING. CEILINGS MUCH LOWER
WITH AREAS OF SNOW AROUND KPNA ALSO. THE UPPER GREEN RIVER BASIN
WAS ALWAYS A FORECAST CONCERN WITH ALL MODELS SHOWING QPF MINIMUM
THERE WITH DOWNSLOPE WINDS DOMINATING.  AT THIS TIME LOOKS LIKE
DYNAMICS AND DEEP CONVECTION WILL TRUMP LOW LEVEL DOWNSLOPE FLOW
AND HAVE INCREASED CHANCES FOR PRECIPITATION ACROSS MUCH OF THE
SOUTHWEST.

.SHORT TERM...TODAY THROUGH MONDAY NIGHT

WV IMAGERY STILL SHOWING OMEGA PATTERN ACROSS THE CONUS...WITH
AMPLIFIED RIDGE OVR THE PLAINS. BOOKEND TROFS OVR EACH COAST. JET
AND ACCOMPANYING VORT MAX JUST ROUNDING THE HORN/BASE OF THE
AMPLIFYING TROF...CONTINUING TO TAKE ON A SLIGHTLY NEGATIVE TILT.
SFC ALSO STILL SHOWS HIGH P TO THE SOUTH OVR CO...WITH LEE TROF NOW
EXTENDING FROM NERN TO SERN WY. MAINLY MID/UPR LVL CLOUDINESS OVR
MOST OF THE FA THIS MORNING...WITH LOWERING CLOUD BASES WEST OF THE
STATE.

DEEPENING MOISTURE PUSHES INTO THE WEST THIS MORNING AHEAD OF SLOWLY
DEEPENING TROF TO THE WEST. LIGHT TO OCCASIONALLY MODERATE SNOWFALL
WILL OCCUR THRU THIS MORNING ACROSS THE WEST. OTHERWISE...THE
STRONGER PORTION OF INCREASINGLY NEGATIVE TILTED TROF (BY TONIGHT)
WILL STILL DEVELOP WELL TO THE SOUTH AND OVER SRN CO/NRN NM BY SUN
MORNING. THE FOCUS OF STRONG CYCLOGENESIS WILL TAKE PLACE FAR FROM
OUR NECK OF THE WOODS...WITH CLASSIC LEE CYCLOGENESIS SPEEDING INTO
AN INSTANT OCCLUDING STORM ON SUNDAY OVR THE CNTRL HIGH PLAINS AND
INTO THE CNTRL PLAINS. FOR WY...THE SPEED...STRENGTH OF
DEVELOPMENT...AND SEPARATION OF THIS SRN PORTION FROM THE NRN BRANCH
OF THE TROF LOOKS TO STILL ALLOW THE NRN PORTION TO DEVELOP SLOWER
AND SEPARATELY...PROVIDING GOOD FRONTOGENESIS AND SOME ASSOCIATED LL
CONVERGENCE THAT WILL ENHANCE PRECIP/SNOW ACROSS MUCH OF THE
CNTRL...SRN...AND ERN FA OVERNIGHT SAT THRU SUNDAY. THIS FRONT WILL
DEVELOP SAT NIGHT AND MOVE BRISKLY SWD ACROSS THE CWA WHILE A
RELATIVELY DEEP AREA OF MOISTURE FROM THE EPAC...NOW OVR CA AND
NV...RIDES INTO THE REGION FROM S TO N OVR THE FRONT SATURDAY NIGHT.
MOST RECENT MOISTURE TRAJECTORY HAS REVERSED A BIT OVR THINKING 24
HRS AGO...TRENDING BACK TO THE EAST SLIGHTLY. HOWEVER...MAIN
FORCING...LOW TO MID LVL CONVERGENCE...AND PRECIP STILL BULLSEYE THE
AREA BOUNDED BY LANDER TO HILAND...TO CASPER...TO MUDDY GAP AND WEST
BACK TO THE SOUTH PASS REGION OF THE EASTERN WIND RIVER
MOUNTAINS.

THE TRANSITION TO PRECIP SHIFTING ACROSS THE SOUTHERN
ZONES...LIFTING  NWD AND ACROSS THE REGION EAST OF THE DIVIDE BEGINS
LATER THIS MORNING. THE RENEWED MOIST FLOW OUT OF THE SOUTH WILL
ALSO BEGIN TO ARRIVE...BEING LIFTED BY INCREASING UPR FLOW AT
FIRST...FOLLOWED BY TERRAIN...AND EVENTUALLY BY THE DEVELOPING AND
RATHER STEEP FRONT LATER TONIGHT. MUCH OF THE AREA E OF THE
DIVIDE...IF/WHEN PRECIP FALLS BEFORE DARK...WILL BY OF THE LIQUID
LIQUID TYPE BEFORE MIXING WITH AND TURNING TO ALL SNOW THIS EVENING.
THERE MAY ALSO BE A FEW WEAKLY CONVECTIVE ELEMENTS AT THE
BEGINNING...WITH A FEW ISOLATED HEAVIER SNOW SHOWERS THRU THE
EVENING. THE AREA CONVERGENCE LOOKS TO SET UP BETWEEN THE FARSON
AREA...TO SOUTH PASS/BEAVER RIM...AND THEN EAST OR NORTHEAST INTO
NATRONA COUNTY BY SUNDAY MORNING. THIS WILL BE THE REGION WHERE AN
ENHANCED POTENTIAL FOR HEAVIER SNOWFALL WILL OCCUR ON SUN. ANOTHER
AREA OFF LL CONVERGENCE FROM PIECES OF THE STALLED FRONT AND SOME
LOCAL ENHANCEMENT FROM N/NERLY UPSLOPE FLOW WILL ALSO BE PRESENT
ACROSS PORTIONS OF THE ERN HALF OF FREMONT COUNTY AND THE LANDER
FOOTHILLS. WINTER ADVISORY LEVELS LOOK TO COVER MOST OF THESE AREAS
JUST MENTIONED...INCLUDING THE I80 AND I25 CORRIDORS. ONLY REAL
CHANGE TO CURRENT FCST INCLUDES THE ADDITION OF ZONE 17 INTO THE
ADVISORY MIX...PINGING ON THE AREA MAINLY FROM RIVERTON EWD. ALSO
EXTENDED THE ADVISORY TIME FOR ZONES 20 AND 22 BY A COUPLE OF HRS.
FURTHER DETAILS ARE PRESENT IN THE CURRENT WINTER WEATHER
PRODUCT...SPECIAL WEATHER STATEMENT FOR EVERYONE ELSE.

OTHERWISE...SEASONAL TEMPERATURES RETURN ALONG WITH IMPROVING
CONDITIONS FROM LATE SUNDAY NIGHT THRU MONDAY
NIGHT.

.LONG TERM...TUESDAY THROUGH SATURDAY

SYNOPSIS...SPLIT FLOW TO CONTINUE ACROSS THE WESTERN U.S. THROUGH
THIS PERIOD WITH THE MAIN STORM TRACK REMAINING SOUTH OF WYOMING.
NEXT PACIFIC TROUGH WILL SPREAD CHANCES OF SNOW SHOWERS INTO THE
WEST ON TUESDAY WITH SOME OF THIS ACTIVITY SPREADING EAST OF THE
DIVIDE WEDNESDAY AND THURSDAY.  UPPER RIDGE IS EXPECTED TO BRING
DRIER WEATHER ON FRIDAY.  THE NEXT PACIFIC TROUGH WILL BRING
INCREASED CHANCES OF SNOW INTO THE WEST NEXT WEEKEND.  TEMPERATURES
WILL GENERALLY BE NEAR OR SLIGHTLY ABOVE NORMAL THROUGH THE
PERIOD...OR HIGHS MOSTLY IN THE UPPER 20S AND 30S ACROSS THE LOWER
ELEVATIONS.

DISCUSSION...SYNOPTIC PATTERN STARTS OUT WITH BLOCKING HIGH OVER
CENTRAL CANADA UNDERCUT BY VERY ENERGETIC PACIFIC FLOW PUSHING INTO
THE WESTERN U.S.  BOTH ECMWF AND GFS SHOW THE NEXT PACIFIC TROUGH
PUSHING INTO THE WEST ON TUESDAY WITH THE SOUTHERN STREAM OUTRUNNING
THE NORTHERN STREAM AS NORTHERN STREAM RUNS INTO BLOCKING HIGH OVER
CENTRAL CANADA.  THE SOUTHERN TROUGH WILL PUSH ACROSS THE SOUTHERN
ROCKIES ON WEDNESDAY AND INTO THE SOUTHERN PLAINS WEDNESDAY NIGHT
ACCORDING TO THE PREFERRED ECMWF...WITH THE GFS ON THE FAST END OF
GUIDANCE HERE.  GFS AND ECMWF IN BETTER AGREEMENT WITH A NORTHERN
STREAM SHORTWAVE DIVING SE ACROSS THE INTERMOUNTAIN WEST/GREAT BASIN
WEDNESDAY NIGHT AND THURSDAY...CARVING OUT BROAD TROUGH ACROSS THE
ROCKIES AND CENTRAL PLAINS.   THIS SYSTEM WILL BRING CHANCES OF SNOW
SHOWERS MAINLY DOWN THE SW 2/3RDS OF WYOMING WHILE THE NORTHEAST IS
EXPECTED TO REMAIN DRY UNDER A MORE STABLE AIRMASS TOWARD BLOCKING
UPPER HIGH.  A TRANSITORY RIDGE IS EXPECTED TO BUILD OVER THE AREA
FRIDAY...BOTH THE GFS AND ECMWF HAVE RIDGE AXIS ALONG THE DIVIDE BY
00Z SATURDAY.  IT LOOKS LIKE THE LAST IN THIS SERIES OF PACIFIC
STORMS WILL MOVE INTO THE WEST EARLY NEXT WEEKEND BEFORE THE
MEDIUM-RANGE MODELS SHOW A STRONG BROAD RIDGE BUILDING ACROSS THE
EASTERN PACIFIC AND EXPANDING ACROSS THE WESTERN U.S. LATE NEXT
WEEKEND INTO THE FIRST PART OF THE CHRISTMAS WEEK.  SO THE OUTLOOK
IS DRY AND MILD FOR MUCH OF THE CHRISTMAS WEEK RIGHT NOW.

.AVIATION.../12Z ISSUANCE/

EAST OF THE DIVIDE...KCOD/KCPR/KLND/KRIW/KWRL ROUTES

MVFR TO IFR CEILINGS AND VISIBILITIES REDUCED IN SNOW AND FOG WILL
SPREAD NORTH AND EAST FROM THE WESTERN MOUNTAINS AND SOUTHWEST
THIS AFTERNOON AND EVENING.  CONDITIONS ALREADY IMPACTING KJAC...KRKS
AND KPNA. RADAR AND SATELLITE INDICATE THESE CONDITIONS PREVAILING
ALONG AND WEST OF THE CONTINENTAL DIVIDE...RAPIDLY SPREADING OVER THE
DIVIDE INTO THE UPPER WIND RIVER AND WESTERN BIGHORN BASINS.
EXPECT ALL AIRPORTS TO BE IMPACTED BY LOWER CE LINGS AND VISIBILITIES
BY 07Z TONIGHT CONTINUING THROUGH 18Z SUNDAY. WIDESPREAD MOUNTAIN
OBSCURATIONS WILL BE ENCOUNTERED ACROSS WESTERN AND CENTRAL
WYOMING.

.FIRE WEATHER...

A GRADUAL RETURN TO WINTER BEGINS LATER TODAY. IN THE MEAN TIME
HOWEVER...IT WILL STILL BE RELATIVELY WARM WITH SOME BREEZY
CONDITIONS FOUND ALONG THE FOOTHILLS EAST OF THE DIVIDE AS WELL AS
FROM SOUTH PASS TO CASPER TODAY. AS MOISTURE SLOWLY INVADES THE
REGION...RH VALUES WILL ALSO INCREASE. SMOKE DISPERSION WILL
GENERALLY BE POOR TO FAIR TODAY. AS A NEW WINTER STORM SYSTEM WORKS
ITS WAY ACROSS THE FORECAST AREA FROM SOUTHWEST TO NORTHEAST TODAY
AND SUNDAY...MEASURABLE SNOW...MUCH LOWER TEMPERATURES AND HIGHER
RELATIVE HUMIDITY WILL OCCUR AREA WIDE THROUGH SUNDAY NIGHT.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
WINTER WEATHER ADVISORY FROM MIDNIGHT TONIGHT TO 5 PM MST SUNDAY
WYZ020-022.

WINTER WEATHER ADVISORY FROM MIDNIGHT TONIGHT TO 3 PM MST SUNDAY
WYZ015-017>019-030.

&&

$$

SHORT TERM...BRAUN
LONG TERM...AEM
AVIATION...C.BAKER
FIRE WEATHER...BRAUN






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