Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 241756
AFDRIW

AREA FORECAST DISCUSSION...UPDATE
NATIONAL WEATHER SERVICE RIVERTON WY
1056 AM MST WED DEC 24 2014

.SHORT TERM...WEDNESDAY THROUGH FRIDAY NIGHT

IN 1942 BING CROSBY SANG I`M DREAMING OF A WHITE CHRISTMAS. AND...AT
THIS POINT IT LOOKS LIKE WE WILL HAVE A VERY GOOD CHANCE OF THAT
OCCURRING. HOWEVER...AS FOR TODAY...MOST AREAS...ESPECIALLY EAST OF
THE DIVIDE WILL HAVE MAINLY DRY CONDITIONS WITH HIGH TEMPERATURES
NEAR TO SOMEWHAT ABOVE SEASONAL NORMALS. THE EXCEPTION WILL BE
ACROSS THE WEST...WHERE SOME WEAK ISENTROPIC LIFT MAY BRING SOME
SNOW SHOWERS LATER IN THE DAY. WITH THE SNOW EXPECTED TO BREAK OUT
IN THE AFTERNOON AND WITH A LOT OF HOLIDAY TRAVEL TODAY...WE WILL
HOIST A WINTER WEATHER ADVISORY FOR MOST AREAS WEST OF THE DIVIDE
STARTING AROUND 3 PM. HOWEVER...THE HEAVIEST SNOW WILL WAIT UNTIL
TONIGHT AND TOMORROW. EAST OF THE DIVIDE...WITH THE PRESSURE
GRADIENT TIGHTENING THERE WILL BE SOME GUSTY WINDS IN SOME OF THE
FAVORED AREAS LIKE NATRONA COUNTY. RIGHT NOW...IT DOES NOT LOOK LIKE
HIGH WIND BUT WE WILL ISSUE A SPECIAL WEATHER STATEMENT FOR GUSTS TO
55 MPH NEAR OUTER DRIVE.

SNOW WILL INCREASE TONIGHT ACROSS THE WEST AS THE COLD FRONT DROPS
SOUTHWARD. THIS LOOKS TO BE ENHANCED WITH SOME JET DYNAMICS WITH THE
RIGHT REAR QUAD OF A JET MOVING ACROSS MONTANA. WITH THE BEST
DYNAMICS ACROSS THE NORTH...THIS IS WHERE WE EXPECT THE BEST
SNOWFALL ACROSS THE WESTERN MOUNTAINS WITH LESSER AMOUNTS FURTHER TO
THE SOUTH. MEANWHILE...SNOW WILL BEGIN BREAKING OUT EAST OF THE
DIVIDE AFTER MIDNIGHT. THE MODELS SHOW SOME LEE CYCLOGENESIS
OCCURRING ACROSS NORTHEASTERN COLORADO WHICH IS FAVORABLE FOR MUCH
OF THE AREA EAST OF THE DIVIDE. THE MODELS ARE IN FAIRLY GOOD
AGREEMENT WITH WINDS STARTING TO TURN MORE NORTHEASTERLY FOLLOWING
THE FRONTAL PASSAGE FROM NORTH TO SOUTH WITH SNOW FOLLOWING THAT
PATH AS WELL. BOTH THE GFS AND NAM SHOW FAVORABLE NORTHEAST UPSLOPE
FLOW THROUGH MUCH OF CHRISTMAS DAY. SO AS A RESULT WE HAVE THE
HIGHEST SNOW AMOUNTS ACROSS THE FAVORED AREAS FROM THE EAST SLOPES
OF THE WIND RIVER RANGE THROUGH NATRONA COUNTY.

THE UPPER LEVEL LOW WILL THEN SLOW DOWN NEAR THE FOUR CORNERS LATER
IN THE DAY AND AT NIGHT. THE BEST MOISTURE AND UPPER LEVEL SUPPORT
SHOULD BE GONE BY THEN. HOWEVER...LIGHT SNOW IS EXPECTED TO CONTINUE
THROUGH MUCH OF THE NIGHT. MOST OF THE SNOW SHOULD END BY FRIDAY IF
THE UPPER LEVEL LOW MOVES OUT. HOWEVER...IT LOOKS TO BE VERTICALLY
STACKED. AND TO PARAPHRASE THE BEASTIE BOYS...SLOW MOVES THE UPPER
LEVEL LOW THAT IS THE TEMPO. SO...SOME FLURRIES MAY LINGER INTO
FRIDAY BUT WITH THE UNCERTAINTY WE MADE FEW CHANGES. THE NEXT
QUESTION IS HOW COLD WILL IT GET FRIDAY NIGHT. THE LONG RANGE
GUIDANCE IS QUITE COLD. THINGS IN FAVOR OF IT INCLUDE LIGHT
WIND...FRESH SNOW AND THE LONGEST NIGHTS OF THE YEAR.
HOWEVER...THERE IS STILL SOME QUESTION AS TO WHETHER IT WILL CLEAR
OR NOT. FOR NOW...WE LOWERED TEMPERATURES A FEW DEGREES BUT MAY HAVE
TO BE LOWERED MORE IF THINGS CLEAR OUT.

.LONG TERM...SATURDAY THROUGH WEDNESDAY

SATURDAY LOOKS DRY FOR MOST OF THE REGION. MOISTURE MOVES INTO NW
WY SATURDAY AFTERNOON AND LIGHT SNOW WILL DEVELOP IN THE HIGHER
ELEVATIONS. SNOW INCREASES SATURDAY NIGHT OVER THE WEST AS THE
NEXT WEATHER SYSTEM TRACKS INTO THE CENTRAL ROCKIES AND GREAT
BASIN. SUNDAY WILL BE COLD WITH LIGHT SNOW SPREADING ACROSS THE
LOWER ELEVATIONS AND CONTINUING INTO SUNDAY NIGHT. THE ARCTIC
AIRMASS WILL BE PUSHING SOUTH FROM MONTANA SUNDAY NIGHT INTO
MONDAY. GFS/ECMWF ARE SIMILAR ON THE 00Z RUNS AGAIN WITH THIS
BITTER COLD AIR. SOME FLURRIES OR FLUFFY SNOW WILL OCCUR
MONDAY...DECREASING IN THE AFTERNOON. HIGH TEMPS ON MONDAY MAY
VERY WELL OCCUR IN THE MORNING HOURS OVER THE NORTH AND
CENTRAL...THEN SLOWLY FALL IN THE AFTERNOON. WEST OF THE DIVIDE
HIGH TEMPS WILL OCCUR AT EARLY AFTERNOON WITH RAPID COOLING TOWARD
SUNSET AS THE COLDER AIR POURS INTO THESE AREAS. VERY COLD MONDAY
NIGHT WITH CLOUDS DECREASING LATE. CLOUD COVER IS CERTAIN TO
INFLUENCE THE LOW TEMPS MON NIGHT. TUESDAY LOOKS DRY AND VERY COLD
WITH THE COLDEST 700MB TEMPS OVER THE REGION. I HAVE LOWERED HIGHS
A BIT MORE FOR TUE...WORSE CASE SCENARIO IS THAT SOME PLACES
BARELY MAKE ZERO FOR A HIGH. STAY TUNED. VERY COLD TUESDAY NIGHT
IN THE VALLEYS AND BASINS WHERE INVERSIONS WILL DEVELOP. GFS IS
HINTING AT SOME WARMING OF 700MB TEMPS TUE NIGHT WHICH WOULD ALLOW
FOR THE HIGHER ELEVATIONS TO MODERATE AND POSSIBLY SOME FOOTHILL
LOCATIONS LATER IN THE NIGHT. THE FIRST OUTLOOK FOR NEW YEARS EVE
DAY IS FOR DRY CONDITIONS THROUGH THE LAST HOUR OF 2014. SOME
MODERATION IN HIGH TEMPS. HOWEVER SOME BASINS AND VALLEYS WILL
STRUGGLE TO WARM DUE TO THE INVERSION AND NO MIXING. HAVE LOWERED
THE HIGHS IN THESE LOCATIONS.

&&

.AVIATION.../18Z ISSUANCE/

A TROUGH OF LOW PRESSURE WILL MOVE ONTO THE WEST COAST
TODAY AND DIG INTO THE GREAT BASIN WEDNESDAY NIGHT.

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

A LEE SIDE SURFACE TROUGH WILL CONTINUE TO INTENSIFY TODAY
WITH CONTINUED SW SFC WINDS...25-35KTS WITH GUSTS 40-45KTS FROM
VCNTY 50NE KRKS-KCPR.  GUSTY W-SW WINDS ALONG THE EAST SLOPES OF THE
DIVIDE WILL ALSO RESULT IN SOME LLWS FROM VCNTY KCOD-KLND.

A COLD FRONT WILL PUSH SOUTH INTO NORTHERN WYOMING 3-6Z THURSDAY AND
TO A VCNTY  KLND-KCPR LINE 10Z-12Z THURSDAY.  WIDESPREAD MVFR/IFR
CIGS AND VSBYS IN -SN...HIGHER TERRAIN OBSCD...WILL SPREAD ACROSS
THE AREA WITH THE FRONTAL PASSAGE.

WEST OF THE DIVIDE...KBPI/KJAC/KPNA/KRKS ROUTES

MVFR CIGS WILL CONTINUE TODAY. VCNTY KJAC-KAFO...-SN OVR THE MTNS
BECOMING INCREASINGLY OBSCURED.

MORE WIDESPREAD IFR SN WILL SPREAD NW-SE ACROSS THE WEST AFTER 21Z
THIS AFTERNOON...SPREADING SOUTH OF A KEVW-KBPI LINE MAINLY AFTER
06Z THURSDAY.

&&

.FIRE WEATHER...

GUSTY WINDS WILL DEVELOP IN NATRONA COUNTY AS A COLD FRONT
APPROACHES WYOMING. SNOW WILL DEVELOP ACROSS THE WEST THIS
AFTERNOON AND SPREAD EAST OF THE DIVIDE LATER TONIGHT AND
CONTINUE THROUGH CHRISTMAS DAY. MUCH COLDER WEATHER WILL FOLLOW
ON FRIDAY AND INTO THE WEEKEND.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
WINTER STORM WATCH FROM LATE TONIGHT THROUGH THURSDAY EVENING
WYZ015-017>020-022-028>030.

WINTER WEATHER ADVISORY FROM 3 PM THIS AFTERNOON TO NOON MST
THURSDAY WYZ001-002-012>014-023-024.

WINTER WEATHER ADVISORY FROM 8 PM THIS EVENING TO 5 PM MST
THURSDAY WYZ025-026.

WINTER STORM WATCH FROM LATE TONIGHT THROUGH THURSDAY EVENING
WYZ027.

WINTER STORM WATCH FROM LATE TONIGHT THROUGH THURSDAY EVENING
WYZ005-007.

&&

$$

SHORT TERM...HATTINGS
LONG TERM...AR
AVIATION...AEM/LIPSON
FIRE WEATHER...HATTINGS









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