Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 242207
AFDRIW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RIVERTON WY
307 PM MST WED DEC 24 2014

.SHORT TERM...TONIGHT THROUGH FRIDAY NIGHT

THE LONG AWAITED CHRISTMAS SNOW STORM IS NIPPING AT OUR HEELS WITH
SNOW ALREADY FALLING IN CENTRAL IDAHO AS OF EARLY WEDNESDAY
AFTERNOON. THIS SWATH OF INITIAL SNOW WILL OVERSPREAD THE WESTERN
MOUNTAINS BY MIDNIGHT TONIGHT WITH SOME SNOW BEGINNING TO WRAP
AROUND INTO NORTHERN WYOMING SHORTLY THEREAFTER. THERE WILL BE A
GOOD BURST OF SNOW WITH THE FRONT AS IT DROPS SOUTH INTO THE
REGION OVERNIGHT...THROUGH THE BIGHORN BASIN EARLY IN THE MORNING
AND THEN THROUGH THE CENTRAL BASINS AROUND SUNRISE CHRISTMAS
MORNING. THERE WILL BE SEVERAL HOURS OF STEADY SNOW BEHIND THE
FRONT AS REINFORCEMENTS ARRIVE AS THE UPPER WAVE ARRIVES ALONG
WITH A GOOD SWATH OF STRONG QG FORCING. LAYER TEMPERATURES ALL THE
WAY THROUGH THE ATMOSPHERE WILL BE FAVORABLE FOR DENDRITIC GROWTH
AS THE FRONT PASSES AS WELL AS SEVERAL HOURS AFTERWARD. HOWEVER
THE ATMOSPHERE BECOMES COOLER AND DRYER AFTER 21Z THURSDAY WHICH
WILL INCREASE SNOW RATIOS BUT IT WILL END THE FAVORABLE HEAVY SNOW
ENVIRONMENT. WINDS WILL BE FAIRLY STEADY WITH THE FRONT AT 15 TO
25 MPH IN MOST AREAS WITH SOME GUSTY SPOTS...SPECIFICALLY AROUND
COWLEY AND BUFFALO AS WELL AS THROUGH WIND RIVER CANYON EARLY IN
THE MORNING. THE GUSTY WIND WILL THEN SHIFT TO SOUTH PASS AND INTO
EASTERN SWEETWATER COUNTY...LIKELY IMPACTING TRAVEL ALONG
INTERSTATE 80 EAST OF ROCK SPRINGS THROUGH THE DAY. IN
ADDITION...THERE WILL BE SIGNIFICANT BLOWING SNOW THROUGH JOHNSON
AND NATRONA COUNTIES IN THE AFTERNOON AS NORTH WINDS BECOME STEADY
AT AROUND 25MPH.

THE SNOW WILL TAPER OFF CHRISTMAS NIGHT INTO FRIDAY WITH SNOW
CLEARING FROM NORTH TO SOUTH. WINDS WILL BE RELATIVELY LIGHT ON
FRIDAY WITH SKIES REMAINING PARTLY TO MOSTLY CLOUDY AS ANOTHER
WEAK DISTURBANCE SWINGS SOUTH. IT WILL NOT RESULT IN MUCH
ADDITIONAL SNOWFALL BUT THERE COULD BE SOME FLURRIES IN THE
MOUNTAINS WITH THIS WAVE. IT WILL HOWEVER KEEP THE TEMPERATURES
FROM BOTTOMING OUT BEHIND THE EXITING SYSTEM FRIDAY NIGHT.

.LONG TERM...SATURDAY THROUGH THURSDAY
BY 00Z FRIDAY...THE SNOWFALL WILL SLOWLY BEGIN TO END FROM NORTH TO
SOUTH AND DECREASE IN INTENSITY SOMEWHAT...EXCEPT IN THE UPSLOPE
FAVORED REGIONS EAST OF THE DIVIDE. THE MID LEVEL CIRCULATION WILL
STILL BE OVER SOUTHWEST WY AT 00Z FRIDAY ALONG WITH LINGERING
ISENTROPIC LIFT FROM NORTH TO SOUTH AS WELL AS LINGERING SUFFICIENT
QG FORCING. WE WILL BEGIN TO LOSE THESE DYNAMICS AFTER FEW HOURS
AFTER SUNSET AS THE SYSTEM PULLS AWAY TO THE SOUTHEAST. HOWEVER...AS
THE 700MB LEVEL LOW MOVES OFF TO OUR SOUTH OVER COLORADO...THE WINDS
WILL BACK TO THE NORTHEAST AT 700MB AND INCREASE FOR ENHANCED
UPSLOPE FLOW IN AREAS LIKE LANDER...CASPER AND THERMOPOLIS. AFTER
06Z...THE SNOWFALL SHOULD DECREASE EVERYWHERE. ON FRIDAY...EXCEPT
FOR SOME LINGERING ISOLATED MTNS FLURRIES...EXPECT DRY WEATHER. DRY
WEATHER IS EXPECTED FRIDAY NIGHT. ON SATURDAY...LIGHT SNOW WILL ONCE
AGAIN DEVELOP IN FAR WEST WYOMING AS A VORT LOBE IN THE NORTHWEST
FLOW ON THE BACKSIDE OF THE MAIN EXITING TROUGH TRACKS ACROSS THE
AREA AND THE INCREASINGLY DIFLUENT NW FLOW INCREASES THE UPSLOPE
POTENTIAL. THIS UPSLOPE FLOW WILL INCREASE SATURDAY NIGHT WITH SNOW
INCREASING IN THE WEST AS THE TROUGH...NOW JUST EAST OF THE
ALEUTIANS...DIVES SOUTHEAST AND DEEPENS OVER THE PACIFIC NORTHWEST.
BY SUNDAY MORNING...THE TROUGH WILL BEGIN TO CLOSE OFF ALONG THE
WASHINGTON OREGON BORDER AND SNOWFALL WILL BECOME MORE WIDESPREAD
AND INTENSIFY OVER THE WEST. BY SUNDAY AFTERNOON...SNOWFALL WILL
BEGIN TO SPILL OVER EAST OF THE DIVIDE WITH THE H5 AND THE H7 LOW
OVER IDAHO PER GFS SOLN. THEN THE ENERGY WITH THE ASCT TROUGH WILL
SPLIT WITH THE IDAHO LOW TRACKING DOWN TO VEGAS BY 12Z TUESDAY. BY
THIS TIME THE TROUGH AXIS WILL BE OVERHEAD. THE NORTHERN STREAM
ENERGY WILL PULL DOWN THE COLDEST ARCTIC AIR OF THE SEASON INTO
WYOMING TUESDAY. BITTER COLD ADVECTION WILL ACCOMPANY THE FRONT
MON/TUESDAY. IN THE MEANTIME...CHC POPS ARE IN THE FORECAST FOR MOST
AREAS WITH THE APPROACHING TROUGH FOR SUNDAY NIGHT WITH LIKELY POPS
IN THE WEST AND BIG HORN MTNS. SNOW WILL DECREASE MONDAY AS THE
ENERGY SPLITS. AS ALLUDED TO ABOVE...TUE WILL FEATURE STEADY OR
FALLING TEMPERATURES AFTER A FRIGID MORNING WITH VERY COLD WIND
CHILLS AS THE POST FRONTAL AIR WORKS ITS WAY INTO THE NORTH AS A
1048MB SFC HIGH DRIVES THE FRIGID AIR SOUTH. TEMPS WILL CONTINUE TO
DROP TUESDAY NIGHT AS SKIES CLEAR AND WINDS DIMINISH. WARM AIR
ADVECTION ALOFT IS EXPECTED FROM THE NW WEDNESDAY WHICH WILL ONLY
STRENGTHEN THE COLD AIR INVERSIONS IN THE BASINS AND VALLEYS FOR
WEDNESDAY MORNING WITH SOME AREAS EAST AND WEST OF THE DIVIDE SEEING
TEMPS AS LOW AS 30 BELOW OR A LITTLE COLDER. THE EURO...ON THE OTHER
HAND...UNLIKE YESTERDAYS RUN...IS DIVERGING AWAY FROM THE GFS SOLN
WHICH IS TO BRING A SHORTWAVE TROUGH THROUGH SUNDAY AND ANOTHER ONE
TUESDAY. AM IGNORING THIS SOLN ATTM BUT...IF NOTHING ELSE...IS
ANOTHER REASON TO SAVE THE COLDEST AIR FOR WEDNESDAY MORNING IN THE
FORECAST AFTER THE COLDEST WIND CHILLS OCCUR TUESDAY MORNING. THE
GFS IS ADVERTISING H7 TEMPS OF -28C WITH THE EURO SHOWING -32C
MOVING INTO WY FROM THE NORTH AT H7 ON TUE.

&&

.AVIATION.../00Z ISSUANCE/

A TROUGH OF LOW PRESSURE WILL MOVE ONTO THE GREAT BASIN TONIGHT.

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

A LEE SIDE SURFACE TROUGH WILL CONTINUE TO COINCIDE WITH SOUTHWEST
WINDS OF 25 TO 35 KTS MPH WITH GUSTS OF 40 TO 45 KTS THROUGH 02Z
THIS EVENING 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 DURING THIS TIME FRAME.

A COLD FRONT WILL PUSH SOUTH INTO NORTHERN WYOMING 3-6Z TONIGHT 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

VCNTY KJAC- KAFO...-SN EXPECTED OVR THE MTNS BECOMING
INCREASINGLY OBSCURED THROUGH 01Z THIS EVENING AS THE SNOWFALL
MAKES ITS WAY ONTO THE VALLEY FLOORS DURING THIS TIME FRAME.

MORE WIDESPREAD IFR SN WILL SPREAD NW-SE ACROSS THE WEST AFTER 01Z
THIS EVENING...SPREADING SOUTH OF A KEVW-KBPI LINE MAINLY AFTER
06Z THURSDAY.

&&

.FIRE WEATHER...

WIDESPREAD SNOW AND GUSTY WIND THROUGH EARLY FRIDAY MORNING. BASINS
AND VALLEYS WILL QUICKLY BECOME TRAPPED ON FRIDAY NIGHT WITH VERY
POOR SMOKE DISPERSION. ON SATURDAY...MILDER AIR MOVES IN ALONG WITH
SOME STRONGER WINDS THAT WILL ENCOURAGE SOME MIXING. ANOTHER SNOW
STORM IS POSSIBLE ON SUNDAY INTO MONDAY.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
WINTER STORM WARNING FROM 5 AM TO 11 PM MST THURSDAY
WYZ015-017>020-022-027>030.

WINTER WEATHER ADVISORY UNTIL 8 PM MST THURSDAY
WYZ001-002-012>014-023-024.

WINTER WEATHER ADVISORY FROM MIDNIGHT TONIGHT TO 8 PM MST
THURSDAY WYZ003-004-006-008>011-016.

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

WINTER STORM WARNING FROM MIDNIGHT TONIGHT TO 8 PM MST THURSDAY
WYZ005-007.

&&

$$

SHORT TERM...ALLEN
LONG TERM...LIPSON
AVIATION...LIPSON
FIRE WEATHER...ALLEN






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