Area Forecast Discussion
Issued by NWS Riverton, WY

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000
FXUS65 KRIW 262144
AFDRIW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RIVERTON WY
244 PM MST FRI DEC 26 2014

.SHORT TERM...TONIGHT THROUGH SATURDAY

AS THE MAIN UPPER LEVEL LOW PRESSURE CENTER NEAR THE FOUR CORNERS
AREA EJECTS EAST TONIGHT...HIGH PRESSURE OVER THE PACIFIC
NORTHWEST WILL RIDGE EASTWARD INTO WESTERN AND CENTRAL WYOMING
WITH A CLEARING TREND.  WITH WIDESPREAD FRESH SNOW COVER AND
STRONG RADIATIONAL COOLING...EXPECT AREAS OF FOG TO DEVELOP IN THE
VALLEYS AND BASINS TONIGHT CONTINUING SATURDAY MORNING.  AS A LEE
SIDE LOW PRESSURE TROUGH DEVELOPS...STRONG WINDS WILL DEVELOP
OVER AND NEAR THE EAST SLOPES OF THE ABSAROKA MOUNTAINS AND CODY
FOOTHILLS...AS WELL AS ACROSS THE WIND CORRIDOR OF SOUTHERN
FREMONT...AND SOUTHERN NATRONA COUNTIES AND EASTERN SWEETWATER
COUNTY. SOME GUSTS AROUND 50 MPH ARE EXPECTED AT THIS TIME.
THE WINDIER AREAS WILL SEE RAPID WARMING WHILE THE TRAPPED VALLEYS
AND BASINS WILL REMAIN COLD AS THE FOG SLOWLY DISSIPATES
AROUND MID DAY. AS AN APPROACHING PACIFIC LOW PRESSURE TROUGH
MOVES EAST...SOME LIGHT SNOW WILL DEVELOP OVER THE WESTERN MOUNTAINS
SATURDAY AFTERNOON.

.LONG TERM...SATURDAY NIGHT THROUGH FRIDAY

THE TRANSIENT RIDGE WILL BE EXITING TO THE EAST SATURDAY NIGHT AS
THE NEXT UPSTREAM TROUGH NOW OVER THE GULF OF ALASKA RIDING UP AND
OVER THE RIDGE OVER THE EASTERN PACIFIC APPROACHES THE PAC NW THUS
INCREASING THE WEST NORTHWEST BACKING DIFLUENT FLOW OVER THE CWA.
THERE WILL STILL BE ABOUT AN 8C SFC DEGREE SPREAD FROM YNP TO CLARK
RESULTING IN SOME GAP FLOW ENHANCED SOMEWHAT BY THE PRESSURE
GRADIENT SO WILL KEEP WINDS GOING UP THERE SATURDAY NIGHT. THIS
WESTERLY DIFLUENT FLOW WILL ALSO AID IN INCREASING SNOWFALL ACROSS
THE FAR WEST SATURDAY NIGHT. ON SUNDAY THE UPPER FLOW WILL SPLIT
WITH THE SOUTHERN STREAM ENERGY BECOMING MORE FOCUSED OVER UTAH.
WITH THE LOW IN THIS POSITION...THE WEST SHOULD RECEIVE SEVERAL
INCHES OF SNOWFALL SUNDAY. DURING THIS TIME MUCH OF THE VORT ENERGY
WILL TRACK SOUTH SOUTHEAST ACROSS IDAHO AND UTAH WHICH...AGAIN...IS
A GOOD TRACK FOR LANDER TO EVENTUALLY RECEIVE ACCUMULATING SNOWFALL.
THE NORTHERN STREAM ENERGY WILL SEND THE MUCH ADVERTISED ARCTIC COLD
FRONT SOUTH ACROSS WYOMING SUNDAY NIGHT. WINDS WILL BECOME FAIRLY
STRONG ACROSS JOHNSON COUNTY EARLY SUNDAY EVENING WITH THE POST
FRONTAL WINDS AS 3 HOURLY SURFACE PRESSURE RISES ARE PROGGED TO BE
9MB THERE. WITH SNOWFALL ACCOMPANYING THIS FRONT...AND THE SNOW
BEING VERY FLUFFY IN NATURE WITH THE COLD AIR THAT WILL ACCOMPANY
THE SNOW...AND THE 30 TO 45 MPH POST FRONTAL WINDS TO
BOOT...CONSIDERABLE BLOWING AND DRIFTING IS ANTICIPATED AT LEAST FOR
A COUPLE OF HOURS IN JOHNSON COUNTY EARLY SUNDAY EVENING ALONG WITH
BITTER WIND CHILLS. AS MENTIONED EARLIER...ONCE THE FRONT SWEEPS
SOUTH ACROSS THE CWA...UPSLOPE SNOWS SHOULD GET UNDERWAY FROM LANDER
TO CASPER WITH THE MAXIMUM SNOWFALL OCCURRING FROM 06Z THROUGH 18Z
MONDAY. INCREASED SNOW AMOUNTS THERE A LITTLE IN THOSE AREAS. BY
MONDAY...MOST OF THE SOUTHERN STREAM ENERGY WILL QUICKLY PROPAGATE
TOWARD VEGAS AS A STRONG 1051MB SFC HIGH PUSHES SOUTH TOWARD
WYOMING. AS A RESULT...THIS STORM SYSTEM WILL HAVE A MUCH SHORTER
RESIDENCE TIME SO SNOW AMOUNTS SHOULD NOT AMOUNT TO MORE THAN
ADVISORY AMOUNTS EAST AND WEST OF THE DIVIDE. TUESDAY MORNING SHOULD
FEATURE WIND CHILLS APPROACHING 25 BELOW IN MANY AREAS WITH ACTUAL
AMBIENT BASIN AND VALLEY TEMPS AS COLD OR A COUPLE OF DEGREES COLDER
THAN THAT BY WEDNESDAY MORNING AS -25C H7 ARCTIC AIR SPILLS IN
BEHIND THE FRONT. FOG IS LIKELY LATE TUESDAY NIGHT AND WEDNESDAY
MORNING WITH THE FRESH SNOW COVER AND THE WARM AIR ADVECTION ALOFT
FROM THE NORTHWEST ENHANCING THE TEMPERATURE INVERSIONS. TEMPS WILL
RECOVER SOMEWHAT IN THE BASINS ON THU/FRI WITH TEMPS STRUGGLING TO
GET ABOVE 20 BUT A LITTLE MORE WARMING WILL TAKE PLACE IN THE AREAS
THAT MIX OUT. THE GFS AND THE EURO ARE STILL DISAGREEING ON WHAT TO
DO WITH THE NEXT POTENTIAL ARCTIC OUTBREAK LATE NEXT WEEK WITH THE
EURO SPILLING THE COLD AIR MUCH FARTHER SOUTH THAN THE GFS SO
LOWERED TEMPS ONLY BY A FEW DEGREES NEXT FRIDAY NIGHT. KEPT IDEA OF
INTRODUCING CHC OF SNOW TO THE FAR WEST LATE IN THE WEEK FROM THE
SYSTEM THAT PULLS DOWN THIS NEXT COLD AIR MASS.

&&

.AVIATION.../00Z ISSUANCE/

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

LCL MVFR CONDITIONS DUE TO CIGS ARE EXPECTED THROUGH 03Z WITH
FLURRIES AND WIDELY SCT SNOW SHOWERS THIS EVENING.  AREAS OF IFR
CEILINGS AND VISIBILITIES REDUCED IN FOG WILL DEVELOP FROM 09Z TO
18Z.

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

AREAS OF LOW CLOUDS WILL CONTINUE THROUGH 03Z WITH AREAS OF CIGS
AROUND 030-040 CREATING AREAS OF MTN OBSCURATIONS. AREAS OF IFR
CEILINGS AND VISIBILITIES REDUCED IN FOG WILL DEVELOP FROM 09Z TO
18Z.

&&

.FIRE WEATHER...

COLD WINTER WEATHER WITH WIDESPREAD SNOW COVER WILL CONTINUE ACROSS
WESTERN AND CENTRAL WYOMING THROUGH THE WEEKEND.  LOW MIXING HEIGHTS
WILL COMBINE WITH GENERALLY LIGHT WINDS BELOW INVERSIONS FOR POOR
SMOKE DISPERSAL.

&&

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

$$

SHORT TERM...C.BAKER
LONG TERM...LIPSON
AVIATION...C.BAKER
FIRE WEATHER...C.BAKER









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