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000
FXUS65 KRIW 010531
AFDRIW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RIVERTON WY
1031 PM MST SAT NOV 30 2013

.SHORT TERM...TONIGHT THROUGH SUNDAY (ISSUED AT 405 PM MST)

CLOUDS WILL LOWER AND THICKEN ACROSS WESTERN WY THIS EVENING WITH
CLOUDS SPREADING ACROSS THE REST OF THE STATE LATER TONIGHT.
SNOWFALL WILL DEVELOP FIRST IN THE WESTERN MOUNTAINS THIS EVENING
EVENTUALLY REACHING THE VALLEY FLOOR BY 9 PM. THE SNOWFALL WILL
BECOME MORE WIDESPREAD ACROSS WESTERN WYOMING TOWARD MORNING AS A
LEAD SHORTWAVE TRACKS ACROSS THE REGION 12Z SUNDAY. THE LONG
ADVERTISED MAIN TROUGH...NOW OFF THE PACIFIC COAST AT 140W...WILL BE
ALONG 130W JUST OFF THE BC COAST BY LATE SUNDAY. AS THIS TROUGH
APPROACHES...THE UPPER FLOW WILL BACK AND INCREASING IN SPEED AS
WELL AS BECOME INCREASINGLY DIFLUENT INCREASING THE SNOWFALL
INTENSITY IN THE WEST SUNDAY. ISENTROPIC LIFT WILL ALSO GET UNDERWAY
AS THIS STRONG WEATHER SYSTEM APPROACHES AIDING SNOWFALL. 4 TO 5
INCHES OF SNOWFALL IN THE WESTERN MOUNTAINS ALONG WITH 2 TO 3 INCHES
ARE  EXPECTED IN THE WESTERN VALLEYS BY LATE SUNDAY AFTERNOON.
SNOWFALL AND WIND WILL INCREASE AFTER 00Z MONDAY.

IN THE MEANTIME...THE PRESSURE GRADIENT WILL BEGIN TO INCREASE AHEAD
OF THIS APPROACHING WEATHER SYSTEM WITH A 12MB SURFACE GRADIENT
EXPECTED BETWEEN GCC AND RKS EXPECTED SUNDAY KICKING THE SW WINDS UP
FROM EAST OF RKS TO CPR AS WELL AS ALONG THE EAST SLOPE OF THE
CONTINENTAL DIVIDE AND ADJACENT FOOTHILLS. TEMPS WILL BE RELATIVELY
MILD.

SOME SNOWFALL WILL DEVELOP LATER IN THE AFTERNOON SUNDAY OVER THE
BIG HORN MTNS WITH PRECIP JUST BEGINNING TO SPREAD SOUTH INTO THE
FAR NORTHERN REACHES OF THE BIG HORN BASIN.

.LONG TERM...SUNDAY NIGHT THROUGH SATURDAY

MAJOR CHANGE TO SNOWY AND BITTER COLD CONDITIONS WILL PREVAIL
THROUGH MUCH OF THIS PERIOD.

SNOW WILL BE ONGOING OUT WEST WITH ISENTROPIC UPGLIDE AND INCREASING
WIND SPEEDS. A PERIOD OF VERY HEAVY SNOW AND WIND WILL LIKELY OCCUR
MONDAY AFTERNOON AND EVENING WITH COLD FRONTAL PASSAGE. AS
TEMPERATURES DROP...THERE WILL BE MORE AND MORE BLOWING AND DRIFTING
SNOW...ESPECIALLY AT PASS AND PEAK LEVEL BUT EVEN THE VALLEYS. THE
DURATION OF THE EVENT IS UNCERTAIN ...ESPECIALLY FOR THE NWRN
MOUNTAINS AS THE NEXT LOW REDEVELOPS OVER IDAHO TUESDAY INTO
WEDNESDAY. THE DEVELOPING SW FLOW FAVORS THE SW MTNS MORE AND THE
MODERATE TO HEAVY SNOW WILL PROBABLY SHIFT SOUTH. OPTED TO KEEP THE
HIGHLIGHTS THROUGH TUESDAY NIGHT FOR THE ENTIRE FAR WEST BUT WE MAY
NEED TO SHORTEN THE NWRN ZONES WHEN WE UPGRADE OR EVENTUALLY JUST
CANCEL THEM EARLY. ADDED IN THE WEST SIDE OF THE WINDS FOR THE SAME
TIMEFRAME. THE BEST PERIOD FOR THE PINEDALE ZONE AND ADJACENT
MOUNTAINS WILL NOT START UNTIL SUNDAY NIGHT AND THEN PEAK MONDAY
NIGHT. THE DURATION OF THIS EVENT...THE AMOUNT OF WIND...FALLING
TEMPERATURES AND DYNAMIC SUPPORT MAKES THIS ONE STORM TO TAKE VERY
SERIOUSLY. SNOWFALL AMOUNTS IN THE WEST COULD REACH 8 TO 16 INCHES
IN THE WESTERN VALLEYS WITH 2 TO 3 FEET IN THE MOUNTAINS.

THE ARCTIC COLD FRONT THEN BECOMES THE MAJOR CHANGE FOR THE NORTH
MONDAY NIGHT AND THEN ALL AREAS TUESDAY THROUGH THE PERIOD.
THE ARCTIC FRONT WILL MOVE INTO THE NORTH MONDAY NIGHT WITH FALLING
TEMPERATURES AND GENERALLY LIGHT SNOW AND AREAS OF STRONG NORTH
WIND. THE BITTER COLD AND STRONG NORTH WIND COMBINED WITH AT LEAST
LIGHT SNOW WILL MAKE TUESDAY A POTENTIALLY DANGEROUS DAY TO BE
OUTSIDE OR TRAVELLING. EVEN THOUGH SNOWFALL AMOUNTS ACROSS THE LOWER
ELEVATIONS LOOK TO BE RATHER LIGHT INITIALLY THEY COULD INCREASE IN
THE SOUTHERN AND CENTRAL AREAS LATER TUESDAY THROUGH WEDNESDAY. THE
MODELS HAVE TRENDED STRONGLY BACK TOWARDS THE SECOND SHORTWAVE BEING
THE MAIN ONE OVER IDAHO TUESDAY INTO WEDNESDAY. THIS TREND WILL
PRODUCE A LONGER POTENTIAL OF SNOWFALL IN THE CENTRAL AND SWRN ZONES
THAT COULD LAST INTO WEDNESDAY NIGHT WITH FLURRIES INTO THURSDAY AND
BEYOND AT TIMES. SIGNIFICANT SNOWFALL IS STILL A POSSIBILITY IN THE
CENTRAL AND SWRN ZONES AS STRONG JET STREAK DYNAMICS...QG FORCING
AND ISENTROPIC UPGLIDE OVERRUNS THE BITTER LOW LEVEL AIRMASS IN
PLACE. WINTER STORM WATCHES MAY BE NEEDED AT SOME POINT TUESDAY OR
TUESDAY NIGHT THROUGH WEDNESDAY OR WEDNESDAY NIGHT FROM FREMONT AND
NATRONA COUNTIES SOUTH AND SWWD. THE ECMWF IS MOST BULLISH WITH THIS
SCENARIO BUT THE GFS IS TRENDING THAT WAY. THE STRENGTH OF THE
DYNAMICS CAN EASILY OVERCOME THE COLD LOW LEVELS TO PRODUCE MODERATE
TO HEAVY SNOW IF THEY ARE AS STRONG AS EXPECTED.

THE OTHER CONCERN IS THE POTENTIALLY STRONG TO HIGH WINDS FROM THE
CODY FOOTHILLS SWD SUNDAY NIGHT THROUGH MONDAY. ISSUED HIGH WIND
WATCHES FOR THE CODY FOOTHILLS LATE SUNDAY NIGHT THROUGH MONDAY
MORNING DUE TO POTENTIAL MOUNTAIN WAVE ACTION AS THE RIGHT FRONT
QUAD OF THE JET SAGS SWD. FURTHER SOUTH...HIGH WINDS ARE QUITE
POSSIBLE IN THE WIND CORRIDOR LATER SUNDAY NIGHT INTO MONDAY AS MID
LEVEL WINDS BACK AND BECOME SW AT 55-70KTS AND THE RIGHT FRONT QUAD
OF THE JET SAGS SWD. IN ADDITION...RAPID SFC CYCLOGENESIS IS
EXPECTED NE OF CASPER WHICH IS OFTEN A HARBINGER OF HIGH WINDS IN
CASPER. WINDS PEAK DURING THE DAY MONDAY AHEAD OF THE ARCTIC COLD
FRONT.

A BITTER COLD AIRMASS WILL REMAIN ENTRENCHED THROUGH NEXT WEEKEND
WITH H7 TEMPERATURES GENERALLY -20 TO -25C LEADING TO SUB-ZERO HIGHS
IN MANY MOUNTAIN LOCATIONS AND SINGLE DIGITS ACROSS MANY LOWER
ELEVATIONS. SOME BASINS WILL STRUGGLE TO EVEN GET TO ZERO THURSDAY
AND FRIDAY. THE WEEKEND HAS SOME LARGE MODEL VARIANCES WITH THE
ECMWF SETTING UP ANOTHER SNOWSTORM WITH BITTER ARCTIC AIR WHILE THE
GFS SHOWS A FLAT WLY FLOW AND SOME MOUNTAIN SNOW OUT WEST. LEAVING
THAT PERIOD UNTOUCHED FOR NOW WITH SLIGHT CHANCE VALLEY/BASIN POPS
AND A CHANCE OF SNOW MOUNTAINS.

&&

.AVIATION.../06Z ISSUANCE/

EAST OF THE CONTINENTAL DIVIDE...

VFR CONDITIONS WILL OCCUR AT THE TERMINAL SITES THROUGH 06Z MONDAY.
SNOW WILL INCREASE OVER THE ABSAROKA AND WIND RIVER MOUNTAINS
OVERNIGHT AND CONTINUE THROUGH 06Z MONDAY. EXPECT MVFR TO IFR
CONDITIONS IN THESE MOUNTAINS. WIND WILL INCREASE AT THE KCPR
BY 15Z AND CONTINUE DURING MUCH OF THE PERIOD.

WEST OF THE CONTINENTAL DIVIDE...

SNOW WILL OCCUR OVER MOST OF THE WESTERN MOUNTAINS OVERNIGHT THROUGH
06Z MONDAY. MVFR TO IFR CONDITIONS WILL PREVAIL WITH FREQUENT
MOUNTAIN OBSCURATION. THE KJAC TERMINAL SITE WILL BE IMPACTED BY
MVFR AND IFR CONDITIONS AS WELL. AT KRKS TERMINAL SITE WIND WILL BE
THE MAIN IMPACT AFTER 16Z WITH WIND SPEEDS INCREASING AFTER 20Z. VFR
CONDITIONS ARE FORECAST TO PREVAIL AT KRKS TERMINAL SITE.

&&

.FIRE WEATHER...

MILD AND DRY CONDITIONS ARE EXPECTED EAST OF THE DIVIDE THROUGH
MONDAY. VERY STRONG WEST SOUTHWEST WINDS ARE POSSIBLE FROM ROCK
SPRINGS TO CASPER AS WELL AS ALONG THE EAST SLOPE OF THE CONTINENTAL
DIVIDE AND ADJACENT FOOTHILLS MONDAY. 10 TO 15 INCHES OF SNOWFALL
ALONG WITH 1 AND A HALF TO TWO AND A HALF FEET OF SNOWFALL ARE
EXPECTED IN THE WESTERN MOUNTAINS SUNDAY THROUGH TUESDAY NIGHT.
SIGNIFICANT SNOWFALL WILL LIKELY SPREAD TO AREAS EAST OF THE DIVIDE
MONDAY NIGHT INTO WEDNESDAY. BITTERLY COLD WEATHER WILL FILTER INTO
THE AREA MONDAY NIGHT THROUGH TUESDAY NIGHT. BY THE END OF THE
WEEK...HIGHS WILL RANGE FROM 10 BELOW TO 10 ABOVE ZERO ALONG WITH
LOW TEMPERATURES OF 15 TO 25 BELOW ZERO.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...

HIGH WIND WATCH FROM SUNDAY EVENING THROUGH MONDAY AFTERNOON
WYZ019-030.

WINTER STORM WATCH FROM LATE TONIGHT THROUGH LATE TUESDAY NIGHT
WYZ001-002-012>014-023>025.

HIGH WIND WATCH FROM SUNDAY EVENING THROUGH MONDAY MORNING
WYZ003.

HIGH WIND WATCH FROM LATE SUNDAY NIGHT THROUGH MONDAY AFTERNOON
WYZ020.

&&

$$

SHORT TERM...LIPSON
LONG TERM...SKRBAC
AVIATION...AR
FIRE WEATHER...LIPSON