Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 220640
AFDRIW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RIVERTON WY
1140 PM MST SUN DEC 21 2014

.UPDATE...

CANCELLED REMAINING HIGH WIND WARNINGS FOR SOUTHERN
WYOMING...SO NO REMAINING WINTER STORM OR HIGH WIND WARNINGS ARE IN
EFFECT ACROSS THE CWA.

AVIATION.../06Z ISSUANCE/

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

AREAS OF GUSTY NORTHWEST WIND WILL REMAIN...ESPECIALLY VCNTY
KDUB-KRIW AND KBYG...THROUGH THE PERIOD.  -SHSN ACTIVITY WILL REMAIN
MOSTLY ISOLD-WDLY SCT WITH MOST OF THIS ACTIVITY VCNTY AND EAST OF A
KCOD-KWRL-KCPR LINE THROUGH MONDAY MORNING WITH MORE NMRS SHSN
OBSCURING THE SURROUNDING MTNS.  MOISTURE AND INSTABILITY WILL
INCREASE MONDAY AFTERNOON WITH SHSN BECOMING MORE SCT ACROSS THE
CENTRAL BASINS.  SOME WRAPAROUND MOISTURE IS EXPECTED TO PUSH BACK
FROM AN UPPER LOW OVER THE PLAINS INTO EASTERN WYOMING MONDAY
NIGHT...AND THIS MAY BRING MORE WIDESPREAD MVFR/IFR -SN AS FAR WEST
AS THE BIGHORN RANGE TO VCNTY KCPR.

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

A MOIST AND UNSTABLE NW FLOW WILL REMAIN OVER THE AREA THROUGH
MONDAY AFTERNOON.  AN UPPER LEVEL DISTURBANCE IN THIS FLOW IS
EXPECTED TO DIVE SOUTH ALONG THE DIVIDE LATE MONDAY MORNING AND
MONDAY AFTERNOON...EXITING SOUTH OF INTERSTATE 80 MONDAY EVENING.
SNOW SHOWER ACTIVITY WILL INCREASE AHEAD OF THIS SYSTEM FROM NW-SE
MONDAY MORNING WITH IMPROVING CONDITIONS AND GUSTY NW WINDS
FOLLOWING THE DISTURBANCE MONDAY AFTERNOON AND MONDAY EVENING.
CLEARING CONDITIONS AND DIMINISHING WIND WILL FOLLOW OVERNIGHT
MONDAY.

&&

.PREV DISCUSSION... /ISSUED 304 PM MST SUN DEC 21 2014/

SHORT TERM...TONIGHT THROUGH WEDNESDAY

SNOW ACROSS THE WESTERN MOUNTAINS CONTINUES WITH MORE VALLEY
LOCATIONS CHANGING OVER FROM A RAIN/SNOW MIX OR FROM A WET SNOW TO A
DRIER ACCUMULATING SNOW TONIGHT. AS THE STRONG WESTERLY FLOW
SWITCHES TO MORE OF A NORTHWESTERLY FLOW OVERNIGHT THERE COULD BE
PERIODS OF GUSTY WIND MOVING INTO ARES MORE PRONE TO STRONG
NORTHWEST FLOW...SPECIFICALLY INTO THE WIND RIVER BASIN AND
BUFFALO/PINEY CREEK AREA. THERE DOES NOT APPEAR TO BE MUCH DOWNWARD
FORCING AS THE LEFT FRONT QUAD OF THE JET REMAINS IN THE
AREA...THERE ARE SOME 50 TO 60KT BARBS AT 700MB BUT THERE IS NOT
EXPECTED TO BE WIDESPREAD TRANSLATION TO THE SURFACE AS FORCING FOR
ASCENT WILL LIKELY WIN OUT. HOWEVER THERE WILL BE A FEW SPOTS
FLIRTING WITH HIGH WIND CRITERIA...ESPECIALLY NORTHWEST OF BUFFALO.
THE NORTHWEST FLOW WILL ALSO BRING DOWN A COLD POOL
ALOFT...INCREASING LOW AND MID LEVEL LAPSE RATES TO OVER 8.5DEG/KM
IN SOME CASES...ESPECIALLY ALONG AND EAST OF THE DIVIDE...AS WELL AS
IN SOME LOCATIONS WEST OF THE DIVIDE LIKE THE TETONS AND YELLOWSTONE.
COULD BE AN INTERESTING EVENING FOR SNOW BANDING. ONE BAND HAS
ALREADY MOVED ACROSS DUBOIS...DROPPING ABOUT AN INCH OF SNOW IN LESS
THAN ONE HOUR.

ON MONDAY...THE MAIN TROUGH MOVES EAST AND UNDERGOES RAPID
CYCLOGENESIS IN THE PLAINS. AROUND THE BACK OF THE TROUGH WE ARE
LOOKING AT A SHORT WAVE THAT IS SET TO MOVE THROUGH LATE TOMORROW
AFTERNOON. LOOKING AT STRICTLY THE MODEL QPF...IT WOULD APPEAR AS IF
NOT MUCH WILL COME OV IT. HOWEVER...THERE WILL STILL BE SOME DECENT
MOISTURE HANGING AROUND ALONG WITH INCREASING LAPSE RATES AS A
SECOND COLD POOL ALOFT SETTLES IN COMBINING WITH A SHORT WAVE OR
TWO...THERE COULD BE SOME SURPRISING ACCUMULATIONS OF SNOW ALONG AND
EAST OF THE DIVIDE...AS IS COMMON IN STRONGLY FORCED UNSTABLE
NORTHWEST FLOW EVENTS. EXACTLY WHERE IS HARD TO PIN POINT BUT POPS
WERE INCREASED MORE THAN THEY WOULD IN AN EVENT WITH LESS FORCING
AND MORE PREDICTABLE FEATURES IN PLACES LIKE THE NORTHERN BIGHORN
BASIN AND CODY FOOTHILLS...ALONG THE BIGHORNS INTO JOHNSON
COUNTY...POPS WERE ALSO INCREASED FOR POTENTIAL SNOW BANDING
ACTIVITY ALONG THE SOUTHERN BIGHORNS FROM SHOSHONI THROUGH CASPER
AND AREAS SOUTHEAST OF THE EASTERN TIP OF THE WIND RIVER MOUNTAINS.
STAYED FAIRLY CONSERVATIVE WITH QPF AND SNOW AMOUNTS GIVEN THE
CHAOTIC NATURE OF A SYSTEM LIKE THIS.

ON TUESDAY...HIGH PRESSURE BUILDS INTO THE WEST WITH ANOTHER SNOW
SHOWER OR TWO POSSIBLE ALONG THE I25 CORRIDOR AND IN THE BIGHORNS.
ELSEWHERE...CLEARING AND COOL WITH DECREASING WINDS. ON
WEDNESDAY...A RIDGE BUILDS IN ALOFT WHILE A LEE TROUGH DIGS AT THE
SURFACE...INCREASING WINDS FROM ROCK SPRINGS TO CASPER IN THE
AFTERNOON. OTHERWISE...INCREASING CLOUDS WITH MORE SNOW EXPECTED
ACROSS THE WEST AND NORTH LATE IN THE AFTERNOON.

LONG TERM...WEDNESDAY NIGHT THROUGH SUNDAY

GOING INTO THE CHRISTMAS DAY FORECAST STILL BRINGS
CHALLENGES...ESPECIALLY EAST OF THE DIVIDE. ONE THING IS CERTAIN...
A COLD FRONT WILL BRING COLDER TEMPERATURES TO THE REGION BY EARLY
CHRISTMAS MORNING.

ECMWF MODEL STILL DIGS SYSTEM TO THE SOUTHWEST OF FORECAST AREA ON
CHRISTMAS MORNING. THE LOW COMES OVER THE ROCKIES AS AN OPEN WAVE.
BUT THERE SHOULD BE A LONG ENOUGH PERIOD OF UPSLOPE FLOW ON BOTH
SIDES OF THE DIVIDE FOR A FEW INCHES OF SNOW. THIS IS TYPICAL MOST
LOCATIONS EAST OF THE DIVIDE AFTER FROPA. HOWEVER...LOCALLY HIGHER
AMOUNTS WILL BE POSSIBLE ALONG FAVORED NORTHEAST MOUNTAIN SLOPES
ON CHRISTMAS. DID NOT CHANGE POPS MUCH ON THURSDAY INTO THURSDAY
NIGHT. REGARDLESS...THIS IS NOT SHAPING UP TO BE A BIG SNOW EVENT.

FLOW ALOFT THEN SUPPORTS SCATTERED SNOW SHOWERS ON FRIDAY. THINKING
IS THAT THERE WILL BE ENOUGH INSTABILITY ALOFT...ESPECIALLY OVER THE
WESTERN AND NORTHERN MOUNTAIN RANGES. BOTH THURSDAY AND FRIDAY
NIGHTS SHOULD BE QUITE COLD...ESPECIALLY WITH ANY FRESH SNOW COVER.

MOSTLY DRY W-NW FLOW THEN SETS UP OVER THE FORECAST AREA SATURDAY
INTO SUNDAY. DETAILS VARY A BIT WITH EACH MEDIUM RANGE MODEL...BUT
LOOKS LIKE TEMPERATURES WILL MODERATE NEXT WEEKEND. POSSIBLY BETTER
CHANCE FOR SNOW NORTHWEST WYOMING MOUNTAINS BY SUNDAY AFTERNOON.

AVIATION.../00Z ISSUANCE/

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

A STRONG STORM SYSTEM CONTINUES TO MOVE ACROSS THE REGION...BASINS
ARE MIXING OUT. EXPECT PERIODS OF 20 TO 30KT WINDS WITH GUSTS MAINLY
IN THE 35-45KT RANGE BUT A FEW MORE WIND PRONE AREAS COULD SEE GUSTS
UP TO 55KT...BUT NOT AT ANY OF THE FORECAST TERMINALS. A STRONG NW
FLOW WILL FOLLOW THIS SYSTEM TONIGHT WITH ISOLD-SCT MVFR/IFR SNOW
SHOWERS POSSIBLE ALONG THE FRONT ACROSS THE LOWER ELEVATIONS THIS
EVENING...ESPECIALLY VCNTY AND EAST OF INTERSTATE 25. DRIER AIR
PREVAIL NW-SE BUT GUSTY NW SFC WINDS WILL REMAIN AFTER 06Z MONDAY.

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

STRONG MTN TOP WINDS RESULTING IN AREAS MDT-SVR TURBULENCE AND
LLWS...WILL CONTINUE THROUGH THE NIGHT. A COLD FRONT WILL CONTINUE
TO PUSH SOUTH ACROSS THE AREA CROSSING INTERSTATE 80 AROUND 03Z
MONDAY.  THE FRONT WILL SPREAD NMRS MVFR/IFR -SHSN SOUTH ALONG
I-80...VCNTY KRKS...THROUGH THE EVENING.  DRIER AIR AND IMPROVING
CONDITIONS WILL PREVAIL BEHIND THE FRONT FROM NW-SE 06Z-12Z MONDAY.

FIRE WEATHER...

ONGOING WINTRY CONDITIONS WILL KEEP SNOWFALL AND AREAS OF STRONG
WIND ACROSS WESTERN AND SOUTHERN WYOMING. WIND IS EXPECTED TO
INCREASE ACROSS CENTRAL AND NORTHEASTERN WYOMING LATER TONIGHT WITH
POTENTIALLY STRONG WIND LINGERING THROUGH MONDAY AFTERNOON. WINTER
STORM AND HIGH WIND WARNINGS ARE CURRENTLY IN EFFECT.

&&

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

&&

$$

UPDATE...AEM
SHORT TERM...ALLEN
LONG TERM...KPL
AVIATION...AEM
FIRE WEATHER...ALLEN









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