Area Forecast Discussion
Issued by NWS Riverton, WY

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000
FXUS65 KRIW 210624
AFDRIW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RIVERTON WY
1124 PM MST SAT DEC 20 2014

.SHORT TERM...TONIGHT THROUGH SUNDAY

UPGRADED HIGH WIND AND WINTER STORM WATCHES TO WARNINGS. INCLUDED
SOUTHERN LINCOLN COUNTY IN HIGH WINDS WARNING AND STARTED THE HIGH
WIND WARNING ACROSS THE SOUTH AT 5 AM MST. OVERALL TREND WAS FOR
MORE SNOW IN THE WEST WITH LITTLE IF ANY IN THE BASINS EAST OF THE
CONTINENTAL DIVIDE. IMPRESSIVE MOISTURE PLUME EXTENDING TO NEAR THE
DATELINE ALREADY SPREADING SNOW ACROSS THE WEST WITH THE SOUTHWEST
GETTING SOME SHOWERS AS WELL. TEMPERATURES WARMER TONIGHT AND SUNDAY
WITH BETTER MIXING...CLOUD COVER. LOWER ELEVATIONS WILL LIKELY SEE SOME
RAIN AT TIMES BELOW 7300 FEET.

.LONG TERM...SUNDAY NIGHT THROUGH SATURDAY

SNOW ACROSS THE WESTERN MOUNTAINS CONTINUES INTO SUNDAY NIGHT WITH
MORE VALLEY LOCATIONS CHANGING OVER FROM A RAIN/SNOW MIX TO ALL SNOW
SUNDAY NIGHT. THE STRONG WESTERLY FLOW OPEN TO THE PACIFIC WILL
BEGIN TO SWITCH TO MORE OF A NORTHWESTERLY FLOW OVERNIGHT ON SUNDAY
WITH PERIODS OF GUSTY WIND SHIFTING TO BUFFALO...POWELL AND COWLEY
AREAS AND THEN DOWN INTO THE UPPER WIND RIVER BASIN. AS THE FLOW
SHIFTS NORTHWEST...THE LEFT EXIT REGION OF AN INCOMING 170 KNOT JET
BEGINS TO INFLUENCE THE REGION. IF YOU LOOK STRICTLY AT MODEL QPF
DURING THE DAY ON MONDAY...IT WOULD LOOK LIKE NOT A WHOLE LOT IS
GOING TO HAPPEN. HOWEVER...THERE WILL STILL BE SOME DECENT MOISTURE
HANGING AROUND ALONG WITH INCREASING LAPSE RATES AS THE COLD POOL
ALOFT SETTLES IN COMBINING WITH A POTENTIAL 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...ESPECIALLY AROUND 18Z MONDAY WHERE THE
MODELS TRY TO PLACE A STRONG DEFORMATION BAND WITH A SECONDARY BAND
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.

ONCE THIS STORM CLEARS THE AREA ON TUESDAY THE NEXT STORM OF
INTEREST WILL ARRIVE ON CHRISTMAS DAY...POTENTIALLY AS EARLY AS LATE
CHRISTMAS EVE ACROSS THE WEST. THIS LOOKS LIKE A VERY FAVORABLE
PATTERN FOR SNOW ON BOTH SIDES OF THE DIVIDE CHRISTMAS DAY AND THE
MODELS APPEAR TO BE IN DECENT AGREEMENT ON THE TIMING AND PLACEMENT
OF FEATURES THIS RUN. HOWEVER...BASED ON THE PREVIOUS RUNS THEY HAVE
NOT ALWAYS BEEN IN AGREEMENT ON EVERYTHING. RELIED HEAVILY ON THE
PREVIOUS FORECAST BUT ALLOWED POPS TO GO INTO THE HIGH CHANCE TO
LIKELY CATEGORY IN FAVORED UPSLOPE LOCATIONS EAST OF THE DIVIDE AS
WELL AS IN THE MOUNTAINS FOR CHRISTMAS DAY GIVEN THE FAVORABLE
PATTERN ADVERTISED BY THE CURRENT MODEL RUNS.

THE FRESH SNOW COVER SHOULD MAKE FOR A CHILLY DAY ON FRIDAY
ESPECIALLY WITH HIGH PRESSURE BUILDING IN BEHIND THE EXITING
SYSTEM...SO ALLOWED A FEW BASIN AND VALLEY TEMPERATURES TO DROP
BELOW ZERO OVERNIGHT. LOOKS LIKE ANOTHER POTENTIALLY WINDY DAY NEXT
SATURDAY AS ANOTHER STORM SYSTEM APPROACHES...THIS SHOULD HELP MIX
OUT THE BASINS AND COULD LEAD TO BORDERLINE TO STRONG WIND IN THE
FOOTHILL LOCATIONS AGAIN. MORE SNOW WILL BE POSSIBLE LATE SATURDAY
NIGHT.

&&

.AVIATION.../06Z ISSUANCE/

A TROUGH OF LOW PRESSURE WILL MOVE ACROSS MONTANA ON SUNDAY AND INTO
THE NORTHERN PLAINS SUNDAY EVENING.  A STRONG JET STREAM 120-130KTS
WILL CROSS THE WYOMING DIVIDE SOUTH OF THIS SYSTEM SUNDAY MORNING
CAUSING AREAS OF MDT-SVR TURBULENCE...LLWS OVER SHALLOW CENTRAL
BASIN INVERSIONS...AND STRONG SFC WINDS FROM VCNTY SOUTH PASS TO
VCNTY KCPR AND SOUTH ALONG THE INTERSTATE 80 CORRIDOR.  THE LEE
SLOPES ALONG THE DIVIDE AND BIGHORN RANGE WILL LIKELY REMAIN MOSTLY
OBSCURE ABOVE 9KFT.  A STRONG NW FLOW WILL FOLLOW THIS SYSTEM SUNDAY
NIGHT WITH ISOLD-SCT MVFR/IFR SNOW SHOWERS POSSIBLE ALONG THE FRONT
ACROSS THE LOWER ELEVATIONS SUNDAY 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

WIDESPREAD IFR...AREAS OF LIFR CONDITIONS IN SN/BLSN WITH MTN
OBSCURED ARE EXPECTED VCNTY AND NORTH OF A KEVW-20SM SW KLND LINE
THROUGH 0Z MONDAY.  STRONG MTN TOP WINDS WILL ALSO RESULT IN AREAS
MDT-SVR TURBULENCE AND LLWS.  A COLD FRONT WILL PUSH SOUTH ACROSS
THE AREA SUNDAY AFTERNOON...AND ACROSS INTERSTATE 80 00Z-03Z
SUNDAY.  THE FRONT WILL SPREAD NMRS MVFR/IFR -SHSN SOUTH ALONG
I-80...VCNTY KRKS...THROUGH SUNDAY EVENING.  DRIER AIR AND IMPROVING
CONDITIONS WILL PREVAIL BEHIND THE FRONT FROM NW-SE 06Z-12Z MONDAY.


&&

.FIRE WEATHER...

PERIODS OF HEAVY SNOW FOR THE WEST AND STRONG WINDS EAST TONIGHT
AND SUNDAY. VARIOUS WINTER STORM WARNINGS AND HIGH WIND WARNINGS
HAVE BEN POSTED. PLEASE REFER TO THEM FOR MORE DETAILS.  OVERALL
LOW MIXING HEIGHTS WILL RESULT IN POOR SMOKE DISPERSAL.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
HIGH WIND WARNING FROM 5 AM SUNDAY TO 2 AM MST MONDAY WYZ027>030.

HIGH WIND WARNING FROM 5 AM TO 11 PM MST SUNDAY WYZ019.

WINTER STORM WARNING FROM 11 PM THIS EVENING TO 11 AM MST MONDAY
WYZ002-014-015-025.

WINTER STORM WARNING UNTIL 8 AM MST MONDAY
WYZ001-012-013-023-024.

HIGH WIND WARNING FROM 8 PM THIS EVENING TO 11 AM MST SUNDAY
WYZ003.

&&

$$

SHORT TERM...C.BAKER
LONG TERM...ALLEN
AVIATION...AEM
FIRE WEATHER...C.BAKER











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