Area Forecast Discussion
Issued by NWS Riverton, WY

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000
FXUS65 KRIW 200531
AFDRIW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RIVERTON WY
1030 PM MST FRI DEC 19 2014


.SHORT TERM...TONIGHT THROUGH SATURDAY

IMPRESSIVE WEEK FOR FOG ACROSS PARTS OF THE WIND RIVER BASIN OF
WEST CENTRAL WYOMING. THE ENVIRONMENT IS STILL CONDUCIVE FOR
FREEZING FOG TO CONTINUE ONE MORE NIGHT...ESPECIALLY NEAR
RIVERTON. IT MAY NOT BE AS WIDESPREAD AS PAST FEW NIGHTS.
NONETHELESS...WILL REISSUE DENSE FOG ADVISORY UNTIL 8 AM MST
SATURDAY MORNING. THINKING IS THAT MORE WESTERLY FLOW ABOVE THE
SURFACE AND WEAK SHORTWAVE ALOFT WILL HELP ERODE ALL FOG/LOW
STRATUS BY MID MORNING SATURDAY. EXPECT UNLIMITED VISIBILITY BY
SATURDAY AFTERNOON.

OTHERWISE...SHORTWAVE RIDGING THIS EVENING GIVES WAY TO WESTERLY
FLOW ALOFT. THIS FLOW ALOFT WILL INCREASE DURING THE DAY SATURDAY.
ISENTROPIC LIFT OVER FAR WESTERN MOUNTAINS THROUGH MUCH OF THE DAY
SATURDAY WILL RESULT IN AREAS OF LIGHT TO MODERATE SNOW.
THIS WILL BE THE APPETIZER TO WHAT IS ABOUT TO OCCUR WEST OF THE
DIVIDE FOR THE REMAINDER OF WEEKEND. THESE WINTRY DETAILS ARE
INTRODUCED IN THE LONG TERM DISCUSSION.

.LONG TERM...SATURDAY NIGHT THROUGH FRIDAY

THE BEGINNING OF THE LONG TERM MARKS THE ONSLAUGHT OF MOISTURE TO
WESTERN WYOMING FOR THE NEXT 36 HOURS.  AS NOTED BY THE PREVIOUS
SHIFT...A WINTER STORM WATCH IS CURRENTLY IN EFFECT FOR FAR WESTERN
WYOMING STARTING 800PM SATURDAY NIGHT...AND THEN INCLUDES THE
ABSAROKAS...WIND RIVERS...AND GREEN RIVER FOOTHILLS ZONES AT 1100PM.
THE WEST TO SOUTHWEST SATURATED FLOW WILL FIRST IMPACT THE WEST SIDE
OF THE TETONS AND SW PORTION OF YNP.  AFTER MIDNIGHT...THE INTENSITY
SHOULD INCREASE IN THE WESTERN MOUNTAINS AS THE LAPSE RATES INCREASE
ALONG WITH THE MID LEVEL FLOW AROUND 40 KNOTS.  THE MODELS ARE ALL
IN QUITE GOOD AGREEMENT WITH THE OVERALL STORM MOVEMENT AND
PRECIPITATION EXPECTATIONS.  FOR THE WESTERN MOUNTAINS...THE
OROGRAPHIC SNOW MODEL ESTIMATES 18-30 INCHES POTENTIAL SNOW ON THE
WESTERN SLOPES OF THE TETONS UNDER PRIME CONDITIONS.  THE
NEIGHBORING VALLEYS ARE LIKELY TO SEE 8-10 INCHES WITH 12-14 IN
LOCALLY FAVORABLE LOCATIONS LIKE MOOSE AND SMOOT. AS THE FLOW
BECOMES MORE WEST NORTHWEST LATER ON SUNDAY...THE MODELS ARE
INDICATING BETTER OROGRAPHICS ALONG THE SALT AND WYOMING RANGES AS
WELL AS INTO THE BONDURANT AND CORA AREAS.

THE GFS HOWEVER INDICATES MORE POTENTIAL FORCING WITH A STRONGER
SHORTWAVE TROUGH MOVING THROUGH MONTANA INTO THE NORTHERN PLAINS ON
SUNDAY.  THE RESULTING MID LEVEL BAROCLINIC BOUNDARY FROM SOUTHERN
MT INTO NE WY ALLOWS FOR ENHANCED PRECIPITATION IN THE NORTHERN BIG
HORN BASIN AND BIGHORN MOUNTAINS INTO JOHNSON COUNTY.  IF THIS
SITUATION PANS OUT WITH THE NORTHWEST FLOW...SOME MORE INTERESTING
HIGH SNOW AMOUNTS COULD OCCUR IN THE NORTHERN BIGHORNS AND ALONG THE
NORTH AND EAST SIDES OF THE BIG HORN BASIN.

THE ACCOMPANYING 120+ KNOT JET WITH THIS STORM SYSTEM WILL MAKE FOR
VERY INTERESTING CONDITIONS AT THE MID AND HIGHER ELEVATIONS.
MOUNTAIN TOP AND SLOPE WINDS ACROSS WESTERN WYOMING ARE LIKELY TO BE
WINDY THROUGHOUT THE PERIOD.  HAVE EVALUATED THE POTENTIAL FOR
STRONG WINDS IN THE CODY FOOTHILLS FROM SAT NIGHT INTO SUN MORN.
CROSS SECTIONS OF SEVERAL MODELS INDICATE AT THIS TIME THAT STRONGER
WINDS OF 50 KNOTS SHOULD STAY IN THE HIGHER ELEVATIONS OF THE
EASTERN ABSAROKAS WITH AROUND 30-40 KNOTS IN THE FOOTHILL
LOCATIONS.  AGREE WITH THE PREVIOUS SHIFT THAT WEST TO NORTHWEST
WIND WILL STRENGTHEN ON SUNDAY FROM DUBOIS INTO THE WIND RIVER
BASIN...AS WELL AS ACROSS SWEETWATER COUNTY INTO CENTRAL WYOMING.

BOTH THE GFS AND ECMWF MOVE THE SYSTEM INTO THE PLAINS BY MONDAY
NIGHT WITH RESIDUAL SHOWERS OVER EASTERN WYOMING MONDAY NIGHT.
WHILE BOTH MODELS DEVELOP THE RIDGE ACROSS THE WESTERN US ON
TUESDAY...THE GFS SOLUTION IS SHARPER...AND THE FLATTER ECMWF ALLOWS
OVER-THE-TOP MOISTURE AND WEAK ENERGY TO SLIDE INTO NW WY TUESDAY
NIGHT.  NOT CONFIDENT IN EITHER SOLUTION TO HAVE INDICATED SLIGHT
CHANCE OF SNOW IN THE FORECAST.  BOTH MODELS THEN BRING ANOTHER
PACIFIC TROUGH INTO OUR AREA LATE WEDNESDAY NIGHT INTO CHRISTMAS
DAY. THE ECMWF STILL DIGS THE SYSTEM FURTHER SOUTH THAN THE
GFS...YET IS SLIGHTLY FURTHER NORTH THAN 24 HOURS AGO.  THE GFS IS
KEEPING WITH A MORE OPEN PROGRESSIVE TROUGH THROUGH THE NORTHERN
ROCKIES.  AND THE PARALLEL GFS IS EVEN MORE PROGRESSIVE.  EITHER
WAY...A STORM WILL IMPACT WYOMING ON CHRISTMAS...BUT THERE IS MUCH
UNCERTAINTY AS TO THE PRECIPITATION...TEMPERATURE..AND WIND IMPACTS
AT THIS TIME DUE TO THE LOCATION AND TIMING DIFFERENCES.
REGARDLESS...FRIDAY LOOKS TO BE COLDER THAN EARLIER IN THE WEEK.

&&

.AVIATION.../06Z ISSUANCE/

INCREASING MOISTURE PER MODELS INTO WESTERN WYOMING THIS WEEKEND.
SNOW WILL INCREASE AND BECOME MODERATE TO HEAVY BY LATE SATURDAY.
THIS WILL RESULT IN MOUNTAIN OBSCURATION FOR ALL MOUNTAIN RANGES.
IN ADDITION...WESTERN VALLEYS WILL BECOME IFR SATURDAY NIGHT AND
REMAIN THAT WAY THROUGH SUNDAY WITH LOW STRATUS AND SNOW.

OUTSIDE OF THE WIND RIVER BASIN OF CENTRAL WYOMING...VFR CONDITIONS
CAN BE EXPECTED THIS WEEKEND EAST OF THE DIVIDE. WITH A TIGHTENING
PRESSURE GRADIENT WINDS WILL INCREASE SATURDAY...ESPECIALLY ACROSS
THE SOUTHWESTERN WIND CORRIDOR FROM KRKS TO KCPR.

FOG HAS YET TO FORM AS OF 5Z AT KLND AND KRIW. LOCAL MODELS STILL
INDICATE FOG TO FORM AFTER MIDNIGHT AND LAST INTO THE EARLY MORNING
ALTHOUGH NOT AS WIDESPREAD AS PREVIOUS NIGHTS. WILL HAVE TO BE
MONITORED HOWEVER.

&&

.FIRE WEATHER...

NO FIRE WEATHER CONCERNS OR ACTIVITIES SHOULD BE CONSIDERED THIS
WEEKEND. A POTENT WINTER STORM WILL MOVE INTO WESTERN WYOMING...
BRINGING MODERATE TO HEAVY SNOW BY SATURDAY NIGHT. IN FACT...A WINTER
STORM WATCH REMAINS IN EFFECT AND DETAILS ON THIS WATCH CAN BE
SEEN IN OUR WINTER WEATHER PRODUCT. WESTERLY SURFACE WIND WILL
ALSO INCREASE MOST LOCATIONS BY SATURDAY AFTERNOON. UNSETTLED
WEATHER WITH SNOW AND VERY GUSTY WIND WILL CONTINUE ACROSS WEST
CENTRAL WYOMING ON SUNDAY.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
WINTER STORM WATCH FROM SATURDAY EVENING THROUGH MONDAY MORNING
WYZ002-014-015-025.

WINTER STORM WATCH FROM SATURDAY EVENING THROUGH MONDAY MORNING
WYZ001-012-013-023-024.

DENSE FOG ADVISORY FROM 8 PM THIS EVENING TO 8 AM MST SATURDAY
WYZ017-018.

&&

$$

SHORT TERM...KPL
LONG TERM...MCDONALD
AVIATION...HATTINGS
FIRE WEATHER...KPL






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