Area Forecast Discussion
Issued by NWS Riverton, WY

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260
FXUS65 KRIW 031757
AFDRIW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RIVERTON WY
1057 AM MST WED FEB 3 2016

.SHORT TERM...TODAY THROUGH FRIDAY NIGHT

IMAGERY SHOWS UPR TROF ACROSS THE WRN 2/3RDS OF THE CONUS WITH
UPSTREAM SW TROF ACROSS WA/OR AND RIDGE AXIS POSITIONED JUST
OFFSHORE THE WCST. STRONGER UPSTREAM SW TROF AXIS LOCATED AROUND
140W. MINOR SW TROF MOVING THRU NW FLOW OVR WY WITH A FEW FLURRIES
AROUND AS THE RESULTANT. SFC HAS GENERAL HIGH P WEST OF
WY...LOWERING P EAST WITH WEAK P GRAD IN BETWEEN.

EARLY TODAY...FLURRIES/LIGHT SNOW EXITING ERN PORTION OF FA THIS
MORNING WITH SOME ISOLATED TERRAIN INDUCED -SHSN/FLURRIES ACROSS THE
WRN MOUNTAINS/VALLEYS ALSO DECREASING IN COVERAGE. MAIN EVENT FOR
TODAY/TONIGHT/THU MORNING WILL BE A BOUT OF WIND OVR PORTIONS OF THE
CODY FOOTHILLS AS THERMAL GRAD AND LEE SIDE TROF FORM THE NRN BIG
HORN BASIN IN RESPONSE TO DOWNSLOPING LATE THIS AFTERNOON. WINDS
WILL LIKELY RUN HIGHEST FROM AROUND MIDNIGHT THRU THE FIRST FEW HRS
OF THE AFTERNOON ON THURSDAY AS RELATIVELY STRONG RIDGE TOP
STABILITY AND A GOOD OMEGA GRAD FORM. MOUNTAIN WAVES WILL ALSO HELP
TO ENFORCE THE ACTION BY AROUND SUN-UP THURSDAY. HAVE CONVERTED AND
EXTENDED THE HIGH WIND WATCH TO A WARNING BEGINNING LATE THIS
AFTERNOON...NOW LASTING THRU 1400 MST THU. GUSTY WEST TO SOUTHWEST
WINDS ACROSS THE WIND CORRIDOR WILL MIX WITH RECENT HEALTHY SNOWFALL
TO GIVE AREAS OF BLOWING SNOW AND REDUCED VIS. SO ISSUING A WINTER
WEATHER ADVISORY FOR BLOWING SNOW BEGINNING TONIGHT AND LASTING THRU
THURSDAY MORNING. ADDITIONALLY...THE RELATIVELY WEAK UPR SW THAT
WILL HELP IN FLATTENING THE FLOW AND INDUCE THOSE MTN WAVES WILL
ARRIVE ACROSS THE WRN FA BY THU MORNING ALONG WITH DEEPER BUT
DECREASING MOISTURE. WHILE SNOWFALL RATES SHOULD BE ON THE LIGHT
SIDE...24 HR SNOWFALL ACCUMULATION ACROSS THE WRN MOUNTAINS THRU
THURSDAY NIGHT SHOULD RANGE FROM 2 TO 6 INCHES...WITH GENERALLY 1
TO 3 INCHES IN THE VALLEYS. AS THE UPR WAVE MOVES ACROSS WY ON
THURSDAY...LIGHT SNOW WILL TRY TO SPREAD ACROSS THE SRN FA AND E
OF THE DIVIDE. BEST UNIMPEDED LIFT AND LL CONVERGENCE WILL LAY OVR
THE WIND CORRIDOR REGION OF MAINLY SWEETWATER AND NATRONA
COUNTIES. OTHER AREAS/BASINS IN THE LEE OF THE MOUNTAINS SUCH AS
THE WIND RIVER AND BIG HORN BASINS WILL ONLY HAVE VERY LOW CHANCES
FOR PRECIP/SNOW THRU THE DAY/EVENING THURSDAY AS NO LOW TO MID LVL
UPSLOPE FLOW SETS UP WITH THIS SYSTEM AS IT SPEEDS THRU THE
REGION. THE UPR LVL SYSTEM AND ASSOCIATED CLIPPER TYPE FRONT
WILL MOVE THRU AND OUT OF THE ERN FA BY FRIDAY MORNING WITH
CLEARING SKIES AND HIGH P IN CONTROL OF THE FA AND ONLY LINGERING
-SHSN REMAIN ACROSS PORTIONS OF THE W CNTRL/NWRN MOUNTAINS. THEN
UPR LVL NW FLOW RETURNS ALONG WITH GOOD RIDGE TOP STABILITY...A
LEESIDE SFC TROF...AND AN INCREASING THERMAL/OMEGA GRAD ACROSS THE
CODY FOOTHILLS ONCE AGAIN FRIDAY NIGHT. THIS COULD OFFER UP
ANOTHER CHANCE FOR LOCALLY HIGH WIND.

.LONG TERM...SATURDAY THROUGH WEDNESDAY

SYNOPSIS...A COLD FRONT WILL PUSH ACROSS THE AREA SATURDAY BRINGING
SNOW SHOWERS ACROSS THE MOUNTAINS AND NORTH...AND STRONG WINDS
ACROSS THE I-80 WIND CORRIDOR.   A RIDGE OF HIGH PRESSURE WILL THEN
BUILD INTO THE AREA SUNDAY THROUGH WEDNESDAY...BRINGING MAINLY DRY
CONDITIONS WITH MUCH ABOVE NORMAL TEMPERATURES BY TUESDAY AND
WEDNESDAY EXCEPT IN SNOW COVERED WESTERN VALLEYS AND CENTRAL BASINS
WHERE STEEPENING INVERSIONS WILL BE THE RULE.

DISCUSSION...GFS AND ECMWF SHOW SIGNIFICANT SPREAD WITH LAST IN A
SERIES OF PACIFIC SHORTWAVES DIVING ACROSS THE NORTHERN ROCKIES ON
SATURDAY.  GFS TRACK IS FURTHER SOUTH AND MORE SPLIT RESULTING IN
CHANCES OF PRECIPITATION ACROSS THE ENTIRE STATE WHILE THE ECMWF
FURTHER NORTHWARD TRACK WOULD KEEP THE SOUTH DRY...ROUGHLY SOUTH AND
WEST OF A KJAC-KRIW-KRWL LINE.  FORECAST REFLECTS FURTHER NORTHWARD
TRACK OF THE ECMWF AS THIS HAS BEEN THE OVERALL TREND OVER THE LAST
FEW RUNS.  MODELS IN GOOD OVERALL AGREEMENT WITH UPPER RIDGE
BUILDING ALONG THE WEST COAST SUNDAY AND MONDAY WITH BUILDING
HEIGHTS ACROSS THE NORTHERN ROCKIES THROUGH MID-WEEK.

SATURDAY...SHORTWAVE AND ASSOCIATED COLD FRONT WILL SPREAD SNOW
SHOWERS INTO THE NW MTNS SATURDAY MORNING...WITH SCT SNOW SHOWERS
DEVELOPING ACROSS THE NORTHERN BIGHORN BASIN AND DOWN THE I-25
CORRIDOR FROM BUFFALO TO CASPER IN THE AFTERNOON AND EVENING. AREAS
OF STRONG W-NW WINDS WILL LIKELY FOLLOW THE FRONT ACROSS FAVORED
AREAS FROM DUBOIS TO RIVERTON AND ALONG THE INTERSTATE 80 CORRIDOR.
SNOW SHOWERS COMBINED WITH STRONG POST-FRONTAL NW WINDS MAY PRODUCE
BLOWING SNOW AND LIMITED VISIBILITY ALONG THE I-90 CORRIDOR IN THE
AFTERNOON AND EVENING.

SUNDAY AND MONDAY...WARM ADVECTION AND MOIST NW FLOW MAY PRODUCE
SOME LIGHT SNOW IN THE NORTHERN MTNS.  GFS MUCH WETTER THAN ECMWF
AND FORECAST ONCE AGAIN LEANS TOWARD ECWMF WITH SLIGHT CHANCE POPS
IN THE NORTHERN MTNS.

TUESDAY AND WEDNESDAY...HERE COMES THE WARM AIR.  ECWMF HAS H7 TEMPS
NEAR 5C OVER CENTRAL WY BY TUESDAY...GFS BY WEDNESDAY.  FOR THOSE IN
SNOW COVERED WESTERN VALLEYS AND LOWER PORTIONS OF THE WIND RIVER
BASIN...THIS WILL LIKELY MEAN STEEPENING INVERSIONS WITH SOME
MORNING FOG.  FORECAST KEEPS THESE AREAS NEAR OR BELOW FREEZING FOR
HIGHS WHILE MIXED AREAS SEE HIGHS IN THE 40S AND 50S.

&&

.AVIATION.../18Z ISSUANCE/

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

VFR CONDITIONS EXPECTED THROUGH 09Z THURSDAY. AFTER 09Z MVFR TO
IFR CEILINGS WILL BECOME MORE AND MORE WIDESPREAD OVER THE
MOUNTAINS AS A PACIFIC LOW PRESSURE TROUGH SPREADS MOISTURE EAST
INTO WESTERN WYOMING. AREAS OF LIGHT SNOW WILL DEVELOP AS WELL.
MVFR TO IFR CEILINGS AND VISIBILITY REDUCED IN SNOW AND FOG WILL
LOWER INTO THE WESTERN VALLEYS AROUND 12Z THURSDAY WITH KJAC AND
KAFO MOST LIKELY TO HAVE IFR CONDITIONS. KBPI...KEMM AND KPNA MAY
SEE MVFR CEILINGS AFTER 15Z THURSDAY AS THE MOISTURE SPREADS EAST.
WEST TO SOUTHWEST WINDS WILL INCREASE AFTER 12Z THURSDAY WITH
AREAS OF LOW LEVEL WIND SHEAR POSSIBLE OVER AND NEAR THE
MOUNTAINS.

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

VFR CONDITIONS ARE EXPECTED TO PREVAIL AT THE AIRPORTS EAST OF THE
CONTINENTAL DIVIDE THROUGH 18Z THURSDAY. ALONG AND EAST OF THE
CONTINENTAL DIVIDE IN THE MOUNTAINS ABOVE 8500 FEET EXPECT
IFR TO MVFR CEILINGS TO DEVELOP AFTER 12Z THURSDAY WITH AREAS OF
LIGHT SNOW AND FOG OBSCURING MOUNTAINS. ALONG AND NEAR THE EAST
SLOPES OF THE MOUNTAINS AREAS OF LOW LEVEL WIND SHEAR WILL BE
ENCOUNTERED...INCREASING AFTER 12Z THURSDAY.

&&

.FIRE WEATHER...

FIRE DANGER REMAINS LOW ACROSS THE FORECAST AREA THROUGH THE REST OF
THE WORK WEEK WITH SEASONALLY COLD TEMPERATURES...SNOW COVERING MOST
SURFACES....AND A NEW WINTER WEATHER SYSTEM ROLLING THROUGH THE
FORECAST AREA THURSDAY THROUGH FRIDAY MORNING. HOWEVER...WINDS WILL
BE QUITE GUSTY AT TIMES ON THURSDAY AND AGAIN FRIDAY NIGHT ACROSS
THE USUAL AREAS. MOST AFTERNOONS ACROSS WYOMING...EXCEPT FOR THE
BASINS...SHOULD SEE FAIR TO GOOD SMOKE DISPERSION.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
WINTER WEATHER ADVISORY FROM 5 PM THIS AFTERNOON TO 11 AM MST
THURSDAY FOR WYZ019-020-022-030.

HIGH WIND WARNING FROM 5 PM THIS AFTERNOON TO 2 PM MST THURSDAY
FOR WYZ003.

&&

$$

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



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