Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 222146
AFDRIW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RIVERTON WY
246 PM MST THU JAN 22 2015

.SHORT TERM...TONIGHT THROUGH FRIDAY

A WEAK FAST MOVING LOW PRESSURE TROUGH WILL SWEEP SOUTH ACROSS
THE AREA FRIDAY WITH LOCAL LIGHT SNOW OVER AND NEAR THE NORTHERN
AND WESTERN MOUNTAINS WITH VERY LIGHT ACCUMULATIONS EXPECTED.
OVERNIGHT SOME FOG AND LOW STRATUS IS EXPECTED TO FORM ALONG AND
NEAR THE RIVERS AND LAKES IN THE BASINS AND VALLEYS.  THIS WILL BE
HAMPERED BY INCREASED MID AND HIGH CLOUDS. LOCAL WINDY AREAS WILL
OCCUR OVER AND NEAR THE EAST SLOPES OF THE MOUNTAINS AND THROUGH THE
WIND CORRIDOR FROM ROCK SPRINGS TO CASPER. AS THE TROUGH PASSES
SOUTH THE WINDS WILL SHIFT TO THE NORTH AND GENERALLY DECREASE.

.LONG TERM...FIRDAY NIGHT THROUGH THURSDAY

WEAK FRONTAL BOUNDARY WILL EXIT SOUTHWARD FRIDAY EVENING...ENDING ANY
LINGERING PRECIPITATION BY 06Z SATURDAY. A BRIEF LULL IN
PRECIPITATION IS THEN EXPECTED OVERNIGHT FRIDAY NIGHT INTO EARLY
SATURDAY MORNING...AS DRY AIR MOVES ACROSS THE STATE. CLEAR SKIES
COULD LEAD TO AREAS OF PATCHY FOG SATURDAY MORNING...MAINLY ACROSS
THE WIND RIVER AND BIGHORN BASIN. A WEAK DISTURBANCE...COMBINED WITH
ANOTHER WEAK FRONTAL BOUNDARY...WILL BRING ANOTHER SHOT OF LIGHT
SNOW DURING THE DAY SATURDAY...MAINLY ACROSS THE NORTHERN AND
WESTERN MOUNTAINS. ECMWF HAS THE WEAK FRONTAL BOUNDARY DIPPING
SLIGHTLY FURTHER SOUTH THAN THE GFS...WHICH COULD BRING A BRIEF SHOT
OF LIGHT SNOW TO LOWER ELEVATIONS OF THE NORTH...AND TO THE EASTERN
CWA SATURDAY AFTERNOON AND EVENING. AN INCREASED GRADIENT WILL ALSO
BRING SOME GUSTY WINDS TO THE ABSAROKA MOUNTAINS AND THE CODY
FOOTHILLS ON SATURDAY. WINDS COULD ALSO INCREASE ACROSS THE WIND
CORRIDOR FROM ROCK SPRINGS TO CASPER ON SATURDAY. HIGH PRESSURE WILL
BUILD ACROSS THE REGION SATURDAY NIGHT...ENDING ANY LINGERING PRECIP
AND DIMINISHING WINDS.

SUNDAY THROUGH TUESDAY...A LARGE RIDGE OF HIGH PRESSURE WILL
POSITION ITSELF ACROSS ROCKY MOUNTAIN REGION DURING THIS TIME
PERIOD. MAINLY CLEAR SKIES AND NO PRECIP IS EXPECTED. MAIN FORECAST
CHALLENGE IS HOW WARM WILL IT GET. H7 TEMPS WILL CLIMB NEAR 5-6 C.
SURFACE TEMPS WILL REALLY DEPEND ON TWO FACTORS...SNOW COVER AND
MIXING. WITH LIGHT WINDS EXPECTED UNDER THE RIDGE...FULLY MIXING OUT
WILL BE TOUGH TO COME BY...ESPECIALLY ACROSS THE WEST. CURRENT NOHRSC
SNOW COVER ANALYSIS SHOWS WHAT ONE MAY EXPECT FOR SNOW IN THE
MOUNTAINS AND WESTERN VALLEYS...WITH LITTLE SNOW COVER ACROSS THE
LOWER ELEVATIONS OF THE CWA. IF SATURDAYS DISTURBANCE PROVIDES
LITTLE NEW SNOW...AND GUSTY WINDS CAN PROVIDE ENOUGH WARM TEMPS TO
MELT SOME SNOW...THIS AIR MASS IS WARM AND DRY ENOUGH TO PRODUCE
HIGHS GENERALLY IN THE 40S...WITH SOME MID TO UPPER POSSIBLE 50S
NEAR THE CODY FOOTHILLS AND THE EASTERN FOOTHILLS OF THE BIGHORNS.

WEDNESDAY THROUGH THURSDAY...HIGH PRESSURE RIDGE CONTAINING WARM DRY
AIR...WILL BE PUSHED OUT BY A SHORTWAVE TROUGH OUT OF THE BAJA AREA.
SLIGHT DIFFERENCES REMAIN WITH THIS DISTURBANCE. GFS WAS A MORE
PROGRESSIVE WAVE...AS THE ECMWF HAS THE WAVE WASHING OUT IN WESTERLY
FLOW...THEN LINGERS THIS MOIST FLOW AS A RIDGE BUILDS TO THE
SOUTHWEST. EITHER SOLUTION BRINGS CHANCES FOR MAINLY SNOW ACROSS THE
WEST...WITH THE ECMWF SOLUTION BRINGING SOME SCATTERED LIGHT
SNOW/RAIN ACTIVITY DURING THE AFTERNOON AND EVENING ON WEDNESDAY
ACROSS THE SOUTH AND PORTIONS EAST OF THE DIVIDE. GUSTY WINDS WILL
ALSO BE POSSIBLE WITH THIS DISTURBANCE...ESPECIALLY ACROSS SWEETWATER
COUNTY UP THROUGH NATRONA COUNTY. MODELS THEN START TO HINT TOWARDS
AN ACTIVE END TO NEXT WEEK AS THE GFS BRINGS A LARGE TROUGH ACROSS
THE DESERT SW...AND PUSHES A COLD FRONT ACROSS THE STATE. THE ECMWF
SOLUTION KEEPS THIS TROUGH FURTHER SOUTHWEST OFF THE CA COAST..BUT
STILL DROPS A COLD FRONT AND STRONG JET ACROSS THE REGION. EITHER
SOLUTION CAUSED FOR SLIGHT CHANCES FOR LIGHT SNOW TO BE PLACED IN
FOR A FEW AREAS...ALTHOUGH THE MODELS MAY NEED A FEW DAYS TO IRON
OUT THE DETAILS...AND INSPIRE HIGHER CONFIDENCE.

&&

.AVIATION.../00Z ISSUANCE/

AN APPROACHING WEAK LOW PRESSURE TROUGH IN NORTHERLY FLOW ALOFT WILL
RESULT IN INCREASING MID/HIGH CLOUDS FROM BETWEEN 00Z AND 06Z.
VFR CONDITIONS WILL CONTINUE. BETWEEN 06Z AND 18Z...EXPECT
CONSIDERABLE MID CLOUDS OVER THE LOWER ELEVATIONS WITH CEILINGS
BETWEEN 080-120 KFT AGL. ISOLATED SNOW SHOWERS WILL SPREAD ACROSS
THE NORTHWEST AND NORTHERN MOUNTAINS WHERE MVFR/IFR CONDITIONS
WILL BE POSSIBLE. THE BASINS AND VALLEYS WILL SEE FAIRLY LIGHT
WINDS THROUGH THE PERIOD...WHILE THE WIND CORRIDOR FROM KRKS TO
KCPR...AS WELL AS KDUB WILL BE BREEZY TO WINDY. LOCAL MVFR TO IFR
CEILINGS AND VISIBILITIES REDUCED IN FOG MAY BE ENCOUNTERED IN THE
VALLEYS AND BASINS FROM 08Z TO 16Z FRIDAY.

&&

.FIRE WEATHER...

GENERALLY TYPICAL WINTER CONDITIONS WILL CONTINUE TONIGHT AND FRIDAY
WITH SOME LOCAL LIGHT SNOW COMING SOUTH ACROSS THE NORTH AND WEST
MOUNTAINS ON FRIDAY.  SOME WARMING WILL MOVE INTO THE WEST ON SATURDAY
WITH WARMING SPREADING EAST INTO MONDAY. THE LOWER AIR MASS WILL REMAIN
STABLE WITH LOW MIXING HEIGHTS AND POOR SMOKE DISPERSAL.

&&

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

$$

SHORT TERM...C.BAKER
LONG TERM...ANGLIN
AVIATION...C.BAKER
FIRE WEATHER...C.BAKER





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