Area Forecast Discussion
Issued by NWS Riverton, WY

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000
FXUS65 KRIW 260936
AFDRIW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RIVERTON WY
236 AM MST THU FEB 26 2015

.SHORT TERM...TONIGHT THROUGH SATURDAY NIGHT

THE SNOW DISTRIBUTION PATTERN EAST OF THE DIVIDE EARLIER THIS
AFTERNOON WAS AN INTERESTING UPSLOPE/CONVECTIVE SNOW BANDING HYBRID
PATTERN WITH MOIST LOW LEVEL NORTHEAST SFC FLOW AND AN UNSTABLE NW
FLOW A FEW THOUSAND FEET ABOVE THAT AT 700MB. A REINFORCING PUSH OF
COLD AIR WHICH INCREASED THE NORTHEAST WINDS FOR A COUPLE OF HOURS
AROUND SUNSET EARLIER THIS EVENING HAS BLOWN THROUGH WHICH
TEMPORARILY ENHANCED THE SNOW BANDING ACTIVITY...ESPECIALLY IN THE
WIND RIVER BASIN WHERE WIND SPEED CONVERGENCE WAS TAKING PLACE
WHERE THE POST FRONTAL WINDS WERE SLOWING DOWN. CURRENTLY...AN
INTERESTING PHENOMENON IS TAKING PLACE OVER THE WIND RIVER BASIN.
THE SURFACE WINDS HAVE VEERED TO THE EAST BUT THE WINDS AT 700MB
ARE STILL OUT OF THE NORTHWEST. A PERSISTENT AREA OF CONVERGENCE
AT 700MB OR SO ORIGINATING FROM NEAR ANCHOR DAM HAS BEEN STREAMING
SNOW SHOWERS TOWARD THE SOUTHEAST TOWARD RIVERTON ALL EVENING AND
SINCE MIDNIGHT. HOWEVER...THE EAST WINDS ARE CAUSING THE SNOW
SHOWERS TO ENHANCE FROM SOUTHEAST TO NORTHWEST TOWARD CROWHEART
CAUSING THE SNOW SHOWERS TO PROPAGATE BACKWARDS TOWARD THE
NORTHWEST EVEN THOUGH THE INDIVIDUAL SNOW SHOWER ELEMENTS ARE
TRACKING SOUTHEAST. OVERALL...THE REMAINING SNOW BANDS ARE
DWINDLING NOW THAT THE H7 TEMPS ARE DECREASING AND ARE NOW TOO
COLD TO SUPPORT IDEAL DENDRITIC GROWTH.

THEN THE NEXT SHORTWAVE IN THE NORTHWEST FLOW WILL SCOOT ACROSS
NORTHEAST WY THIS EVENING WITH PVA OUT AHEAD OF IT TODAY. THIS WILL
EXPAND THE COVERAGE OF LIGHT SNOW ACROSS AREAS EAST OF THE DIVIDE
TODAY BUT LESS ACCUMULATION IS EXPECTED TODAY THAN ON WEDNESDAY
SINCE LESS MOISTURE WILL BE AVAILABLE ADVECTING IN FROM INTERIOR
CANADA AND JET DYNAMICS WILL BE LACKING. WEST OF THE DIVIDE...ENOUGH
WEAK INSTABILITY SHOULD BE AROUND FOR MORE SNOW SHOWERS THERE. THEN
TONIGHT THE FOCUS OF ANY SNOWFALL WILL PIVOT TO SOUTHWEST WY CLOSER
TO THE SOUTHWESTWARD MOVING SOUTHERN JET ENERGY. ON FRIDAY...SNOWFALL
WILL RETURN TO THE FAR WEST AS THE NEXT TROUGH IN THE PACIFIC
NORTHWEST MOVES SOUTHEAST TOWARD THE GREAT BASIN. THE MODELS HAVE
BEEN WAFFLING WITH THE DYNAMICS OUT AHEAD OF THIS LOW FOR SATURDAY
WITH THE GFS ADVERTISING A BROAD FIELD OF ISENTROPIC LIFT ACROSS
CENTRAL AND SOUTHERN WY SATURDAY BUT NOT THE ECMWF. THEN BOTH MODELS
TOOK ALL THE ENERGY TO THE SOUTH AND EAST OF OUR CWA. NOW THE GFS
WANTS TO PAINT A BROAD AREA OF ISENTROPIC LIFT OVER THE AREA ONCE
AGAIN AND THE ECMWF ONLY SOUTHWEST WY. THIS ISENTROPIC LIFT WOULD
SEED A LOWER DECK OF CLOUDS RESULTING FROM LOW LEVEL UPSLOPE EAST
OF THE DIVIDE. WILL KEEP THE IDEA OF A BROAD AREA OF CHC SNOW
OVER THE CWA EAST AND WEST OF THE DIVIDE FOR SATURDAY FOR NOW. ANY
SNOWFALL WILL END FROM NORTHWEST TO SOUTHEAST SATURDAY NIGHT.

THIS MORNING SHOULD BE THE COLDEST PERIOD FOR SOME LOCATIONS THAT
MANAGE TO CLEAR OUT 24 HOURS AFTER THE 700MB TEMPS BOTTOMED OUT AT
ROUGHLY -23C OVER JOHNSON COUNTY THURSDAY MORNING...THE TYPICAL
RADIATIONAL COOLING SURFACE COLD TEMPERATURE LAG. SEASONABLY COLD
WEATHER CAN BE EXPECTED FOR THE REST OF THE WEEK. THE COLD FRONT
ASCT WITH THIS SATURDAYS WEATHER FEATURE WILL LEAVE BEHIND -17C
700MB AIR OVER NORTH CENTRAL WY BY 12Z SUNDAY RESULTING IN A FEW
MORE BELOW ZERO READINGS BY LATE SATURDAY NIGHT.

.LONG TERM...SUNDAY THROUGH THURSDAY

MODELS IN EXCELLENT AGREEMENT TONIGHT ALTHOUGH THE EURO SLOWS OUR
TUE/WED SYSTEM DOWN AND TRIES TO CLOSE IT OFF NRN UT BRIEFLY EARLY
WED. SATURDAY`S SYSTEM DIGS FURTHER INTO FAR SRN CA WITH MOST OF
THE AREA CLEARING OUT BY THE START OF THE PERIOD. ONLY THE FAR
SOUTH COULD REMAIN ON THE NRN FRINGES OF THIS SYSTEM. CLOUDS IF
NOTHING ELSE WITH A LITTLE POTENTIAL FOR SOME LINGERING SNOW IN
THE FAR SRN SECTIONS. COLD START MONDAY IF THERE`S ENOUGH OF A
BREAK IN CLOUD COVER BEFORE OUR NEXT SYSTEM COMES QUICKLY IN LATER
MONDAY NIGHT THROUGH TUESDAY. THIS SYSTEM ORIGINATES OVER
KAMCHATKA AND COMES IN AT A COLD...HIGH LATITUDE. GFS/EURO IS
BRINGING A FAIRLY SHARP BUT NOT QUITE CLOSED OFF TROUGH INTO THE
GREAT BASIN TUESDAY...HEADING FOR COLORADO BY WEDNESDAY.
INCREASING FORCING AND DECENT LAPSE RATES AHEAD OF THE FRONT
SHOULD GET SOME SNOW GOING AS THIS SYSTEM DROPS SWD. COULD BE A
DECENT PERIOD OF FORCING ACROSS THE AREA WITH DECENT CENTRAL ZONE
UPSLOPE. STILL ALONG WAY AWAY BUT POTENTIAL FOR AT LEAST SOME
HIGHLIGHTS ARE CERTAINLY THERE...AT LEAST WINTER WX ADVISORIES. IF
IT SLOWS DOWN/TRIES TO CLOSE OFF MORE LIKE THE EURO HINTS AT...COULD
BE EVEN MORE INTERESTING. VERY COLD...CYCLONIC FLOW PEAKS ON
WEDNESDAY WITH GFS AND ECMWF SHOWING -24C TO -25C AT H7 IN CENTRAL
WYO. WARMING ALOFT WEDNESDAY NIGHT WHICH MAY RESULT IN SOME FOG IN
THE NORMAL AREAS. INCREASING NW FLOW POSSIBLE BY THURSDAY WITH
SOME INCREASE MOUNTAIN WIND AND POSSIBLY THE WIND CORRIDOR/CODY
FOOTHILLS BUT STILL RATHER CHILLY TEMPS ALOFT.

&&

.AVIATION.../12Z ISSUANCE/

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

VFR TO MVFR CONDITIONS WILL OCCUR TODAY THROUGH 02Z FRIDAY AS AREAS
OF LIGHT SNOW AND LOWER CLOUDS ARE EXPECTED. SOME MOUNTAIN
OBSCURATION AT TIMES THROUGH 02Z FRIDAY. AFTER 02Z VFR CONDITIONS
WILL PREVAIL OVER THE AREA.

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

VFR TO MVFR CONDITIONS WILL OCCUR OVER SOUTHWEST WY TODAY THROUGH
01Z FRIDAY AS AREAS OF LIGHT SNOW AND LOWER CLOUDS ARE EXPECTED.
SOME MOUNTAIN OBSCURATION AT TIMES THROUGH 01Z FRIDAY. AFTER 01Z VFR
CONDITIONS WILL PREVAIL OVER THE REGION.

&&

.FIRE WEATHER...

TODAY WILL FEATURE AREAS OF LIGHT SNOWFALL EAST OF THE DIVIDE
TODAY WITH SOME SNOW SHOWERS WEST WITH NORTHWESTERLY FLOW
CONTINUING. HOWEVER...PINPOINTING EXACTLY WHERE THE SNOW WILL FALL
WILL BE RATHER DIFFICULT. WITH FRESH SNOW COVER IN SOME OF THE
BASINS...TEMPERATURE INVERSIONS WILL BE IN PLACE THIS MORNING RESULTING
IN LOW MIXING HEIGHTS AND POOR SMOKE DISPERSAL. THERE IS A GOOD
CHANCE OF SNOW IN THE WEST FRIDAY...POTENTIALLY SPREADING TO
AREAS EAST OF THE DIVIDE SATURDAY. MORE COLD WEATHER IS EXPECTED
BEHIND SATURDAYS WEATHER FEATURE.

&&

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

&&

$$

SHORT TERM...LIPSON
LONG TERM...PS
AVIATION...AR
FIRE WEATHER...LIPSON











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