Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 181903
AFDRIW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RIVERTON WY
103 PM MDT MON MAY 18 2015

.SHORT TERM...TONIGHT THROUGH TUESDAY

COOL WET WEATHER WILL INCREASE OVER WESTERN AND CENTRAL WYOMING
TONIGHT AND TUESDAY. LOW LEVEL COOL NORTHEAST FLOW WILL BE OVERRAN
BY WARMER MOIST AND SOMEWHAT UNSTABLE AIR FROM AN UPPER LEVEL LOW
OVER THE GREAT BASIN. THIS EVENING ISOLATED THUNDERSTORMS WILL BE
POSSIBLE OVER THE SOUTH AND WEST. THROUGH TUESDAY MORNING CLOUDS
AND SHOWERS WILL BECOME MORE WIDESPREAD WITH SNOW ABOVE 8500 FEET.
SOME MOUNTAIN AREAS CAN EXPECT 3 TO 5 INCHES OF SNOW...MAINLY
ABOVE 9000 FEET...IN THE WIND RIVER...BIGHORN AND SOUTHERN
ABSAROKA MOUNTAINS. RAINFALL OF A QUARTER TO A HALF OF AN INCH IS
EXPECTED OVER AND NEAR THE EAST SLOPES OF THE MOUNTAINS WITH LOCAL
ONE INCH AMOUNTS POSSIBLE WITH ISOLATED INCH AND A HALF AMOUNTS
IN THE FOOTHILLS OF THE BIGHORN AND WIND RIVER MOUNTAINS.

.LONG TERM...TUESDAY NIGHT THROUGH MONDAY

THE LARGE HIGH OVER WESTERN CANADA WILL CONTINUE TO BLOCK A
SUCCESSION OF PACIFIC LOWS AND SPLIT THEM UP AS THEY MOVE EAST
TOWARD THE CANADIAN MAINLAND. THE PORTION OF THE ENERGY THAT GETS
DIVERTED SOUTH WILL CONTINUE TO BE OUR CONCERN AND WILL KEEP US
RELATIVELY COOL AND WET THROUGH THE REST OF THE WEEK.
PRESENTLY...THE PROMINENT LOW PRESSURE CENTER IS SPINNING OVER
48N/152N AND HAS SPUN OFF A SOUTHERN STREAM CIRCULATION NOW OVER
SOUTHERN CALIFORNIA. THIS CLOSED LOW WILL AGAIN TRACK TO WYOMING BY
TUESDAY NIGHT. THE MODELS SEEM TO BE TRACKING THIS LOW ABOUT 6 HOURS
FASTER THAN PREVIOUS RUNS. HAVE PEGGED THE PERIOD FROM 00Z THROUGH
06Z WED AS THE APEX OF THE EVENT. THE PRECIPITATION FROM THIS EVENT
WILL CONTINUE THROUGH TUESDAY NIGHT AND GRADUALLY END FROM NORTHWEST
TO SOUTHEAST AS THE LOW EXITS THE AREA TO THE EAST. A COUPLE OF
CIRCULATIONS AT 700MB ON BOTH SIDES OF THE DIVIDE WILL ENHANCE
CONVERGENCE AND LIFT TUE NT. THE OLD STATIONARY FRONT THAT HAS BEEN
LODGED ALONG THE EAST SIDE OF THE DIVIDE DURING THE LAST COUPLE OF
DAYS WILL BE REINFORCED AS MORE COLD AIR WRAPS AROUND THE BACKSIDE
OF THE LOW SUFFICIENT ENOUGH TO BRING THE SNOW LEVEL DOWN TO ABOUT
6K OVER THE BIG HORN MTNS AND 7K OVER THE WIND RIVER MTNS TUESDAY
EVENING IF ONE AVERAGES THE H7 TEMPS OF THE NAM/GFS/EURO AND THAT
AVERAGE IN AND OF ITSELF HAS BEEN FAIRLY CONSISTENT. AS FOR SNOW
AMOUNTS...IT LOOKS LIKE 3 TO 5 INCHES OF SNOW COULD FALL IN THE WIND
RVRS/BIG HORNS ABOVE THE AFOREMENTIONED ELEVATIONS BY WED MORNING.
WILL CONTINUE TO HANDLE THIS ELEVATED SNOW EVENT WITH AN SPS FOR
NOW. ON WEDNESDAY AREAS EAST OF THE DVD WILL BEGIN TO WARM UP AND
DRY OUT AS A TRANSITORY RIDGE BUILDS INTO THE AREA. BUT A SOUTHERLY
DIFLUENT FLOW WILL QUICKLY BECOME ESTABLISHED BY WED AFTERNOON ON THE
BACKSIDE OF THE RIDGE AND AHEAD OF THE NEXT LOW CENTER WHICH BY THEN
WILL BE OFF THE OREGON COAST. THIS WILL RESULT IN LATE DAY
THUNDERSTORMS MAINLY WEST AND SOUTH OF THE DIVIDE. THEN BY
THURSDAY...THE OLD SURFACE BOUNDARY WILL RETREAT TO THE NORTHEAST OF
THE CWA ALLOWING AREAS EAST OF THE DIVIDE TO WARM UP INTO THE 60S.
WITH INSTABILITY SPREADING ACROSS THE REST OF THE CWA TO INCLUDE
AREAS EAST OF THE DVD SO WILL MENTION LATE DAY T STORMS EVERYWHERE
THU. ON FRIDAY THAT NEXT LOW CENTER WILL AGAIN TRACK ACROSS THE
GREAT BASIN AND TOWARD THE CENTRAL ROCKIES FOR LATER FRIDAY...FRIDAY
NIGHT AND SATURDAY. NUMEROUS SHOWERS CAN BE EXPECTED FRIDAY AND
SATURDAY WITH THIS WEATHER SYSTEM...HOWEVER IT WILL BE A WARMER
SYSTEM THAN THE ONE THIS TUESDAY NIGHT. THIS LOW WILL BE OVER THE
FOUR CORNERS REGION 12Z SATURDAY AND SOUTHEAST WY BY 06Z SUNDAY.
THEN WE GET A SHORT RESPITE FROM THE PRECIP LATER SUNDAY AND SUNDAY
NIGHT BEFORE THE NEXT LOW BRINGS MORE SHOWERS AND THUNDERSTORMS FOR
MONDAY BUT FOR NOW HAVE LEFT POPS RELATIVELY LOW FOR MONDAY DUE TO
THE UNCERTAINTY OF THE PLACEMENT OF THE LOW AS THE GFS PLACES IT
OVER THE PAC NW AND THE EURO IN IDAHO. ALL BETS ARE THAT MONDAY WILL
BE WET KEEPING WITH CONSISTENCY OF THE BLOCKING HIGH REMAINING OVER
WESTERN CANADA CONTINUING TO DIVERT THE LOW CENTERS TOWARD THE
CENTRAL ROCKIES. THIS ALSO GOES IN LINE WITH KEEPING WITH
TRADITION...HENCE A COOL WET MEMORIAL WEEKEND FOR WESTERN AND
CENTRAL WYOMING. AS A SIDE NOTE...ANY THUNDERSTORMS THAT FORM THIS
WEEK WILL LIKELY PRODUCE SMALL HAIL CONSIDERING THE LOW WET BULB
ZERO.

&&

.AVIATION.../00Z ISSUANCE/

AREAS OF MVFR TO IFR CEILINGS WITH RAIN SHOWERS AND PATCHY FOG OVER
AND NEAR THE MOUNTAINS WILL OBSCURE THE HIGHER MOUNTAINS. SHOWERS
AND ISOLATED THUNDERSTORMS WILL BE ALONG AND WEST OF THE DIVIDE AND
OVER THE SOUTH THROUGH 06Z TUESDAY. THROUGH TUESDAY MORNING...MVFR
TO IFR CEILINGS AND VISIBILITY REDUCED IN RAIN AND FOG WILL
INCREASE WITH WIDESPREAD MOUNTAIN OBSCURATIONS SPREADING ACROSS
THE BASINS AND PLAINS. SOME SNOW CAN BE EXPECTED ABOVE 8500
FEET...ESPECIALLY IN THE WIND RIVER AND BIGHORN MOUNTAINS. MVFR
CONDITIONS ARE MOST LIKELY AT KCPR...KLND...KCOD...KRKS...KBPI
AND KPNA.

&&

.FIRE WEATHER...

COOL WET WEATHER WILL BE OVER WESTERN AND CENTRAL WYOMING TONIGHT
AND TUESDAY. LOW LEVEL COOL NORTHEAST FLOW WILL BE OVERRAN BY
WARMER MOIST AND SOMEWHAT UNSTABLE AIR FROM AN UPPER LEVEL LOW OVER
THE GREAT BASIN. THIS EVENING ISOLATED THUNDERSTORMS WILL BE
POSSIBLE OVER THE SOUTH AND WEST.  THROUGH TUESDAY MORNING CLOUDS
AND SHOWERS WILL BECOME MORE WIDESPREAD WITH SNOW ABOVE 8500 FEET.
MIXING HEIGHTS WILL LOWER THROUGH TUESDAY WITH POOR SMOKE DISPERSAL
OVER AND NEAR THE MOUNTAINS IN THE AREAS OF PRECIPITATION. SOME
AREAS WEST OF THE CONTINENTAL DIVIDE MAY SEE GOOD SMOKE DISPERSAL
WITH HIGHER MIXING HEIGHTS IN THE AFTERNOON.  RELATIVE HUMIDITY WILL
BE HIGH ACROSS THE AREA WITH GENERAL EXCELLENT RECOVERY TONIGHT.
SOME MOUNTAIN AREAS CAN EXPECT 3 TO 5 INCHES OF SNOW...MAINLY
ABOVE 9000 FEET...IN THE WIND RIVER...BIGHORN AND SOUTHERN ABSAROKA
MOUNTAINS.

&&

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

$$

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








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