Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 220550
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
1150 PM MDT SAT MAY 21 2016

.UPDATE...Added 06z Aviation Discussion.

&&

.AVIATION.../06Z ISSUANCE/

West of the Continental Divide...KJAC/KRKS/KPNA/KBPI Routes

An upper level disturbance will spread scattered showers into the
southwest, mainly vicinity and south of a KAFO-KPNA line 12z-18z
Sunday along with areas of MVFR conditions and frequent mountain
obscurations.  Isolated thunderstorms will blossom during afternoon
heating across mainly southwest Wyoming with activity more isolated
vicinity KJAC-KDUB and northward.  Showers and thunderstorms will
diminish in the early evening with some clearing and patchy ifr/lifr
ceilings or fog forming overnight, partially obscuring western
valleys (vcnty KJAC-KPNA-KBPI).

East of the Continental Divide...KCPR/KCOD/KRIW/KWRL/KLND Routes

A line of rain showers with areas of MVFR conditions VCNTY of a KRKS-
KRIW-KGEY line at 06z will slowly drift east and dissipate by 12z,
likely not making it to KCPR beforehand.  The area should be mostly
precipitation free 12z-20z Sunday.  An upper level disturbance will
spread scattered showers and thunderstorms along and south of a KRS-
KLND-KCPR line through the afternoon with activity more isolated
further north.  Brief MVFR conditions are possible KRKS-KLND-KRIW-
KCPR through 01z Monday.  Clearing will occur with VFR conditions
prevailing 3z-12z Monday.

&&

.PREV DISCUSSION... /issued 257 PM MDT SAT MAY 21 2016/

Short Term...Tonight through Sunday

A large low pressure storm system centered over eastern Oregon
will spread strong wind and Pacific moisture north across western
and central wyoming. The air mass will be unstable with strong
dynamics supporting widespread showers and thunderstorms...
especially in the western mountains spreading east tonight and
sunday. Temperatures will be cool in the mountains with most areas
near to below freezing tonight. Sunday conditions will remain
unsettled with temperatures remaining on the cool side with
scattered showers and thunderstorms...especially over the
mountains. Snowfall is expected above 8000 feet during the day
down to 7000 feet at night with accumulations of 1 to 4 inches.
The northern Wind River and southern Absaroka mountains may see up
to 8 inches above 10000 feet. The northern Bighorn and Absaroka
mountains may see up to 6 inches of snow by sunday evening.

Long Term...Sunday night through Saturday

The week will be cool and unsettled with mainly mountain showers
and thunderstorms as a large low pressure trough remains extending
from the eastern Pacific east into the western Great Lakes. There
will be smaller low pressure centers within the large trough and
these smaller troughs will bring some variation in the timing
and coverage each day of shower and thunderstorm activity.
Temperatures will be cold enough in the mountains for snow most
days above 8000 feet...lowering to 7000 to 6500 feet at night...
especially in the western valleys. Overall no major changes to
the extended forecast.

Aviation.../00Z issuance/

West of the Continental Divide...KJAC/KRKS/KPNA/KBPI Routes

A cold front will push into central wyoming this evening with
showers...thunderstorms...gusty winds and MVFR/IFR conditions
decreasing behind the front from west to east across most of western
Wyoming. This improvement will generally take place between 03 and
07z...with mountain obscurations hanging on a bit longer. However,
this will then be followed by local IFR/LIFR ceilings and
-SHRASN developing early by 12z...especially across the Upper Green
River Basin and foothills. Isolated to scattered showers and
thunderstorms with then return to most forecast sites after 18z.

East of the Continental Divide...KCPR/KCOD/KRIW/KWRL/KLND Routes

VFR conditions will prevail through most of the forecast with some
brief periods of MVFR later this evening near TSRA/SHRA and
especially KLND after 05z. Strong south to southwest winds will
decrease this evening after 02z for most terminal locations as a
cold front spreads showers and isolated thunderstorms into central
Wyoming through the evening period. A few strong to severe
thunderstorms with large hail and damaging wind will be possible
vicinity and north and east of a KCOD-KBYG-KCPR line through 03z.
Drier air will push in behind the cold front overnight tonight with
prevailing VFR conditions expected after 04z for most
locations...09z for KLND. Then, showers and thunderstorms return to
the forecast for most sites after 18z.

Fire Weather...

A large low pressure storm system centered over eastern Oregon
will spread strong wind and Pacific moisture north across western
and central wyoming. The air mass will be unstable with strong
dynamics supporting widespread showers and thunderstorms...
especially in the western mountains spreading east tonight and
sunday. Temperatures will be cool in the mountains with most areas
near to below freezing tonight and sunday night. Relative humidity
will be elevated. With an unstable air mass and rather strong
winds most areas will have very good to excellent smoke dispersal
in the afternoon sunday. Conditions will remain unsettled with
temperatures remaining on the cool side with scattered showers and
thunderstorms...especially over the mountains into mid week.
Snowfall is expected above 8000 feet during the day down to 7000
feet at night with accumulations of 3 to 7 inches. The northern
Wind River and southern Absaroka mountains may see up to 15 inches
above 10000 feet. The northern Bighorn and Absaroka mountains may
see up to 10 inches of snow by monday evening.

&&

.RIW Watches/Warnings/Advisories...
None.
&&

$$

SHORT TERM...C.Baker
LONG TERM...C.Baker
AVIATION...AEM
FIRE WEATHER...C.Baker


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