Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 230553 AAA
AFDRIW

Area Forecast Discussion...UPDATED
National Weather Service Riverton WY
1153 PM MDT SUN MAY 22 2016

.UPDATE...Added 06z aviation discussion.

&&

.AVIATION.../06Z ISSUANCE/

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

An upper level low pressure system over south central Canada will
continue to produce isolated to scattered rain and snow showers and
higher mountain obscurations over the northwest mountains, VCNTY of a
KJAC-KDUB line and north through the period.  To the south, an upper
level disturbance will spread scattered showers and possible a
few thunderstorms into VCNTY KRKS-KPNA-KBPI 18z Monday-01z Tuesday
along with occasional mountain obscurations.  This activity will
move out of the area or dissipate with sunset.

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

LLWS will continue in the VCNTY of KCOD through 12z before ridgetop
winds diminish.  The two main culprits of showers on Monday
afternoon will be an upper level low pressure center over south
central Canada.  This system will produce isolated showers in the
VCNTY of KCOD with mountains to the west occasionally obscured.  The
second system will move across the south producing scattered showers
and possibly a few thunderstorms mainly VCNTY and south of a KLND-
KCPR line along with occasional mountain obscurations.  Most of this
activity will dissipate with sunset Monday evening.

&&

.PREV DISCUSSION... /issued 236 PM MDT SUN MAY 22 2016/

Short Term...Tonight THROUGH Monday

A large long wave low pressure trough will dominate the weather
across western and central wyoming through Monday. The trough
extends from the eastern Pacific off the west coast...east across
the great plains into the midwest. Smaller short wave low pressure
troughs rotating through the main trough will bring areas of
enhanced showers and embedded thunderstorms to the area. This
afternoon through Monday...showers and thunderstorms are
expected...especially over the mountains. The daily variation in
coverage will be largely determined by the timing of these short
wave low pressure troughs. This long wave trough will keep cool
air over the area with freezing temperatures expected in the
mountains and at times in the western valleys where lows will
range from the mid 30s to lower 30s. Most of the highs will be in
the 40s to lower 50s in the western valleys and foothills with the
higher mountains remaining in the 30s to lower 40s. These cool
temperatures will retard the snow melt across the western and
northern mountains. The showers...which will be in the form of
snow above 9000 feet with vary from rain to snow from 9000 to 7000
feet through the day Monday. Snow accumulation of 3 to 7 inches
is expected from 8500 feet and above in the Wind River...Teton...
Gros Ventre...Wyoming/Salt River and Absaroka mountains with 2 to
4 inches in the Bighorn mountains.  This pattern will remain
breezy to windy.  Thunderstorms will have small hail local heavy
rain and strong wind gusts.

LONG TERM...Monday night THROUGH Sunday

A large long wave low pressure trough will dominate the weather
across western and central wyoming through the week.  The trough
extends from the eastern Pacific off the west coast...east across
the great plains into the midwest.  Smaller short wave low pressure
troughs rotating through the main trough will bring areas of
enhanced showers and embedded thunderstorms to the area.  Each day
showers and thunderstorms are expected...especially over the
mountains.  The daily variation in coverage will be largely
determined by the timing of these short wave low pressure troughs.
This long wave trough will keep cool air over the area with freezing
to near freezing temperatures expected in the mountains and at
times in the western valleys where lows will range from the mid
30s to lower 30s. Most of the week highs will be in the 40s to
lower 50s in the western valleys and foothills with the higher
mountains remaining in the 30s to lower 40s. lower elevations will
warm into the mid 60s to lower 70s and may warm about ten degrees
to the 70s and lower 80s in the north friday. These cool
temperatures will retard the snow melt across the western and
northern mountains. The showers...which will be in the form of
snow above 9000 feet with vary from rain to snow from 9000 to 7000
feet through the days. Areas of snow accumulation of 2 to 5 inches
is expected from 8500 feet and above each day through the period.
This pattern will remain breezy to windy varying from the
southwest to west.

AVIATION.../00Z ISSUANCE/

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

Mostly VFR conditions to prevail through the forecast period. An
upper level disturbance will continue to spread scattered showers
into the region this evening along with areas of MVFR conditions and
mountain obscurations. Isolated thunderstorms will accompany this
activity...mainly south and southwest Wyoming...with thunderstorm
activity more isolated to the north. the showers and thunderstorms
will then diminish later this evening with some clearing. Patchy
ifr/lifr ceilings or fog may form overnight, particularly vcnty KJAC-
KBPI-KRKS. LLWS may be an issue near KCOD 05 - 12z. Isolated to
widely scattered showers and thunderstorms return to the forecast
after 20z.

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

Mostly VFR conditions to prevail through the forecast period. An
upper level disturbance will continue to spread scattered showers
and thunderstorms south of a KLND-KWRL-KCPR line this evening...with
activity more isolated to the north of this line. Expect brief MVFR
conditions near and south of this line through 01z...with generally
clearing skies prevailing 02z-06z. Isolated showers and
thunderstorms return to the forecast after 22z...mainly over/near
the mountains and along/near/south of a boundary between South
Pass and Midwest...including both KLND and KCPR.

FIRE WEATHER...

A large long wave low pressure trough will dominate the weather
across western and central wyoming through the week.  The trough
extends from the eastern Pacific off the west coast...east across
the great plains into the midwest.  Smaller short wave low pressure
troughs rotating through the main trough will bring areas of
enhanced showers and embedded thunderstorms to the area.  Each day
showers and thunderstorms are expected...especially over the
mountains.  The daily variation in coverage will be largely
determined by the timing of these short wave low pressure troughs.
This long wave trough will keep cool air over the area with freezing
temperatures expected in the mountains and at times in the western
valleys where lows will range from the mid 30s to lower 30s.
Most of the week highs will be in the 40s to lower 50s in the
western valleys and foothills with the higher mountains remaining in
the 30s to lower 40s.  These cool temperatures will retard the snow
melt across the western and northern mountains.  The showers...which
will be in the form of snow above 9000 feet with vary from rain to
snow from 9000 to 7000 feet through the days. Snow accumulation of 5
to 10 inches is expected from 8500 feet and above through tuesday
in the Wind River...Teton...Gros Ventre...Wyoming/Salt River and
Absaroka mountains with 3 to 6 inches in the Bighorn mountains.
Relative humidity will remain elevated. This pattern will remain
breezy to windy. Mixing heights will be high enough during the
afternoons for good smoke dispersal except in areas of more
widespread shower activity where smoke dispersal will be poor.

&&

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

$$

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



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