Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 292202

Area Forecast Discussion
National Weather Service Riverton WY
402 PM MDT SUN MAY 29 2016


The most prominent thunderstorms over the CWA have erupted across
southern Sweetwater County this afternoon where capes of 1000J/Kg
and lifted indices are -3. High and mid level moisture will be
somewhat meager so not expecting much in the way of wetting rains
in those areas in spit of the fact that these storms will be
relatively slow movers. more isolated activity has been occuring
farther north west of the divide.

On Monday, the vort tracking east across Oregon at the moment will
provide some lift for some shower activity for Monday initially.
But then the more significant weather maker, a prominent shortwave
trough being ejected by the strong circulation spinning well to
the north along the BC coast, will track across Montana Monday and
then colse close off resembling the closed low that tracked east
across Montana earlier this week and provided the last cool wet
episode. This will result in increased thunderstorm
activity...mainly over the north half of Wyoming Monday afternoon.
The associated trough and cold front will be coming through the
eastern zones at the most favorable time later on Monday where the
best capes will be positioned. There is only a slight chance of
large hail/damaging winds across eastern johnson County. With the
directional shear associated with this weather feature...a
possible tornado cannot be completely be ruled out in this area.
Drier air will quickly filter in behind the trough passage from
the west, The associated strong cold front swinging around the
backside of the Montana low will track south across northern
Wyoming Monday afternoon and then push through LND/RIW/CPR around
00z Tuesday. Some of the strongest post frontal winds with this
front are expected along the mouth of the Wind River Canyon and
the Wind River Basin where 8mb 3 hourly sfc pressure rises are
progged. BYG will also become blustery. Then it will be a race
between the timing of the diminishing shower activity along with
the cooling temperatures to support snowfall. It appears that the
rain showers will briefly turn to snow in the Wind River Mountains
and Casper Mountains at and above 8000 feet later Monday night.
The snow level should descend to 7000 feet over the Absaroka/Big
Horn Mountains Monday night with the northern Big Horns standing
the best chance to accumulate;ate 2 to 3 inches of snow above
100000 feet where the residence time of the coldest air and
precipitationwill be the longest. Wednesday will see a few more
showers as the low exits to the northeast along with the coolest
high temperatures for the short term. By Wednesday morning, the
1021mb sfc high will be overhead resulting in good radiational
cooling with frost and freezing temperatures likely in the western
valleys and basins and mid 30s expected in portions of the basins
east of the divide. Then as a prominent upper high pressure system
bulges east into the area, we will experience the beginning of the
most significant warming trend of the season thus far which will
eventually likely result in significant melting of our mountain
snow pack.


Medium range covered by gradually building ridge over the Great
Basin and Inter-Mountain West. End result will be warm to very warm
conditions, Isolated late day thunderstorms possible mainly in
and downwind (se) of the mtn ranges, melting snow and rising
streams and rivers. Previous fcst has been indicating the best
areas of tstms in this kind of pattern and made little change to
the going fcst. Isold storms mainly near the nrn mtns late Thu and
Fri with daytime heating and a few ripples rotating down (se) the
front side of the ridge. As the ridge sharpen more towards the end
of the period to the west, we may see a few storms further west in
Western Wyoming as moisture and weak ripples drop down a little
further west. Overall though, just a very warm and dry period with
any storms that do form producing mainly strong, gusty wind
downwind of the mtns and limited mtn pcpn. These weak disturbances
do appear to have some weak fronts associated with them that could
have some `cooling` effect in the far nrn zones late in the period
so we`ll just have to watch the temps in say Johnson County with
time. The central to swrn zones will see little effect from these
shallow fronts with very warm conditions prevailing through the



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

Isolated showers and thunderstorms will prevail across the area into
this evening. The strongest storms could have brief heavy rain,
small hail and wind gusts over 40 mph through 04z. Only a few higher
mountain showers may continue overnight. Overall, Saturday will be
drier with most areas dry. Only a few mainly mountain showers or a
brief thunderstorm are possible Saturday afternoon.

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

Isolated shower and thunderstorm activity will prevail through 04z.
Isolated light rain showers may persist through 06z. Saturday will
be dry for most of the lower elevations with Johnson County being
the main exception as a cold front brings a little better chance of
afternoon showers and thunderstorms.



Fire danger low as green-up and seasonally cool temperatures with
elevated humidity levels continue across most of Wyoming. Isolated
to widely scattered showers and storms possible through the
evening period across mainly southern Wyoming with little overall
rainfall expected with any of this activity and less than a tenth
of an inch of precipitation at the best locations in the
southwestern/west central mountains. Winds will flow out of the
west to southwest west of the Divide at less than 15 mph, except
near stronger showers and storms which could put down gusts up to
40 mph. Wind will be mostly light and variable east of the divide.
Monday will be a little cooler across the forecast
area...especially across northern Wyoming. Chances for showers and
thunderstorms will increase east of the divide as an upper level
disturbance and a couple of cold fronts move across the region. A
few storms may become strong to severe over Johnson county by late
afternoon and early evening. Rain showers will turn to snow
showers briefly in the mountains east of the divide Monday night.
Tuesday will be cooler than Monday with some shower activity east
of the divide. Expect freezing temperatures in the basins and
valleys west of the divide Wednesday morning. Then expect a
warming trend for the rest of the week beginning Wednesday
afternoon through the rest of the week with significant snow melt.


.RIW Watches/Warnings/Advisories...


LONG TERM...Skrbac
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