Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 180840

Area Forecast Discussion
National Weather Service Riverton WY
240 AM MDT Fri May 18 2018

.SHORT TERM...Today through Sunday night

The weather pattern will remain quite messy/busy today, before
quieting down over the weekend. An upper level area of low pressure
will continue to slowly move over northern Utah this morning. This
low will slowly begin to slow during the day today as it gradually
makes its way over Uinta County. At the same time, a leeside low
will develop along the Front Range. This will help turn flow to the
north-northeast and bring upslope flow to areas east of the Divide.
A frontal boundary will also be draped across S WY and will be the
focal point for the best lift and instability. Convection will be
scattered again, with storms over S and SW WY possibly lasting
through the evening hours. A second shallow cold front will also
plunge southward from MT across areas east of the Divide late in the
afternoon into the evening hours. Overall, there will be much cooler
temperatures and just make for a cloudy and cool day for much of the
forecast area. It could even get cold enough for some mountain snow,
but snow levels would be rather high and keep travel problems

As the leeside low in CO develops Friday night into Saturday, an
occluded front will form. The location of this front could bring
precipitation to the eastern portions of the forecast area or keep
it further south over SE WY. For now, will side with rain showers
occurring. The rest of the area will be similar to last Saturday,
with cool and cloudy conditions. Isolated to widely scattered
showers will be possible across the far west. Sunday will remain
mainly dry, with isolated showers and thunderstorms across the far
west to northwest portions.

.LONG TERM...Monday through Friday

The large-scale upper air pattern will remain relatively unchanged
through this extended period.  A mean ridge will remain over west-
central Canada with incoming Pacific systems splitting over/under
the ridge.  The next in this series of Pacific troughs will drop an
upper low down the California coast on Monday with this system
lifting northeast across the Great Basin on Tuesday; across Wyoming
on Wednesday.  Warmer temperatures will precede this system Monday
and Tuesday with most highs in the 70s to lower 80s across the
valleys and basins.

Instability will be slight over most of the area on Monday with
isolated to scattered afternoon/evening thunderstorms moving through
anticyclonic flow across the west and north.  On Tuesday, southerly
difluent flow aloft with low level easterly flow east of the
Continental Divide will bring better moisture, lift and instability
across the area; so some stronger, possibly isolated severe
thunderstorms will be possible across central areas.  Wednesday
looks to be the most active day across the area for shower and
thunderstorm coverage as upper low lifts northward across Wyoming.

A warmer westerly flow will then prevail on Thursday south of this
system as it continues to lift across eastern Montana. Chances of
showers and thunderstorms will remain across the north and west.
Overall trend toward Memorial Day Weekend points to the
strengthening of ridge across the northern Rockies, with incoming
Pacific storms less likely to penetrate into the western U.S.  This
scenario would result in above normal temperatures, with slight
chances of afternoon/evening thunderstorms.


.AVIATION...For the 12Z TAFS through 12Z Saturday

MVFR ceilings vicinity Cody early this morning will gradually
expand across the Big Horn Basin with VCSH through 18Z/Fri as
weak front moves southward. Scattered thunderstorms expected to
develop between 18Z-21Z across central and southern WY as
northerly flow strengthens east of the divide and runs up against
southerly flow in southern WY. By 00Z- 02Z/Sat widespread rain
showers expected across central and southern WY, especially in
upslope areas like vicinity KLND and KCPR. Heavier precipitation
moves to SE WY after 06Z/Sat. Mountains to be generally obscured
after 00Z/Sat, and VFR/MVFR mix could also see localized IFR
conditions near and around heavier precipitation Friday evening.
Prevailing MVFR conditions, local IFR conditions with areas of
light rain and fog are expected across central areas into SW
Wyoming 06z-12z Saturday.



Afternoon showers and thunderstorms will return to much of the
area today. Conditions will become cooler later this afternoon across
northern portions east of the Divide, as a cold front plunges
southward. Wind will mainly be light, with moderate northerly winds
will occur across central and eastern areas. Gusty winds are
possible around any shower or thunderstorm however. Humidity will
will stay well above critical values, but could drop into the
upper teens over extreme SE sweetwater County. Mixing and smoke
dispersal will range from poor to fair over the mountains and the
far west to good and very good central and eastern areas.
Excellent mixing and smoke dispersal will occur in eastern
Sweetwater County once again.




LONG TERM...Meunier
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