Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 201811

Area Forecast Discussion
National Weather Service Riverton WY
1211 PM MDT Sun May 20 2018

.SHORT TERM...Today through Tuesday night
Issued at 200 AM MDT Sun May 20 2018

Overview...Weak steering flow over the region today, with a trough
developing along the CA coast today into Monday. Winds aloft will be
more southwesterly, as minor shortwaves move over the state. The
trough will begin to deepen Monday, with a closed low developing
over CA/S NV. As a consequence, warming temperatures and additional
moisture will impact the forecast area by Tuesday.

Discussion...Patchy fog/low clouds will be possible this morning.
Upper level clouds continue to clear from the area, with low
stratus remaining over for many areas. The best chances for fog
formation will be over portions of Natrona County, but could
develop across Fremont County.

Convective activity will remain isolated today and Monday, with
the focus of any showers/storms being across the west and north. A
shortwave coming up from the south will help generate isolated
showers/storms over the west and the far south. PW`s will remain
near 0.50", so locally heavy rain will be the main threat from
these storms. Similar conditions will be in Place for Monday, with
the focus for afternoon storms being over the west and north.
Locally heavy rain will once again be the main threat, as mid and
upper level winds remain light (5-10kt). By Tuesday, the closed
low slowly lifts northward over E NV, keeping the moist southerly
flow going over WY. The nose of the channel jet will make its way
over southern and eastern portions of the forecast area, with the
left front quadrant over Natrona County by late afternoon. Cape
values of 1000-1500 J/kg and LI`s of -6 to -8, strong thunderstorms
will likely occur over central portions. A scenario similar to
this past Friday afternoon/evening could occur, with an area of
thunderstorms with heavy rain from south-central to northeastern
portions. These storms will then lift northward through the
overnight hours Tuesday night, with brief periods of heavy
rainfall possible.

.LONG TERM...Wednesday through Sunday
Issued at 200 AM MDT Sun May 20 2018

Wednesday still looks to see scattered showers and thunderstorms
over most of the region as the weather system moves across the
region and into Montana Wednesday night. Would expect lingering
showers or storms over the north through late Wed night. A west to
northwest flow aloft for Thursday. Plenty of lingering moisture
around for the formation of showers and afternoon and evening
thunderstorms over the west and north. Less coverage of storms in
the central and south. Northwest flow aloft for Friday with less
coverage of afternoon and evening thunderstorms, mostly over and
near the mountains. The first look at Memorial Day weekend would
feature isolated afternoon and evening showers and thunderstorms
over and near the mountains each day. Lower elevations would be
mostly dry with very isolated thunderstorm activity. As for high
temps, 60s and 70s will occur Wed and Thurs. Then Fri will see 70s
to lower 80s. Saturday and Sunday will see mid 70s to mid 80s
with 70s to lower 80s on Memorial Day (Monday the 28th). Mountain
highs will gradually warm Thurs through next weekend with high
elevation snow melt. Then the expected rises of streams and
rivers would occur. Plenty of time to watch that scenario play


.AVIATION...For the 18Z TAFS through 18Z Monday Issued at 200 AM
MDT Sun May 20 2018

A stratocumulus deck over the Wind River Basin at 3K will
dissipate from south to north through 20Z. Afternoon convection
will fire over the higher terrain of the west after 21Z this
afternoon. KJAC has the best shot at late day and early evening
showers. Have indicated VCSH in terminals west of the Divide
between about 23Z/Sun and 05Z/Mon, with dry conditions region wide
after that time.


Issued at 200 AM MDT Sun May 20 2018

Isolated showers and thunderstorms possible across western and
southern portions later this afternoon and evening. Relative
humidity will not be as high compared to the last couple of days,
but will still be above critical values. Isolated showers and
thunderstorms will develop over western and northern portions
Monday afternoon and evening. Showers and thunderstorms will
develop across southern portions and the higher elevations by mid
afternoon Tuesday, becoming widespread late in the afternoon into
the evening. Locally heavy rain will be possible as these storms
move northward Tuesday night. Mixing and smoke dispersal today
will range from fair to very good across the area.





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