Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 241936
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
136 PM MDT Wed May 24 2017

.SHORT TERM...(This evening through Saturday)
Issued at 137 PM MDT Wed May 24 2017

A Digging Pacific trough tracking across southwest Canada and the
Pacific Northwest will push into the Northern Rockies spreading
clouds and scattered showers to the western and northern mountains
in the evening with isolated thunderstorms. West to southwest
winds will be quite strong ahead of the low pressure trough
being windy across the south and over and near the east slopes of
the mountains. Part of the trough will dig south into the
northern Great Basin while the main closed low circulation tracks
east across the Canadian Prairies. This will keep some showers,
mainly over the mountains, Tonight with a cold front moving into
the north. The Canadian portion of the low pressure trough will
stall out over southern Saskatchewan as a second cold front dives
south across Montana into northern Wyoming on Thursday with
increasing showers and isolated afternoon and evening
thunderstorms. Cooling will be widespread as the cold fronts move
through the state. Thursday night and Friday northeast upslope
flow will extend across the area with low stratus and showers
expected to be rather widespread with snow above 7500 feet.
Isolated thunderstorms cannot be counted out Friday afternoon.
Much of the lower elevations will have highs in the 50s, with
lower to mid 60s over the Bighorn basin. The mountains will have
highs in the 40s with 30s above 9000 feet. Saturday cold northerly
flow aloft will remain but moisture will become more limited with
less coverage of showers.

.LONG TERM...(Saturday night through Wednesday)
Issued at 137 PM MDT Wed May 24 2017

The overall pattern for the extended looks to be fairly similar with
a mean ridge over the intermountain west and a long wave trough over
the northern plains/Great Lakes region.  The result for western and
central WY will be near normal temperatures Sunday with gradual
warming each day through mid week.  In addition, each afternoon and
evening should see a slight chance of showers and thunderstorms,
mainly over the higher terrain. The driving surface conditions will
be northerly flow over the NE part of the state (closer to the low
pressure).  This pattern essentially sets up a stationary boundary
over the continental divide which will be modulated by weak waves of
energy circulating southward out of Canada, which are not well
detailed at this time.

For Memorial Day, conditions should be quite nice across the area
with wind speeds generally below 10-15mph, temperatures near normal,
with some mountain showers and storms, and a higher likelihood from
Johnson into Natrona County.  For Tuesday into Wednesday, the GFS
tries to bring in more energy from the PacNW and dampen the ridge,
while the other models are keeping the ridge fairly stationary.

&&

.AVIATION...(For the 18Z TAFS through 18Z Thursday afternoon)
Issued at 137 PM MDT Wed May 24 2017

As a cold front approaches from the northwest, the surface pressure
gradient has increased across WY, thus strengthening surface wind to
15-20 kts with gusts 25-35 kts.  Wind dir is mostly west, but some
local forcing is from the south (KGEY, KWRL) before the front pushes
through.  Expect a wind shift to the northwest from 01z-03z/Thu
across much of the area, with wind speeds also decreasing after
03z/Thu.  Isolated showers and thunderstorms are likely between
21z/Wed-00z/Thu across SW WY and NE WY - which is decreased from
yesterday`s forecast.  Expecting activity to continue south of line
from KBPI to KCPR through 06Z/Thu and some lingering showers during
the overnight hours.  Some showers will linger after 06Z Thursday,
with the NAM being rather strong with precipitation across southern
WY. While VFR conditions generally expected, some MVFR possible due
to reduced ceilings and visibility. Outflow from the convective
activity likely to create some gusts around 50 kts.  Brief clearing
area moves through from the northwest between 12z-18z/Thu, followed
by another shot of cool moist air from the northwest, increasing
clouds Thu afternoon and bringing some showers to northern WY Thu
afternoon.

&&

.FIRE WEATHER...
Issued AT 137 PM MDT Wed May 24 2017

A Digging Pacific trough tracking across southwest Canada and the
Pacific Northwest will push into the Northern Rockies spreading
clouds and scattered showers to the western and northern mountains
in the evening with isolated thunderstorms. West to southwest
winds will be quite strong ahead of the low pressure trough
being windy across the south and over and near the east slopes of
the mountains. Part of the trough will dig south into the
northern Great Basin while the main closed low circulation tracks
east across the Canadian Prairies. This will keep some showers,
mainly over the mountains, Tonight with a cold front moving into
the north. The Canadian portion of the low pressure trough will
stall out over southern Saskatchewan as a second cold front dives
south across Montana into northern Wyoming on Thursday with
increasing showers and isolated afternoon and evening
thunderstorms. Cooling will be widespread as the cold fronts move
through the state. Thursday night and Friday northeast upslope
flow will extend across the area with low stratus and showers
expected to be rather widespread with snow above 7500 feet.
Isolated thunderstorms cannot be counted out Friday afternoon.
Much of the lower elevations will have highs in the 50s, with
lower to mid 60s over the Bighorn basin. The mountains will have
highs in the 40s with 30s above 9000 feet. Saturday cold northerly
flow aloft will remain but moisture will become more limited with
less coverage of showers. Mixing heights should be high enough in
the afternoon for generally good smoke dispersal Thursday but
should be poor to fair on Friday and Saturday with more widespread
clouds and precipitation and colder temperatures.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
NONE.
&&

$$

SHORT TERM...Baker
LONG TERM...McDonald
AVIATION...McDonald
FIRE WEATHER...Baker



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