Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 242135
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
335 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 00Z TAFS through 00Z Thursday evening)
Issued at 335 PM MDT Wed May 24 2017

An approaching cold front from the west northwest has increased the
surface pressure gradient over western and central WY late this
afternoon and will continue into the evening.  Surface wind is
widespread 20 to 30 kts with gusts over 30kts.  Some showers and
thunderstorms have already developed in west central WY, and this
area is expected to increase in the evening hours and move east
southeast into southwest WY, then eastward into central WY.  Winds
will shift to the north across northern WY between 00z-02z/Thu and
slowly push southward into central WY through 06Z/Thu.  Winds will
shift to the northwest across SW WY by 06Z/Thus as well.  Showers
with isolated thunderstorms may continue through 06Z/Thu and
possibly after midnight as well given the latest model guidance.
Outflow from the storms could create wind gusts of 40 to 50 kts.  As
the cold front moves southeast overnight, there will be a brief
region of clearing before mid-level moisture moves down in the
northwest flow Thursday, and brings a slight chance of showers to NW
WY, including KJAC and KCOD by 00Z/Fri.  VFR conditions generally
expected, with MVFR likely around showers and thunderstorms.  Winds
on Thursday from the north and northwest do not look to be as strong
as Wednesday.

&&

.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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