Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 151720
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
1020 AM MST Mon Jan 15 2018

.SHORT TERM...Today through Wednesday night
Issued at 220 AM MST Mon Jan 15 2018

The Canadian Front is still east of the divide and will continue its
slide to the southeast this morning, exiting the area by mid day.
Snow accumulations still expected to be less than an inch in most
areas east of the divide. Fog in the valleys likely to continue
with inversions in place and limited mixing. Expect it to remain
dry through Wednesday with a high amplitude ridge in place over
WY. Models do have a shortwave move through the ridge Wednesday
evening which could bring snow to the northwest for a short time,
however my confidence is not high in this happening.

.LONG TERM...Thursday through Monday
Issued at 220 AM MST Mon Jan 15 2018

High wind potential will continue across the Cody Foothills
Thursday morning. Models are showing a large temp gradient
between Yellowstone and the Cody Foothills, strong winds aloft,
and the right exit region of a jet over northwest Wyoming. A
trough will approach the West Coast Thursday, with the flow
backing to west-southwest. Precipitation should overspread the
far northwest on Thursday, but most of the area will be dry, with
unseasonably warm temperatures for mid-January. The main Pacific
trough will push into the West Coast Thursday night with a moist
southwest flow, resulting in widespread precipitation developing
over western Wyoming Thursday night. Models are also hinting at
some right entrance region of a jet streak helping to enhance
precipitation over the west Thursday night. Enough warm air will
be in place for precipitation to begin as rain in the western
valleys before turning over to snow during the night, as the cold
front works into western Wyoming. The cold front is then expected
to push across the rest of the area Friday into Friday evening,
as the main energy from the trough pushes into the southern Great
Basin and then into the Four Corners area by 12Z Saturday.
Precipitation chances will overspread areas east of the Divide
Friday into Friday night while gradually decreasing over the west.
Just like the western valleys, the eastern lower elevations could
see rain initially, before most areas east of the Divide change
to snow by around 00Z Saturday. A mid-level circulation moving
along the baroclinic zone across southeast Wyoming, as well as
some upslope flow, will keep a good chance of snow across central
/south-central areas of the forecast area into Saturday morning.
The ECMWF track is a bit farther south than the GFS, keeping
significant snowfall southeast of the forecast area, while the GFS
shows the possibility of decent snowfall over central areas. The
trend has been for a farther south of the main energy with some
splitting, which would be limiting factor of anything major.
Right now, the best shot of significant snowfall would be across
the western mountains Thursday night into the day Friday. Also
windy conditions are expected Thursday and Friday in the central
wind corridor from South Pass to Casper, but looks to remain below
high wind criteria right now.

Except for the possibility some mountain snow showers mainly over
the west, the area dries out Saturday night and Sunday. The next
wave could begin to impact the area by Sunday night/Monday. The
GFS is less amplified and faster than the ECMWF, which brings
some precipitation east of the Divide during the day Monday. For
now the forecast will only have slight valley/low chance PoPs
across the west.

Temperatures will be above average Thursday with temperatures
falling to seasonal to below average temperatures for the weekend
in wake of the cold front on Friday.

&&

.AVIATION...For the 18Z TAFS through 18Z Tuesday afternoon
Issued at 948 AM MST Mon Jan 15 2018

West of the Continental Divide...KJAC, KBPI, KPNA and KRKS

MVFR/IFR ceilings and widespread fog vicinity KJAC continues this
morning. Improving conditions will occur later this afternoon,
similar to Sunday. There remains a slight chance that these
conditions will not improve or at least, quickly return later this
evening. MVFR/IFR ceilings and areas of fog have also developed
near KPNA/KBPI/KRKS and are also expected to last through much of
this afternoon. Skies are expected to clear by this evening,
however, patchy fog could return tonight into early Tuesday
morning.

East of the Continental Divide...KCPR, KCOD, KRIW, KLND, and KWRL

Areas of MVFR, local IFR conditions, will persist over all but
KWRL until late this afternoon 21Z-22Z timeframe. The back edge
of this band (clearing skies) is slow to infiltrate and may not be
able to clear these clouds. There is a slight chance this does not
occur this afternoon and these MVFR ceilings will continue this
evening. There is also a slight chance for patchy fog to form
vicinity KCOD and KWRL before sunrise Tuesday morning.

&&

.FIRE WEATHER...
Issued AT 220 AM MST Mon Jan 15 2018

The Canadian Front is still east of the divide and will continue its
slide to the southeast this morning, exiting the area by mid day.
Snow accumulations still expected to be less than an inch in most
areas east of the divide, and 1 to 2 inches in the eastern areas.
Expect it to remain dry through Wednesday with a high amplitude
ridge in place over WY. Models do have a shortwave move through the
ridge Wednesday evening which could bring snow to the northwest for
a short time, however my confidence is not high in this happening.

&&

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

$$

SHORT TERM...Hulme
LONG TERM...Murrell
AVIATION...LaVoie
FIRE WEATHER...Hulme


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