Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 130554 AAA
AFDRIW

Area Forecast Discussion...UPDATED
National Weather Service Riverton WY
1054 PM MST Sun Feb 12 2017

.UPDATE...Added 06z aviation discussion.

&&

.AVIATION.../06Z ISSUANCE/

Patchy valley fog may partially obscure terrain in the lower river
valleys with all TAF sites except KCOD and KCPR showing VCFG through
16-18z Monday.  Fog may become more extensive along the KRKS-KBPI
route, on the west side of South Pass, Monday morning.  Otherwise
VFR conditions will prevail through 06z Tuesday with a weak, dry
cold front pushing south across the area Monday morning, accompanied
by some mid and high clouds and a subtle wind shift from the north.
High pressure aloft will fold across the area Monday night bringing
in warmer air aloft, steeper inversions over the valleys, and
possibly more extensive areas of fog development Tuesday morning
across the lower central basins (vicinity KRIW-WRL-KGEY) and
southwest Wyoming (KRKS-KBPI route).

&&


.PREV DISCUSSION... /issued 107 PM MST Sun Feb 12 2017/

Short Term...Tonight Through Wednesday

Cold high pressure will remain over the area with generally light
wind, except in the east around Casper where southwest winds will
continue to blow. Temperatures will gradually warm through
Wednesday with southwest wind gradually increasing across the
south and east.  Dry conditions will prevail.  Ice Jam flooding
continues on the Bighorn river at and around Worland with the
potential to spread north towards Greybull through Monday.
Patchy fog will remain a threat over night and in the mornings
in the southwest and western valleys.

Long Term...Wednesday Night Through Sunday

The ridge of high pressure will breakdown and shift eastward Thu, in
response to a long wave trough making its way onshore over the
Pacific Northwest. This trough will weaken as it moves onshore,
ejecting several minor shortwaves. The strongest one will make its
way over the region Thu night into Fri and exit the area by 12Z Sat.
The next major long wave trough will quickly follow behind, but will
dig southward over southern portions of the Desert Southwest and N
Mexico. Both the GFS and ECMWF begin to phase this system with
another trough over the N Pacific. This will create conditions for
several shortwaves to impact western portions Sat and Sun.

Dry conditions through Thu afternoon, as clouds begin to increase
across the far west. Chances for snow will quickly increase and
spread across the rest of areas west of the Continental Divide
Thursday evening and the overnight hours. This activity will
continue Friday, with a slight chance in the Big Horn Basin Friday
afternoon as the trough exits the area. Another round of snow will
impact western and southern portions, as the next shortwave enters
the area from the south. Some of this activity will again spill east
of the Divide. Snow will change over to rain Sat afternoon over
southern portions and east of the Divide. Snow will remain confined
across the far west Sun, with rain mixing with snow over southern
Lincoln and western Sweetwater counties.

Afternoon highs of 50+ degrees could return by Wed along the Cody
Foothills and northern Johnson County. These temps will continue Thu
and Fri, with portions of Johnson County reaching the mid to upper
50s. As per today`s vis satellite images, snowpack remains in these
areas. Flooding could impact these areas, as the snow melts and area
rivers/creeks rise accordingly.

Aviation.../00Z Issuance/

SKC conditions will continue through 06Z, as mid to high level
clouds push south across the area tonight and Monday morning. No
significant impacts are expected. Patchy morning fog will again be
possible Monday morning, partially obscuring terrain in the lower
river valleys through 16z.

Fire Weather...

Cold high pressure will remain over the area with generally light
wind, except in the east around Casper where southwest winds will
continue to blow. Temperatures will gradually warm through
Wednesday with southwest wind gradually increasing across the
south and east.  Dry conditions will prevail.  Ice Jam flooding
continues on the Bighorn river at and around Worland with the
potential to spread north towards Greybull through Monday.
Patchy fog will remain a threat over night and in the mornings
in the southwest and western valleys.  With low mixing heights
and generally light wind, smoke dispersal is expected to remain
poor through Wednesday.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
None.
&&

$$

SHORT TERM...C.Baker
LONG TERM...LaVoie
AVIATION...AEM
FIRE WEATHER...C.Baker



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