Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 170828
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
128 AM MST Wed Jan 17 2018

.SHORT TERM...Wednesday through Friday night

For the short term today, the activity in the weather resembles a
car starting off from a stop light. Starting off fairly inactive and
then increasing in activity through the week. As for today, we will
have the weather car stopped with high pressure in control. Although
we can`t rule out a flurry over the highest elevations, they will be
so inconsequential that we kept the forecast dry through tonight.
Temperatures will continue to modify as well in the Basins, although
inversions will likely hold one more day. Wind should also remain
light to moderate with some breezes across the usual suspects. There
could still be some patchy river fog, but with much less coverage
than previous days.

For tonight, we shift into first gear. (I realize I am using a stick
shift analogy, likely dating me somewhat since it is very hard to
find manual transmissions now.) Across the Absarokas and Cody
Foothills, 700 millibar winds will increase somewhat, but likely
remain to around 40 knots at the most. As a result, there could be
some gusty winds. In other parts of the country, they would be
noteworthy, but in Wyoming we call it Wednesday.

We shift the weather into second on Thursday. Winds will increase
across the area as a Pacific front and trough approach the area from
the west. This could bring some light snow showers to the western
mountains but any amounts would be very light. Winds will also
continue across the Absarokas and Cody Foothills as well. Even in
the basins with increasing mixing, this could be the day that areas
mix out and see temperatures rise above freezing.

The RPMs on the weather really increase as we shift into third and
fourth gear Thursday night. The first concern will be strong and
potentially high wind. The approaching front will continue to
increase the 700 millibar winds, eclipsing 50 knots across the
Jeffrey City to Natrona County corridor and to 60 to 65 knots across
the Absarokas and Cody Foothills. In addition, there will be some
downward forcing courtesy of the right front quadrant of a 140 knot
jet streak passing across Montana. As a result, some high wind
highlights may be needed across Zones 2 and 3. There is a bit more
uncertainty across the Green Mountains and Southwestern Wind
Corridor since downward momentum is not as pronounced. There will
likely be some high winds in places like Camp Creek, but impacts are
still in question. Something to watch though. Meanwhile, snow will
be increasing, especially late at night in the northwestern
mountains as the front approaches.

The active weather will continue into Friday as well when the car is
shifted into fifth gear. At this point, the heaviest snow in the
western mountains looks to fall between around 3 am and noon as the
front passes and upper level divergence is enhanced by the right
rear quadrant of a jet streak. However, southwest flow is not
favored for really heavy snow. Also, some models are showing the
shortwave may split. There will likely be enough for advisories
though. We can`t rule out a few places receiving over a foot, but
impacts would be minimal or even positive in a place like Jackson
Hole. Meanwhile, East of the Divide, mainly dry conditions should
continue through the day. Many places will be breezy to windy
though. One positive would be downsloping flow that could bring
the mildest day of the past several for many of the basins. The
risk of snow will increase Friday night however, especially late
at night as lee cyclogenesis occurs in Colorado and we develop
some upslope flow in areas favored by north to northeasterly flow.
Most of the snow would fall Saturday though. My esteemed
colleague will have more on that.

.LONG TERM...Saturday through Wednesday

Lee cyclogenesis occurs across the Front Range of Colorado
Saturday as the main part of the trough pulls into the Four
Corners Region. The 700mb baroclinic zone is expected to be across
southeast Wyoming Saturday. The central and south-central areas of
the forecast area could see a combination of isentropic lift,
warm conveyor belt and upslope flow for a good chance of
snowfall. Far northern and western areas should be mainly dry. The
track of this system, as all of them, will be critical in how
much snowfall occurs. Right now looks like areas along and
southeast of Rock Springs to Midwest including Casper could see
advisory amounts of snowfall. The snow is expected to come to an
end Saturday night as the activity pulls into the Plains.

A transitory ridge will result in mainly dry conditions Sunday.
This will be short-lived as a quick moving shortwave trough will
pull across the northern Rockies. This system looks to produce a
combination of some diffluence aloft and a moist westerly flow for
a good shot of snowfall across the far west/northwest Sunday
night/Monday. East of the Divide, and Sweetwater County looks to
remain mainly dry in this flow. The flow is expected to become
northwest Monday night/Tuesday with snowfall confined mainly to
the northwestern mountains as well as the Bighorn Mts. Models then
show a transitory ridge Tuesday night, before another trough could
begin to impact the far west on Wednesday with more snow.

Temperatures will be seasonal to slight below average through the
extended with all of the precipitation expected to fall as snow.

&&

.AVIATION...12Z Issuance

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

Fog and lower ceilings will be much less extensive this morning than
the past few mornings as an upper level disturbance brings a band of
mid and high clouds across the area. Patchy fog may redevelop
vicinity KJAC through the early Wednesday afternoon as the mid-level
clouds move off to the east by 15z. Likewise, increasing southwest
flow aloft and high clouds will hinder fog redevelopment on
Wednesday night.

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

VFR conditions will prevail through the period. Some morning fog
along the lower river valleys partially obscuring terrain. Some low
level wind shear will occur vicinity KCOD through this morning and
again Wednesday night.  Surface southwest winds 15-25 knots will
prevail through the wind corridor through 06z Thursday, vicinity
50SM NE KRKS-KCPR.  These winds will increase 06z-12z Thursday.

&&

.FIRE WEATHER...

Mainly dry weather will continue through Thursday. Inversions will
continue over the Basins east of the divide although shallower than
in recent days. A gusty breeze will continue across the southwestern
wind corridor from the Red Desert through Natrona County. Mixing and
smoke dispersal will be generally poor.&&

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

&&

$$

SHORT TERM...Hattings
LONG TERM...Murrell
AVIATION...Meunier
FIRE WEATHER...Hattings



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