Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 182057
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
157 PM MST Mon Dec 18 2017

.SHORT TERM...Tonight through Thursday

We have a few concerns this afternoon. We have had some locally
strong winds in the usual areas due to the combination of strong mid
level winds and a tight pressure gradient. Both are expected to ease
somewhat this evening, so wind should decrease. All in all,
fairly quiet tonight. There will be some light snow at times in
the western mountains, but certainly nothing of consequence.

At this point, Tuesday looks fairly quiet as well. There will be
some gusty winds continuing across the usual areas. There may be
the potential for some strong or locally high winds in the usual
areas in the Cody Foothills late tomorrow or tomorrow evening.
However, at this time this area would be restricted mainly to
areas with very little population, so no highlights at this time.
Temperatures should be fairly similar to yesterday as well, with
some inversions persisting in the deeper valleys and some of the
basins.

Things will begin to change Wednesday and especially Wednesday night
and into Thursday. A cold front will move into the west during the
day and spread more snow into the area. The difference between this
one and the many of the cold fronts we have seen over the past
several weeks or so, is that this one is coming from Canada. As a
result, there will be much more cold air associated with it.
However, the one thing these fronts usually lack is moisture. More
on that in a minute. At this point, while snow will break out in the
northwest, most areas East of the Divide should have one more mild
and mainly dry day. With southwest flow increasing, this looks to be
the warmest day of the week and possibly the warmest day for quite a
while.

Snow will begin to spread into areas East of the Divide Wednesday
night. There is some decent jet support with it that will enhance
upward motion. This may increase snowfall rates. However, one thing
it is missing is a lot of moisture. It is also moving fairly
quickly, so the period of heavier snow looks to be fairly brief.
It is still probably enough for some advisories however. The main
problem will be the consistency of the snow. This will be a
lighter, more powdery snow. There will also be some decent north
wind with the strong cold advection behind the front. This will
likely cause some blowing and drifting of the snow that could lead
to more travel problems. The strongest winds will occur in the
areas that are usually favored by north to northwesterly flow with
strong cold advection, aka the northern Big Horn Basin and
northern Johnson County. Maybe not enough for high wind highlights
but certainly quite gusty.

The main effect will be the shock to the system with the sharply
colder temperatures moving in, certainly the coldest so far this
winter for many areas. Many areas may end up having midnight high
temperatures on Thursday with falling temperatures through the day
on Thursday. Many areas could end up in the single digits by sunset
on Thursday with wind chills below zero. Very fitting as well, given
that winter arrives on Thursday at 9:26 am, the time of the winter
solstice.

.LONG TERM...Thursday night through Monday night

Synopsis...As one system leaves, another enters. The first real
push of arctic air will impact the Cowboy State on Friday,
bringing widespread snow showers with it. This arctic front will
quickly pass through the state through the day Friday, as light
snow continues Friday night. H7 temps of -23 to -28C will
infiltrate over the area behind this front and will moderate
little through 12Z Monday. Models begin to diverge by 12Z Monday,
with the GFS being the warmer solution bringing H7 temps 0f -10
to -15C. Meanwhile, the ECMWF brings a reinforcing shot of arctic
air through the state.

Discussion...The area will be calm briefly Thursday night, before
the arctic front. Moisture recovery will be poor ahead of the
arctic front, with QPF values ranging between 0.15 - 0.25" from
12Z Friday to 12Z Saturday. Most of this will occur east of the
Divide, in northern and central portions. Thus, a widespread
estimate of 3 to 6" with 15 to 20:1 ratios looks to be a good
start. Apart from lingering showers over the north, precipitation
will end Saturday night. Even without this arctic air,
temperatures will be a forecast challenge through the period.
After Wednesday`s system adds to the existing snow cover,
temperatures are going to trend downward. Clouds will be in and
out Thursday night, but will not be surprised if there are sub
zero readings Friday morning. On the flip side, after the arctic
front goes through, clouds hang around Sunday, keeping
temperatures from plummeting into the -20s and -30s. Highs over
the weekend will mainly be in the single digits. Lower to middle
teens will be more widespread on Christmas.

&&

.AVIATION...00Z Issuance

Marginal MVFR conditions KJAC and KPNA with marginal LLWS at KJAC
subsiding after 00Z. Stratus decks into low VFR/marginal MVFR
categories with areas of - SN returning tonight. SCT-BKN clouds
FL120-200 will be in place from KJAC southward after 12Z Tuesday.

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

VFR conditions will prevail across the area through 12Z Tuesday
with mainly broken high clouds. The potential for mountain wave
activity vicinity KCOD has decreased, but LLWS will persist
through 18Z Tuesday.

&&

.FIRE WEATHER...

Other than some light snow in the west, expect dry conditions
through Tuesday. Locally gusty winds will continue this afternoon
and again on Tuesday in the usually windy areas, although winds on
Tuesday should not be quite as strong as Monday. Relative humidity
will remain well above critical levels. Mixing and smoke dispersal
will be generally fair to poor on Tuesday.
&&

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

$$

SHORT TERM...Hattings
LONG TERM...LaVoie
AVIATION...LaVoie
FIRE WEATHER...Hattings



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