Area Forecast Discussion
Issued by NWS Riverton, WY

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399
FXUS65 KRIW 162058
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
158 PM MST Fri Feb 16 2018

.SHORT TERM...(This evening through Monday)
Issued at 158 PM MST Fri Feb 16 2018

Extremely active short term period with both snow, wind, and sub-
zero temperatures to worry about. First shortwave in the nw flow
will drop se across the area tonight with a period of enhanced
snowfall in the nrn mountains and some snow chances across the
lower elevations of the north. Second shortwave moves into the nw
Saturday evening with some increasing upglide ahead of it for
increasing snowfall rates in the nw mountains Sat aftn. Best
snowfall still looks to be Saturday night into Sunday morning
(06-15z) when right entrance region from 2nd shortwave and curved
nose of jet ahead of main trough combine to produce some strong
upper divergence along with increasing mid level QG forcing.
Resulting omega looks pretty strong with snowfall rates over an
inch per hour quite possible in the nw mtns. The snow will drop
south by Saturday evening with heavy snow quite possible
throughout the sw mtns and valleys/foothills through Sunday. Added
in these areas to a solid winter storm potential. Very strong
winds ahead of this storm (55-70kts in the lower 3-5K) will likely
produce a significant high wind event across the wind corridor,
east slope of the Winds (Lander is a challenge) and in the Cody
Foothills. Signature for mountain wave up there and gap flow
doesn`t look all that great but strength of the low level flow and
developing surface low over srn MT is just too impressive to not
have a watch out for now. The arctic front will drop south quickly
on Sunday east of the divide with a period of gusty north wind and
rapidly falling temperatures. Decent upglide behind the front
followed by potential strong jet support (possible couplet like in
the west) will likely produce heavy snow across parts of the area.
The best signature is across Natrona and southeast Fremont County
but other areas will still have to be watched as we`re still
roughly two days away. Many other areas east of the divide may
very well need winter weather advisories due to snow, bitter cold
temps/wind chills and gusty north wind. Johnson County looks like
3-6 inches right now with some advisory amounts from Cody south to
WRL and THP. The Wind River Basin and Lander Foothills are
challenging on exact amounts yet. Jet support will likely be the
deciding factor between a 1 to 3 inch event vs a advisory or ??.
If the support lines up for a 3-6 hour period Sunday night there
could be a period of inch per hour snowfall rates. If not, more
like a quarter inch for roughly 12 hours. The I80 corridor will
see a strong west wind and snow with fropa moving across Sunday
into Sunday evening. Snow will peak in central WY late Sunday into
Sunday night with lighter snow Monday gradually ending from north
to south. Some light snows will continue out west as the trough
axis stays to the west through about 00z Tuesday. Quite cold
behind this system with temperatures struggling to reach zero in
the north Monday and get above the single digits further south in
Fremont and Natrona counties.

.LONG TERM...(Monday night through Friday)
Issued at 158 PM MST Fri Feb 16 2018

This period will start off, in a word, cold. Rather, very cold. With
Arctic air expected to be entrenched over the area, lows will be the
lowest in quite a while. There could be some lingering cloud cover
on Monday night though, and this could hold temperatures up a bit on
Monday night in some locations. If it clears though, with fresh snow
cover, lows will be off to the races. There could be a bit of
lingering light snow as well, but this will decrease through the
night as lift moves away. However, there is a silver lining to this
outbreak of Arctic Air, it is that it is occurring in late February,
not early January. With the longer days, shorter nights and higher
sun angle, it would not be as cold as earlier in the year.

Tuesday at this point looks mainly dry except for some light snow in
the northwest as high pressure builds into the area. Tuesday may end
up being the coldest night with clear skies expected in many areas.
High pressure will then bring dry and cold weather from Tuesday
night through the first part of Thursday. The models do have some
light snow showers in the northwest, but they always tend to have a
wet bias in this situation. The next system looks to approach from
the west for Thursday into Friday, although the models all over the
place with the details. We made few changes to continuity as a
result.

&&

.AVIATION...(For the 00Z TAFS through 00Z Saturday afternoon)
Issued at 158 PM MST Fri Feb 16 2018

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

Increasing MVFR, areas of IFR, ceilings and visibility in areas of
snow over the western mountains in Teton, Lincoln and Sublette
counties will obscure mountains through 00z. MVFR ceilings and
visibilities reduced in snow will be most probable at KJAC, and
possible at KPNA and KBPI. Increasing winds at mountain top level
will result in areas of LLWS, mainly near KJAC. Surface west winds
at 20-25kts with gusts of 35 kts will prevail across the I80
corridor, including KRKS. Conditions should remain VFR at KRKS
through 00Z Sunday.

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

Lower elevations will have prevailing VFR conditions. The exception
will be in vicinity of KCOD where lower ceilings and vicinity showers
may bring local MVFR conditions after 12Z Saturday. Increasing winds
at mountain top level will also result in areas of LLWS. Areas of
LLWS will develop vicinity along the east slopes of the Divide,
vicinity KCOD-KDUB-KLND. Any snow from the incoming Arctic Front
will likely hold off until after 00Z Sunday.

Please see the Aviation Weather Center and/or CWSU ZDV and ZLC for
the latest information on icing and turbulence forecasts.

&&

.FIRE WEATHER...
Issued AT 158 PM MST Fri Feb 16 2018

Very active weather is expected tonight through the weekend.
Very strong winds will blow in the mountains, foothills, wind
corridor (Jeffrey City to Casper) and along the Cody Foothills at
times through Saturday night. The I80 corridor will also be windy
to very windy through Sunday. Snow over the northwest mountains
will increase and spread throughout the west Saturday into
Saturday night. An arctic cold front will drop south into Central
Wyoming on Sunday with falling temperatures, snow and gusty north
wind.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
High Wind Watch from Saturday afternoon through late Saturday
night for WYZ015-018>020-022.

Winter Storm Watch from Sunday morning through Monday morning
for WYZ019-020-022.

Winter Storm Watch from Saturday afternoon through Sunday
afternoon for WYZ001-002-012-013.

High Wind Watch from Saturday afternoon through late Saturday
night for WYZ003.

Winter Storm Watch from Saturday afternoon through Sunday
afternoon for WYZ014-023>025.

&&

$$

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



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