Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 091003
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
303 AM MST Mon Dec 9 2019

.SHORT TERM...(Today through Tuesday)
Issued at 135 AM MST Mon Dec 9 2019

Latest shortwave trough is exiting the forecast area early this
morning, moving from northwest to southeast. Big impact overnight
has been light to moderate snow from NW to SE oriented snowbands,
particularly along Highway 20/26 from Hiland all the way into
Casper, where around 4 inches has been reported. Other snowbands
showed up on radar over the Wind River Canyon and just south of
Riverton. These should dissipate by sunrise, but will result in
some snow covered slick roads for the morning commute. Some fog
has already developed in the Upper Green River Basin, with Big
Piney reporting low visibilities and freezing fog. Areas of fog
should continue in this area through at least mid-morning.

The rest of the day looks to be quite a bit cooler, especially
east of the divide, with the northwest flow continuing over the
area. Skies will generally be partly cloudy and westerly breezy
conditions will develop across SW WY into central WY as well as in
some eastern foothills due to low surface pressure from MT into SD
increase the gradient during the afternoon hours. Clouds will
increase from the north during the afternoon hours as a weak
shortwave trough moves from southern Canada into MT and NE WY
after 00z/Tue. This system should bring light snow to the northern
Bighorns before sunset, and increase the chance of snow from the
northern Absarokas and YNP to the northern Big Horn Basin and
into the Bighorn Mountains and Buffalo area Monday evening
and Tuesday morning. Only light accumulations are expected at this
time, though the northern Bighorns and northwest mountains could
pick up a couple of inches. The shortwave trough will increase the
NW flow during the period east of the divide, while breezy
westerly flow should continue overnight from SW into central WY.
Tuesday will see some light snow showers continue, mainly over the
mountains of northern WY as the trough moves into the Plains and
a ridge builds up from Nevada northward into western Idaho. Model
guidance is still showing breezy to windy conditions from
southwest into central WY on Tuesday as the ridge moves eastward.

.LONG TERM...(Tuesday night through Sunday)
Issued at 300 AM MST Mon Dec 9 2019

Tuesday night will be a dry period across the area, but Wednesday
isolated showers may make a reappearance in the far northwest due
to a passing shortwave. Then, on Thursday, flow becomes more zonal
and ushers moisture into the area. Showers will begin in the west
on Thursday, especially later in the day, but exact timing is
uncertain. Models differ on the duration of this event and on how
much moisture will be brought in. The event looks likely to last
through Friday, impacting mainly the west on Thursday and then
beginning to spread east of the Divide on Friday. A cold front
will push through the area Friday. Models become more uncertain
over the weekend, but currently PoPs look to briefly decrease
going into Saturday before having a resurgence with scattered
showers across the entire area by the end of the day. If the GFS
gets it`s way, activity may last through much of Sunday.
Temperatures gradually warm through the week until Friday`s cold
front pushes through.

&&

.AVIATION...(For the 06Z TAFS through 06Z Monday night)
Issued at 939 PM MST Sun Dec 8 2019

West of the Continental Divide...KBPI/KJAC/KPNA/KRKS Terminals

Any snow should end by around 08Z with mainly dry conditions
expected through the period after that. Patchy fog will be possible
in the Green River Basin at times but difficult to time seeing how
visibilities and ceilings have been bouncing up and down like a
tennis ball. A gusty wind will continue at times across southern
Wyoming, including in vicinity of KRKS.

East of the Continental Divide...KCOD/KCPR/KLND/KRIW/KWRL Terminals

Any snow showers should end by around 10Z with dry conditions then
expected through the period. A weak system brushing by to the north
may bring some snow showers to northern Wyoming but any showers
should remain away from the TAF sites. Patchy fog is possible in the
basins after 05Z Tuesday but not confident enough to put in the
forecast at this point.

&&

.FIRE WEATHER...
Issued AT 140 AM MST Mon Dec 9 2019

Cool northwest flow will dominate the pattern for the next couple
of days. A weather system moved through in this flow late Sunday,
bringing snow showers to central WY and a couple of inches of
snow. Monday will be cooler compared to the weekend as northwest
flow continues under partly cloudy skies. Breezy west wind will
develop in the typical areas Monday afternoon. Another weak system
will move into northern WY Monday night into Tuesday morning,
increasing clouds and bringing another chance of light snow,
mainly to northern and northeast WY. Breezy to windy conditions
should continue Monday night into Tuesday due to this pattern.
Tuesday will warm slightly as weak high pressure from the west
approaches, though NW WY will continue with light snow showers.

&&

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

&&

$$

SHORT TERM...McDonald
LONG TERM...Jones/Bourque
AVIATION...Hattings
FIRE WEATHER...McDonald


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