Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 162125
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
225 PM MST Sun Dec 16 2018

.SHORT TERM...(This evening through Monday night)

It should stay pretty quiet tonight and Monday morning. Winds are
looking a little gusty in the wind corridor from the Red Desert to
Natrona County, but nothing out of the ordinary for this time of
year. The inversion looks to have come back quicker than expected in
the Star and Jackson Valleys so I have dropped high temperatures for
those areas tomorrow. Another weak shortwave will move into the
western portion of the state Monday afternoon and into Monday night.
Much like the previous one, it will have little upper level support
and will be splitting as it moves across Wyoming so only light snow
is expected with well below advisory amounts of accumulation
expected. It could cause some locally strong winds in the usual
suspects like the lee of the Absarokas and near Casper but low level
winds do not look sufficient enough for any wind highlights at this
time. A more potent system will arrive late Tuesday night into
Wednesday which will be covered by my colleague in the long term.

.LONG TERM...(Tuesday through Sunday)

After the weak shortwave and its asct relatively light snowfall
(that my esteemed short term forecaster describes so well),
tracks to the east of the area, a lull in the snowfall should
occur in Western Wyoming Tuesday morning before snowfall gets
underway out west once again from an approaching, fiercer Pacific
weather system. A backing strengthening westerly difluent upslope
flow will initiate snowfall out west Tuesday afternoon, with
isentropic lift adding another layer to the dynamics of this
weather system for steady snowfall, heavy at times, to get
underway by sunset Tuesday evening. Left front quad dynamics will
add to the lift for NW WY Tuesday night. Steady snowfall in
Western Wyoming Tueday night will transition to more showery
snowfall Wednesday morning as the asct Pacific cool front swings
through with the airmass becoming weakly unstable. Have indicated
advisory amounts for the Western Mtns and valleys during the
period Tuesday night through Wednesday. Significant blowing and
drifting snow will likely accompany this storm in exposed
locations of the far west.

Meanwhile, east of the Divide, wind will be the predominant
feature with this weather system. On Tuesday night, there will be
a 13mb sfc pressure gradient from GCC to RKS, with only a slight
decrease in the sfc gradient Wednesday during and after the
fropa, so the Southern Wind Corridor, as well as the foothills
just east of the Divide from Clark to Cody to Dubois and to Lander
should blow fairly strong from the west southwest, with at least
some areas like Casper Outer Drive, south of Casper, blowing at
least at the low end of high wind criteria ahead of and during the
fropa. Behind the front on Wednesday, there are 7 height lines
progged to be spaced apart by 30 decameters over the state of WY,
a good indicator, in this case, of potentially strong post frontal
nw winds in places like the Wind River Basin and Northern Johnson
County Wednesday afternoon. 55 knot winds at 700mb will likely
mix down to the sfc along and behind the front.

Some light snow will fall over the lower elevations of Northern
wyoming Tuesday night/Wednesday morning where accumulations will
be mainly less than an inch along with 1 to 3 inches in the Big
Horn Mtns.

The cool air behind this weather system will be short lived as
high temps only cool off around 5 degrees or so Thursday as the
high builds in from the west along with dry weather. temps will
warm right back up Friday. If there are any central basin
inversions left east of the Divide by the time this weather
system hits mid week, these inversions will likely be wiped out
with the lack of much fresh snow cover by then.

The next northern stream shortwave will likely deliver more snow
to the far west Friday where low pops are indicated attm. And
then a weaker southern stream shortwave Sunday could produce yet
another round of snow to the far west Sunday acdg to the GFS.
There is uncertainty with this system this far out with the Euro
placing a ridge over us Sunday and the Canadian keeping us under
flat nw flow. Dry conditions are expected east of the divide
during the latter portion of the week and next weekend.

&&

.AVIATION...(For the 00Z TAFS through 00Z Monday afternoon)
Issued at 117 PM MST Sun Dec 16 2018

Breezy winds are expected in the wind corridor from South Pass to
Casper, and on lee side of mountain ranges for the rest of this
afternoon, tonight and Monday. Otherwise light winds will be
observed. VFR conditions are expected everywhere through mid
afternoon Monday. By 21Z, clouds will increase, lower, and
thicken, with MVFR conditions expected to get underway, along
with some mtn top obscn in Far Western Wyoming, including the
vcnty of JAC as the next Pacific weather system approaches from
the west.

&&

.FIRE WEATHER...
Issued AT 117 PM MST Sun Dec 16 2018

Dry conditions are expected through Monday morning. A gusty wind
will continue from the Red Desert through Casper, but not as
strong as on Saturday. Wind should remain light to moderate
elsewhere. Inversions in the Star and Jackson Valleys have
reestablished themselves well as of this morning while inversions
elsewhere are still fairly weak. Relative humidity should remain
above critical levels. Mixing and smoke dispersal will be
generally poor.

&&

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

$$

SHORT TERM...Hulme
LONG TERM...Lipson
AVIATION...Lipson
FIRE WEATHER...Hulme


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