Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 170457
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
1057 PM MDT Mon Apr 16 2018

.SHORT TERM...(This evening through Tuesday)
Issued at 310 PM MDT Mon Apr 16 2018

Very strong south winds are occurring out ahead of a strong cold front
now about to pounce upon the Tetons in far Western Wyoming. These south
winds are ushering in unseasonably warm temperatures around 60 in the
far west valleys and in the mid 70s at the lower elevations east of
the divide. Relative humidity is down to near 10 percent east of
the divide and in Sweetwater County. Wind speeds in many areas east
and west of the divide are gusting to between 45 and 55 mph. Tonight
and Tuesday morning, the front will plow southeast across Wyoming
with rain showers associated with the front turning to snow in
the mountains fairly quickly early this evening and then by midnight
in the western valleys. As the showers track east across the divide
tonight and spread north to south, the showers will turn to snowfall
by sunrise. Strong southwest winds shifting to northwest will continue
to blow fairly strong as some of the 60 knot winds at 700mb mix down.
The front will clear the area by 15Z Monday morning. Many areas will
see a couple of hours of clearing skies before the cold northwesterly
unstable flow takes over behind the exiting front. Convective snow
and snow pellet showers mixed with some rain will occur across Wyoming
by late morning across Western and Central Wyoming, but will be a bit
more numerous overall across the north half of the state, with
the exiting mid level circulation tracking east just to the north
of the Owl Creek Mtns. 3 to 5 inches of snowfall is expected across
the Big Horn Mtns as well as the far west mountains. However,
with the favorable unstable northwest flow, areas like Targhee and
Alpine could see 9 and 5 inches of snowfall respectively by late
Tuesday. Many other lower elevations will experience 1 to 2 inches
of snow with local amounts of up to 3 inches. West winds behind
the exiting trough will pick up once again from the west this time
over Sweetwater County at speeds of 30 to 40 mph with gusts of 50
mph. These winds will combine with the expected snow showers,
enabling the snow showers to play out as snow squalls. Have issued
another segmented SPS addressing the critical fire weather
conditions through early this evening. The red flag warning for
Natrona County will expire at 7 pm this evening as that is the
only fire weather zone with critical fuels. Have also addressed
the snow squalls in Sweetwater Co as well as the locally heavier
snow expected over the Tetons and the Northern Star Valley with an
SPS. Isolated lightning strikes are even possible with widespread
lifted indices of minus 2. Post frontal northwest/north winds are
expected to blow at 25 to 35 mph with gusts to 40 mph in quite a
few other areas as well on Tuesday, given the 9-11mb 3 hourly sfc
pressure rises expected along with 5 height lines across WY spaced
apart every 30 decameters. There will be quite a dramatic drop in
high temperatures behind the front Tuesday over what they have
been today. Expect high temperatures to be as much as 30 to 40
degrees colder east of the divide Tuesday and 20 to 25 degrees
west.

A gradual warm up is expected to begin Wednesday and continue
through Thursday. Then the next weather system to affect the CWA
later in the week will begin to affect Western Wyoming by
Thursday afternoon in the form of showers under the increasing ssw
difluent flow out ahead of this next approaching weather system
along with increasing instability. We will not recover to the
temps we experienced today on Thursday.

.LONG TERM...(Thursday night through Monday) Issued at 310 PM
MDT Mon Apr 16 2018

Large upper low is moving into Desert Southwest to start this
period before it heads towards the 4-corners Friday. Upslope
redevelops quickly with the leftover high behind our Tuesday
system and rapidly lowering pressures to our south. A strong
diffluent/divergent flow sets up in the south half into Thursday
night and Friday morning with strong cyclonic nose of the jet
trying to edge up into the srn zones. Showers and isold tstsm will
likely break out in this period and then the front will back into
the Red Desert/I80 corridor Thursday night with strong ne wind.
Temps are still rather warm for snow through the day Friday in all
but the higher elevations of our sern zones. Euro is colder but
not confident that the rain chances will actual be snow chances in
the basin/lower elevation sern zones. Above 6-7K odds increase
significantly. Still a ways out so we can watch it. Most of the
energy stays across the far south with some potential for
wraparound warm sector moisture to back into Natrona County on
Friday. The weekend will be generally dry with a warming trend.
There is a weak disturbance that pushes a cool front down from the
north Sunday with a few showers likely popping up, especially
over and near the mountains. A nw flow starts to develop Monday
into Tuesday with potential clipper impacting us on Monday. Euro
was much more bullish and with potential mean nw flow, we`ll have
to watch for strengthening in the other models with time. Either
way, a front drops down with a chance of showers/isold tstms with
cooler conds behind the front up north Monday into Monday night.

&&

.AVIATION...(For the 06Z TAFS through 06Z Wednesday)
Issued at 1045 PM MDT Mon Apr 16 2018

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

Snow showers will occur overnight through 15Z before becoming more
scattered in nature. MVFR to IFR conditions will occur at times. At
KRKS airport snow showers will be most likely between 12Z and 18Z.
Expect frequnet mountain obscuration A gusty wind will be common at
the terminal sites through 01Z Wed. Then the wind will decrease
after 01Z Wed. VFR conditions will prevail after 00Z Wed at the
terminal sites.

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

A cold front passes through the area overnight. An area of snow will
develop over central WY after 12Z and last until 19Z. This area of
snow will impact the KBYG to KCPR area after 18Z Tue through 00Z
Wed. Lingering snow showers will occur until 03Z Wed. MVFR to IFR
conditions will occur during the snow showers, before returning to
VFR conditions once the area of snow moves out. Mountains will be
frequently obscured at times. VFR conditions will prevail in all
areas after 02Z Wed.

&&

.FIRE WEATHER...
Issued AT 310 PM MDT Mon Apr 16 2018

The rest of today will be a bit of a fire threat for Sweetwater,
Fremont, Natrona Counties, as well as across the Big Horn Basin.
Clearing skies, temperatures around the 70s, and strong gusting
winds are of concern. A Red Flag Warning has been issued for
Natrona County from noon to 7PM this evening. Smoke dispersal will
be very good or better this afternoon, decreasing to fair to good
on Tuesday as clouds lower and showers become more widespread and
temperatures fall with an increasingly strong frontal inversion.
South winds will shift to the west tonight and Tuesday with wind
speeds as high as 30 to 40 mph along with gusts to 50 mph in zone
279 on Tuesday. Accumulating snowfall is likely across much of
Western and Central Wyoming tonight and Tuesday with several
inches of snow expected in the northern and western mountains, as
well as across the Northern Star Valley.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...None.

&&

$$

SHORT TERM...Lipson
LONG TERM...Skrbac
AVIATION...AR
FIRE WEATHER...Lipson


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