Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 220047

Area Forecast Discussion
National Weather Service Riverton WY
647 PM MDT Sat Oct 21 2017

Issued at 643 PM MDT Sat Oct 21 2017

18Z model data continues to show a high wind event across SE
Fremont/Natrona Counties. With the evening updated decided to
upgrade the watch to warning in that area which matches WFO CYS.

Kept the high wind watch going across the eastern slopes of the
Wind Rivers and Lander Foothills. High resolution model data are
showing the high winds not breaking into the Lander Foothills, and
just below criteria for mountain zones. However there remains the
potential so will keep the watch ongoing now, and look at the 00Z
data before making a decision on the morning package.


.SHORT TERM...Tonight through Tuesday

Quite a plethora of active weather for the next 48 hours or so, with
wind and snow being the main concerns. Given the contrast in
conditions, we will split the discussion into West and East of the

West of the Divide...Here the main concern is snow. We are currently
in between systems with only some light snow being reported.
However, we do expect the snow to pick up once again as a shortwave
moves in from the west with some jet support and brings another
round of snow. At this point, the steadiest snow looks to fall
around midnight to around sunrise tonight and also later Saturday
afternoon into early Sunday night as a cold front moves towards the
area. We hemming and hawing over whether or not to issue an
advisory. For now, we will issue one but only for Zones 1 and 12.
Somewhat warmer air will move into the lower elevations later
tonight and tomorrow and possibly mix the snow with rain and limit
accumulations. Wind could be a problem as well with some blowing
snow, although it will be of the wet variety. Any snow should then
end Monday morning. Ridging then builds across the area from Monday
afternoon through Tuesday and bring more settled weather.
Temperatures should also begin a warming trend, especially on
Tuesday with more sunshine.

East of the Divide...Here the main concerns shift to wind. We
already have some gusty winds although no high wind criteria as of
yet. Things will get windier however as we head into tonight and
tomorrow. The Cody Foothills look to be in the prime spot with the
favored right front quadrant of a pair of 100 knot + jet streaks to
enhance downward forcing. 700 millibar winds also become quite
strong, as high as 75 knots on some of the models. This looks to be
a fairly long lived wind event, especially in areas like Clark. Wind
should decrease fairly quickly though following the cold frontal
passage later Sunday night.

Another area of concern will be the southwestern wind corridor. Here
it will be the increasing pressure gradient as the aforementioned
front drops southward. A few of the models show the 50 knots 700
millibar wind barbs as well. The GFS MOS shows high winds as well,
although the NAM does not. Still with a bit of uncertainty, we have
opted to go with a watch and let the night crew make the final
decision. In addition, we are watching for possible mountain wave
development in and around Lander so we issued a high wind watch
there for much of Sunday, although these can be hard to predict.

And finally, we have the potential for high winds Sunday night into
Monday as well. Following the frontal passage, areas across northern
Wyoming favored by north and northwest flow with strong cold
advection may see some high winds. This is mainly the northern Big
Horn Basin and especially northern Johnson County. The mid level
winds again look substantial enough to meet criteria. Still some
time here though so we will hold off on any highlights. Wind should
decrease for all areas later Monday as the pressure gradient
decreases. Tuesday at this point looks to be a nice autumn day with
sunshine and some high clouds along with above normal temperatures.

As for the chance of precipitation East of the Divide, strong
downsloping flow should prevent most of this outside of the
mountains and possibly the northern Big Horn Basin. Any amounts East
of the Divide should be light.

.LONG TERM...Tuesday night through Saturday

Overview...It will be warm, breezy, and dry Wednesday ahead of the
next cold front. Temperatures may fall short of record highs
Wednesday (most set in 2014), but the combination of conditions will
very likely generate elevated fire weather conditions. Cold front
swings rapidly south across areas east of the Continental Divide
Wednesday night with accompanying rain and snow chances Thursday
morning. Lower elevations could see at least light accumulations by
mid-morning Thursday. Temperatures will be 20-30F degrees colder
Thursday from what will be seen Wednesday. The below normal
temperatures will remain Friday along with dry conditions. A better
chance of showers may arrive Saturday as another weather system
pushes south through the Northern Plains.

Discussion...Vigorous shortwave trough will begin its march toward
Wyoming Wednesday, pushing into the northern Rockies during the day.
Surface low develops over the plains of Montana/North Dakota
Wednesday afternoon with the attendant cold front draped just to our
north at 00Z/Thu. Southwest wind will crank up Wednesday afternoon
ahead of this front and 700mb temperatures could be in the +8C to
+10C range. Given abundant sunshine, warm mid-level temperatures,
and the increased wind, maximum temperatures Wednesday should be in
the upper 60s to possibly the mid 70s. Have again gone above blended
model temperatures for Wednesday as well as boosted forecast wind
speed. The combination of conditions will also serve to elevate fire
weather conditions, particularly in the still critical zones of
Johnson and Natrona counties.

12Z ECMWF model run has backed off on its temporary flirtation with
a Four Corners low Thursday into Friday and is now more in line with
the solutions offered by the GFS and Canadian. Those more consistent
solutions show the cold front blasting into northern Wyoming
Wednesday evening and through the central basins east of the
Continental Divide by 12Z/Thu. Snow levels will drop rapidly behind
the front, falling to around 5K feet MSL by sunrise Thursday. Looks
as though there will be some light accumulations across the lower
elevations east of the Divide for the Thursday morning commute.
Precipitation will generally exit the area by 18Z/Thu, with the
possible exception of some lingering light snow around Casper. Other
area of concern Thursday will be the post-frontal gusty north wind.
This wind will be most prevalent across northern Johnson County and
the northern Big Horn Basin. Have increased speeds and gusts
substantially from blended solution and collaborated accordingly.
Could easily see speeds of 30 to 45 mph at Buffalo in this scenario.
Thursday temperatures will be well below normal.

Friday looks to be generally dry, although instability snow showers
off the south end of the western mountains would not be surprising.
Temperatures will remain well below normal. Strong northerly jet
remains overhead as the western U.S. ridge and eastern trough show
little movement. Clipper-like system will slide south through the
northern Plains Saturday possibly producing another chance of snow
and cooler temperatures in Johnson and Natrona counties. Really saw
no obvious targets of opportunity to make forecast adjustments
heading into the weekend.


.AVIATION...00Z Issuance

Wind will continue to be the primary aviation hazard through Sunday.
Gusty westerly surface wind of 18G26kts will be widespread until
around 00Z-01Z/Sun before diminishing. However, 700-800mb layer
winds will increase during the overnight hours creating the
potential for foothill mountain waves and LLWS. KCOD and KLND the
obvious possibilities for potential mountain waves and LLWS of 30-
40kts below 1500-2000 feet AGL. Still have concerns about a brief
period of strong surface wind at KLND between about 09Z/Sun and
maybe as late as 15Z/Sun. Synthetic satellite imagery and cross-
sections indicate this might be the best time for the wind to
materialize at KLND. Regardless, lee slopes of the Absaroka and Wind
River ranges will be impacted by LLWS through most of Sunday. As for
clouds and moisture, KJAC will be the only terminal impacted through
Sunday. Trapped moisture will remain across the western mountains
obscuring mountain tops through at least midday Sunday. Low-end VFR
ceilings at KJAC will lower as mid-level moisture arrives Saturday
evening and spreads across northwest Wyoming overnight. MVFR and
intermittent light snow expected through the night and much of
Sunday morning at KJAC before conditions dry and ceilings rise to



A cold front and upper level disturbance will cross the state
tonight through Sunday night. High winds are likely across the
Absarokas and Cody Foothills tonight and tomorrow and across the
southwestern wind corridor from the Red Desert through Casper
tomorrow. More snow is likely across the northwestern mountains as
well. Relative humidity should stay above critical levels. Mixing
and smoke dispersal tomorrow will range from poor some of the basins
to good in the windy areas.


High Wind Warning until midnight MDT Sunday night for WYZ002-003-

High Wind Warning from 6 AM to 6 PM MDT Sunday for WYZ019-020.

High Wind Watch from 6 AM MDT Sunday through Sunday afternoon
for WYZ015-018.

Winter Weather Advisory until 9 PM MDT Sunday for WYZ001-012.



SHORT TERM...Hattings
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