109
FXUS65 KRIW 171846
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
1246 PM MDT Sun May 17 2026

.KEY MESSAGES...

- Cool today. Rain chances increase through this afternoon as a
gusty north wind increases from north to south through this evening.

- Rain transitions to snow late this evening, and especially
  around midnight, with the heaviest snow occurring between
  midnight and 10am Monday morning. Combined with gusty winds,
  travel will be impacted tonight through Monday morning.

- Greatest travel impacts expected around Lander, near Casper,
  across South Pass, and over Interstate-80. Gusty winds and
  snow will bring reduced visibilities and possibly icy surfaces
  with colder temperatures moving in.

&&

.UPDATE...
Issued at 1245 PM MDT Sun May 17 2026

Some small adjustments to liquid and snow totals were made with the
latest hi-res models. However, the overall message remains
unchanged, with the most impacted area being from a Lander to Casper
line and south, including South Pass and I-80. Snow rates will be
greatest between about midnight and 10 am Monday morning. One area
of greater confidence is Lander, where totals by Monday morning will
impact travel. Have therefore upgraded to a Winter Storm Warning for
the Lander Foothills. Gusty north to northeast winds bring
further impacts as snow falls through Monday morning.

&&

.DISCUSSION...
Issued at 146 AM MDT Sun May 17 2026

In meteorology, the transition seasons, mainly spring and fall, are
usually the most difficult to forecast for. This is because you
still have different air masses of decent strengths still clashing,
and this is no exception over the next two days. In addition, it can
also lead to some strong storms since the weather drives off of a
lack of balance in the atmosphere, and this is especially present in
spring.

A cold front is currently dropping southward through the area as I
write this around midnight, with some strong wind behind it. This is
especially the case in the favored cold advection / northwest flow
areas like the Bighorn Basin and Johnson County, as both Buffalo and
Greybull have seen gusts to around 50 mph. Wind should decrease a
bit today, but will stay gusty through the day. It will also usher
in much cooler temperatures, with the first below temperatures we
have seen in a while. As for precipitation today, coverage will be
increasing as an upper level low heads toward the area. During the
day, the greatest coverage of showers will be across southern
Wyoming, with lesser chances further north. However, most areas
should be dry most of the time during the day today with the
exception of Sweetwater County south of Interstate 80, where showers
will be more numerous.

The real impacts from the system begin tonight. Low pressure will
open into a trough and pass to the south of Wyoming, bring an
impactful storm to much of the area, with an emphasis on the
southeastern half of our forecast area. Guidance is in good
agreement on timing of the maximum impacts, which look to be from
later Sunday evening through the rest of Sunday night and into
Monday morning with some improvement in the afternoon. Confidence is
high with this. However, it is lower in other facets of the storm,
but not all. We will discuss them individually below.

STRONG TO HIGH WIND...Gusty wind will occur through the day today.
However, the strongest wind for many areas looks to occur later
tonight and into Monday morning. Low pressure will pass over
Colorado with a decent area of cooler high pressure building to the
north. The result will be a good pressure gradient across the state,
about 16 millibars from the northern border to the southern border.
However, it is especially notable from south of Lander and Casper
down to the southern border, with an emphasis on the Green and
Rattlesnake Range and Sweetwater County, especially east of Rock
Springs. The 700 millibar wind increases through the night with the
strongest from around 3 am tonight until noon Monday when some
guidance shows as strong as 60 knots. Both the NAM and GFS MOS show
sustained winds at the Rock Springs airport over 40 mph at times
through that period. There could be some gusts to 70 mph at times in
the strongest wind, especially in the higher elevations in eastern
Sweetwater County and along South Pass. Both the HREF and NBM
ensembles show a greater than 1 out of 2 chance of wind gusts over
50 mph through this period in Sweetwater County, including
Interstate 80. Travel is strongly discouraged during this period
along Interstate 80. Wind should decrease in the afternoon as the
low moves into the Plains states and the pressure gradient
decreases, with lighter winds returning by Monday night.
CONFIDENCE...Medium to high.

SNOW... Uncertainty increases significantly here, along with fact it
is mid May, the ground is warm and a lot of the snow that falls
after 10 am would have trouble sticking due to the high May sun
angle, making forecasting accumulations difficult. Models have
come into somewhat better agreement in having the maximum
impacts across southern Wyoming. Timing of the heaviest snow is
also in fairly good agreement, with the maximum impacts from
late this evening through Monday afternoon. There are a lot of
contrasts here, so we will split this into three separate
sections: Southern, Central and Northern Wyoming.

Southern Wyoming...This still looks to be the most impacted
location. This area will be closer the closest to the low and
the best dynamics. Eastern Sweetwater County still looks to be
most impacted from this, probably for the entire area. Both the
NBM and HREF ensembles give a greater than 1 in 2 chance of
greater than 6 inches of snow across the area. However, good
luck measuring it with wind gusts expected to be over 50 mph.
There could be some short periods of blizzard conditions, but
enough confidence to upgrade here. Conditions here shouldn`t be
too bad until after sunset, with the changeover occurring
between 8 pm and midnight as 700 millibar temperatures drop
below minus 5. The most impactful time still looks to be from
midnight Sunday night through Monday morning with improving
conditions after that. Interstate 80 will likely be a mess and
travel is not advised. We did make a couple of changes.
Ensembles are showing a greater than 1 in 2 chance of 3 inches
or more of snow in southern Lincoln County, so we added that
zone to an advisory. We thought about the Salt and Wyoming Range
as well, but with worst impacts will be above pass level.
Otherwise, snowfall amounts look reasonable. And one final
thing, there may be enough lift for some thunder snow.

Central Wyoming...Things get a little more uncertain here, as
terrain effects will have more of an impact here. This area will
also be closer to the edge of the steadiest precipitation. The
maximum impacts will be similar to other areas, mainly late this
evening through Monday morning. But here are the forecast
quandaries. In the Wind River Basin and Natrona County,
elevation and upslope will be the concerns. With elevation,
locations under around 5000 feet or so may stay a bit warmer for
longer, allowing for more mixing and decreasing snowfall
amounts. This includes places like Shoshoni and some areas
around the north side of Casper. And the other is the upslope /
downslope component. This could be especially notable in the
Wind River Basin. Flow will be strong out of the north and
northeast. This should led to a shadow in northern Fremont
County, probably somewhere around Missouri Valley Road, where
accumulations may be 2 inches or less. Contrast this with areas
like Lander and Jeffrey City, where there will be good upslope
flow and some areas may over perform. Where the exact cutoff for
the best upslope will occur is still difficult to pinpoint.
This could be a situation where Shoshoni has a coating to an
inch, Riverton has 1 to 3 inches, 4 to 6 in Lander while Sinks
Canyon has closer to a foot. It is the same story in Natrona
County, where the lower elevations around Casper only have a 1
out of 4 chance of 4 inches or more. Meanwhile, US 20-26 to the
west, which is around 1000 feet higher in some places, has a
greater than 1 in 2 chance of 6 inches or more. Casper Mountain
should also do well out of this, with a greater than 2 out of 3
chance of a foot or more above 8000 feet. The big winner still
looks to be eastern Wind River Range though, where the highest
peaks have a greater than 1 out 2 chance of 18 inches or more.
In the lower elevations, the greatest impacts will be in the
Green and Rattlesnake Range, where chances of over 6 inches of
snow are greater than 4 out 5 and wind gusts over 40 mph will
make for very difficult travel.

Northern Wyoming...Impacts drop somewhat here. But again, elevation
and upslope will play a role here. With elevation, this area
generally has a lower elevation so impacts will be less. The eastern
Big Horn Basin still looks to be the least impacted, with less than
an inch or snow expected. The most impacted area, other than the
mountains, looks to be Cody Foothills. For travel impacts, the
portions of the higher elevations near Meeteetse above 5500 feet
where there is greater than 1 in 2 chance of 3 inches of snow or
more. However, wind is expected to be gusty to strong across much of
the area, The advisories here are more impact related than snowfall
total, as the wind and snow may reduce visibility and there could be
some icy spots on the roads if the snowfall is heavy enough. Far
northwestern Wyoming will be the least impacted, with only minor
accumulations of snow expected.

Starting Monday night, somewhat quieter weather then moves in
for the rest of the week. The Rockies will still be under an
area of cyclonic curvature so some instability showers will
still be possible Tuesday and Wednesday, especially in northwest
Wyoming where general northwest flow will bring some added
upslope component. Most areas should remain dry though. Another
weak system may move through Thursday with another chance of
showers Thursday. With the northwest flow, temperatures should
average below normal through Thursday with warmer temperatures
returning for Friday an the holiday weekend as flow turns more
zonal.

&&

.AVIATION /18Z TAFS THROUGH 18Z MONDAY/...
Issued at 1122 AM MDT Sun May 17 2026

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

Longwave trough approaching form the west will begin to generate a
few Sunday afternoon showers. KRKS is the most likely location for
impacts as showers move northeast in a narrow axis. Attention then
begins to shift to a mid-level low spinning up over northeast Utah.
This low wraps moisture into southwest Wyoming beginning Sunday
evening through at least midday Monday. A tight pressure gradient
leads to increasing northeast wind (20-40kts) at KRKS Sunday night.
Thus, IFR to reign at KRKS after about 03Z/Monday as snow and wind
increases. Light snow provides a 6- to 10-hour period of MVFR light
snow at KPNA and KBPI between 05Z-15Z/Monday. Downsloping northerly
flow should limit impacts at KJAC where forecast now has PROB30
03Z-09Z/Monday for MVFR snow showers. Mountain tops obscured
after 03Z/Monday.

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

MVFR cloud deck at KCPR persists until around 20Z/21Z Sunday before
giving way to VFR conditions. This is short-lived as a mid-level low
takes shape over northeast Utah and induces widespread upslope flow
across the central basins by mid-evening Sunday. Northerly surface
wind gradually increases to 15-30kts as the pressure gradient
tightens across the state. Overrunning precipitation from the
southwest along with favorable jet dynamics and upslope flow will
lead to an extended period of IFR conditions at KCPR, KRIW, and KLND
tonight and Monday morning. Light rain changes to snow between 05Z-
08Z/Monday at all but KWRL. The low slowly begins to pull east
around midday Monday. IFR/MVFR prevails through 18Z/Monday at KCPR,
KRIW, and KLND, although some slow improvement will be seen late in
the forecast period. Mountain tops obscured after 03Z/Monday.

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

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory from 6 PM this evening to 6 PM MDT
Monday for WYZ002-003-005-007>011-014-016-017-026>029.

Winter Storm Warning from 6 PM this evening to 6 PM MDT Monday
for WYZ015-018>020-022-030.

&&

$$

UPDATE...Wittmann
DISCUSSION...Hattings
AVIATION...CNJ