040
FXUS65 KRIW 022054
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
254 PM MDT Thu May 2 2024

.KEY MESSAGES...

- Breezy and dry conditions across Natrona County this afternoon,
  with a Red Flag Warning in effect through this evening.

- A quick-moving weather system will bring a round of moderate
  snow to the Wind River Range tonight into Friday morning.
  South Pass and Sinks Canyon will see accumulating snow. A
  Winter Weather Advisory is in effect tonight through Friday
  early afternoon.

- After a brief warm up this weekend, widespread rain and snow
  impact the region Monday, followed by another cooler than
  average work week.

&&

.DISCUSSION...
Issued at 250 PM MDT Thu May 2 2024

A longwave trough remains over the region today, and an embedded
shortwave will move overhead this evening. Ahead of this
shortwave weather system, dry air and breezy conditions for
central Wyoming have led to elevated fire weather concerns (see
the Fire Weather section for more details).

The shortwave weather system moving through tonight into
tomorrow (Friday) morning looks to be more robust than in
previous model runs. For this reason, snow amounts were
increased, mainly for the Wind River Range and southern
Absarokas. South Pass looks to get 4 to 6 inches of snowfall,
and although much of this may quickly compress given the
relatively strong sun leading to warm ground this late in the
spring, there were enough signals to justify issuing a
localized Winter Weather Advisory for the northeastern side of
the Wind River Range. Expect winter travel conditions over South
Pass tomorrow morning, and those in Sinks Canyon or Dubois are
likely to see a light layer of snowfall in the AM hours.

Saturday will bring a brief warm-up to the area, with
temperatures about 5 degrees above average. On Sunday, the next
weather system starts impacting the western side of the state,
bringing mountain snow, valley rain, and much cooler
temperatures. Those east of the Divide will have one more day of
warmth on Sunday with temperatures about 10 degrees above normal,
but it will come paired with strong winds gusting in the 40 to
55 mph range ahead of the cold front. Precipitation spreads east
of the Divide Sunday night, and Monday will be wet across most
of the state. Mountains could easily pick up another foot or
more of snow from this system, especially the Tetons where
multiple feet look likely (70% confidence). Lower elevations
are likely (60% chance) to see a wetting rain of a tenth or more
of precipitation, with the Bighorn and Powder River Basins
(northern Wyoming) being favored for the heavier and more
widespread rain/snow. This system exits the state Monday night.

On Tuesday and into the end of next week, cooler weather
prevails under another broad trough. Scattered mountain showers
are likely especially during the afternoons, but no other major
weather systems are set to impact the state through next Friday.

&&

.AVIATION /18Z TAFS THROUGH 18Z FRIDAY/...
Issued at 1106 AM MDT Thu May 2 2024

VFR conditions through the day today. A shortwave passes through the
area beginning this evening, with impacts tonight through early
Friday afternoon. Precipitation chances (which will most likely be
in the form of snow for all sites) begin at KJAC around 00Z/Fri,
with the best chances occurring after 06Z/Fri. Models are showing
KJAC on the edge of the best snow chances, so timing and categories
are hard to narrow down; VFR to IFR is possible. The recent NBM run
has peak chances for the worst flight category at about 10Z/Mon (50-
60% chance of at least MVFR). For KBPI and KPNA, snow chances look
to be more banded, making it very difficult to pin down a timing and
duration of snow lasting.

Snow bands pass through the KRKS area around 10Z-14Z/Mon. NBM has
the peak chances for the worst flight category at about 12Z/Mon,
which lines up well with other high-resolution models. If a snow
band does pass, expect a quick drop in visibilities. Snow bands are
difficult for models to place accurately in time, so a change in this
timing is very possible.

Snow impacts begin at KLND and KRIW around 08Z/Mon. The best snow
chances continue through most of the morning and should be ending by
18Z/Mon. Though MVFR currently looks to be the most probable
category, IFR is possible (20%). Snow looks to last longer at KCPR,
potentially continuing a little after the TAF period.

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

&&

.FIRE WEATHER...
Issued at 250 PM MDT Thu May 2 2024

Critical fire weather conditions currently exist across the
lower elevations of Natrona County (Fire Weather Zone 280),
with weather stations showing west winds 20 to 30 mph, gusting
35 to 45 mph. Weather models currently show these conditions
lasting until about an hour before sunset, which seems
reasonable given typical diurnal patterns. With sunset, winds
quickly decrease and RH increases. The air mass will be more
moist tomorrow with widespread light precipitation and lighter
winds, so fire weather concerns are much lower. On Sunday,
strong winds and relatively low RH in the 20 percent range could
again bring elevated fire weather concerns mainly to areas east
of the Continental Divide.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory from 9 PM this evening to 3 PM MDT
Friday for WYZ015.

Red Flag Warning until 8 PM MDT this evening for WYZ280.

&&

$$

DISCUSSION...VandenBoogart
AVIATION...Wittmann
FIRE WEATHER...VandenBoogart