539
FXUS65 KRIW 192219
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
419 PM MDT Sun May 19 2024

.KEY MESSAGES...

- Elevated to critical fire weather conditions today, with gusts
  up to 40 mph and relative humidities 10% to 20%.

- Wet and cool Monday and Tuesday. Some locations east of the
  Divide may see (40% to 60% chances) over an inch of rain.

- Mountains see snow, the heaviest occurring Monday night
  through Tuesday. Over a foot is possible. Expect impacts to
  travel over mountain passes.

- Unsettled weather pattern continue to end the week.

&&

.DISCUSSION...
Issued at 1226 PM MDT Sun May 19 2024

Southwest flow today ahead of the next weather system. Gusty winds
up to around 40 mph develop across much of the area, continuing
through the afternoon. Relative humidities (RH) drop below 20%
for many lower elevation locations, with near 10% for some.
These two factors lead to elevated to critical fire weather
conditions today. Isolated (15% to 50%) showers moves across the
area, mainly for far northern Wyoming and southern areas, where
the best leading moisture and jet support is. A rumble of
thunder is possible with these.

The advancing upper low, and associated cold front, push into Idaho
tonight. This low tracks across Wyoming through Tuesday and will be
the main weather driver the next couple days.

For Monday, precipitation chances will be increasing, even before
sunrise as moisture enters the state. The best chances (15% to 40%)
Monday morning begin across southwest Wyoming, where jet support
will help initiate showers. Chances spread through much of the area
through the morning, however. Widespread chances of 30% to 80%
chances for the rest of Monday. Highs Monday will only be in the 50s
for most, about 10 degrees below normal.

The low tracks eastward Monday night into Tuesday. This brings
heavier precipitation to roughly the Fremont, Natorna, and Johnson
County areas, as well as the eastern Wind River, Absaroka, and
Bighorn Mountains. Starting with those lower elevation counties,
rain total amounts for Monday and Tuesday (most of which occurs
Monday night through Tuesday afternoon) have a 40% to 60% chance to
be above 1 inch. A few favored locations, such as near mountains
where upslope is favored, is closer to a 70% chance. Current
forecast has just under an inch to about an inch for the lower
elevations of roughly Fremont, Natrona, and southern Johnson
Counties. Elsewhere across the area, totals are generally less than
half an inch.

For the mountains Monday through Tuesday, the big day will be Monday
night through Tuesday as east to northeast flow favored upslope on
the eastern side of the mountains. By Tuesday night, snow amounts on
the eastern slopes of the Winds, Absarokas, and Bighorns could be a
foot or more. Expect winter travel conditions if going over the
passes in these areas. Will hold off issuing any winter highlights
given the heavier snow won`t start until Monday night. Snow amounts
in these areas have been fairly consistent run-to-run.

Tuesday will be cold day, with many places around the 50 degree mark
for the high, about 20 degrees below normal. Wind does not appear to
be a big factor Monday or Tuesday with this storm.

Transitory ridging Wednesday brings less precipitation chances and
slightly warmer temperatures, just a little below normal. Another
system looks to impact the area Thursday/Friday, with low confidence
as models have disagreement on the details. In any case, the end of
the week looks to remain unsettled, with cool and wet weather.

&&

.AVIATION /00Z TAFS THROUGH 00Z TUESDAY/...
Issued at 418 PM MDT Sun May 19 2024

A few showers that developed near KCPR this afternoon have swept
east, and should be complete for the evening. VFR conditions
continue through the overnight hours, with mid to high level clouds
starting to increase from the southwest. Gusty winds 25 to 30kts
will be decreasing soon, and will remain light from the north
overnight. Showers increase from the southwest late tonight, but
will initially remain higher based, keeping cigs above MVFR through
18Z/20. All sites will see more numerous showers during the
afternoon, with a few cloud bases falling near 3000 feet as they
pass. Best chances for lower cigs would be east of the Divide in the
afternoon. Winds Sunday will be noticeably lighter that Saturday,
remain AOB 10kts.

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 1226 PM MDT Sun May 19 2024

Elevated fire weather conditions today. Wind gusts to 40 mph occur
over most of the area today. RHs will be down to 20% or lower for
many locations. The Wind River and Green River basin see RHs down to
around 10%. Conditions start improving this evening. Wet and cool
starting Monday, with precipitation chances through the rest of the
week.

&&

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

&&

$$

DISCUSSION...Wittmann
AVIATION...Straub
FIRE WEATHER...Wittmann