525
FXUS65 KRIW 170419
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
1019 PM MDT Sun Jun 16 2024

.KEY MESSAGES...

- A cold front pushes across the forecast area Monday increasing
  the chance for precipitation, especially across the north.
  Gusty west to northwest wind follows the front along with much
  cooler temperatures.

- The colder air leads to light snow over the northwest corner
  of the state Monday night, including lower elevations of
  Yellowstone. Sub-freezing temperatures look likely for the
  western valleys/basins.

- After an unseasonably cool Tuesday, temperatures begin to
  rebound Wednesday and climb to above normal Thursday and
  beyond.

&&

.DISCUSSION...
Issued at 100 PM MDT Sun Jun 16 2024

Mid and high clouds have persisted over the region through early
afternoon Sunday aided by jet-level energy. Dry conditions have
prevailed as stable conditions and limited moisture have been
unfavorable for any convective development. Instability finally
increases late Sunday afternoon in Natrona and Johnson Counties were
surface dewpoints around 40 are at least a tad more favorable. Any
convection that develops will be short-lived and headed eastward by
midnight. Elsewhere, cloud cover will be on the increase overnight
as a shortwave in the Pacific Northwest begins an eastward trek
toward Wyoming. Seasonal overnight temperatures are in store.

The approaching shortwave and associated cold front reach far west
Wyoming by midday Monday. Pre-frontal gusty southwest wind ramps up
in advance of the front during the late morning. Surface winds veer
to the west and northwest following frontal passage. Jet streak
dynamics and increasing mid-level moisture favor a steady increase
in rain shower coverage across the northwest beginning mid-morning
Monday, spreading across mainly the northern-third of the region
through the afternoon. The best instability and strongest convection
looks to be across Johnson County late Monday afternoon, although
all areas east of the Continental Divide are susceptible to isolated
thunderstorms. While western locales trend much cooler Monday, areas
of central and southwest Wyoming will still see seasonal
temperatures ahead of the cold front.

Scattered showers continue across the north and east through Monday
evening as the front progresses east. Attention then shifts to wrap-
around moisture and cold air in northwest flow moving into the
western mountains and Yellowstone Monday evening through at least
sunrise Tuesday. Mid-level temperatures around -6C/-7C favor snow
levels falling to the valley floors in Yellowstone. Despite recent
warm weather, there is a 50-60 percent chance of light snow
accumulations up to around one inch above 7000 ft MSL. The Tetons
and northwest mountains above 9000 ft have a 60-80 percent chance
of 2 to 4 inch accumulations, but a less than 30 percent chance
of 6 inches or more by sunrise Tuesday. Cloud cover plays a
role in how cold the western valleys and basins get Monday
night. Cloud cover should be less extensive in Star Valley,
south Lincoln County, and the Upper Green River Basin, allowing
for better radiational cooling. HREF plots shows 29-32F by
sunrise Tuesday for these areas and Jackson Hole. This is a tad
warmer than what was seen yesterday, so confidence in a hard
freeze <28F is not as high. The better chance for lows <28F
will be at Old Faithful and Canyon in Yellowstone.

Light orographic-driven snow showers continue much of Tuesday over
the northwest mountains with scattered rain showers across lower
elevations of the north. Daytime highs Tuesday will be 8 to 15
degrees below normal. Temperatures across the western valleys and
basins will again be cold Tuesday night given less cloud cover.
Locations east of the Divide mainly see lows in the lower 40s. Weak
ridging begins to build back into the region Wednesday as the early
week trough lifts to the northeast. Temperatures rebound closer to
seasonal readings and many areas will be dry. A warming trend
continues into next weekend with above normal temperatures beginning
Thursday. A general westerly flow aloft will favor late-day
convection along and east of the Divide each day.

&&

.AVIATION /06Z TAFS THROUGH 06Z TUESDAY/...
Issued at 1014 PM MDT Sun Jun 16 2024

VFR conditions are expected through tonight with lingering midlevel
clouds for many terminals. A cold front will begin to push into
western Wyoming around 18Z Monday. Showers will impact the KJAC area
first, with lower chances of showers and thunderstorms then
spreading east through the rest of the afternoon and evening. The
best chances for direct terminal impacts and possibly MVFR
conditions will be at KJAC and KCOD, with lesser chances at KCPR and
KWRL. Otherwise, wind will also be an aviation concern ahead of and
behind the front, with gusts of 25 to 35 knots fairly widespread
through Monday evening.

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

&&

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

&&

$$

DISCUSSION...Jones
AVIATION...Myers