834
FXUS65 KRIW 030359
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
959 PM MDT Sun Jun 2 2024

.KEY MESSAGES...

- A plume of Pacific moisture impacts western Wyoming Monday.
Precipitation totals of one-third to two-thirds of an inch will be
common in the far west mountains by sunset Monday.

- A ridge of high pressure will dominate the western United States
for the latter half of the coming week. Temperatures will be about
10F degrees above normal, with widespread lower elevation
temperatures in the mid 70s to upper 80s.

- The above normal warmth allows for continued melting of the
mountain snowpack. The most notable rises are expected in smaller
tributaries of the Snake River and smaller creeks and streams fed by
snowpack in the northern Bighorn Range.

&&

.DISCUSSION...
Issued at 100 PM MDT Sun Jun 2 2024

Weak troughing is across the forecast area early Sunday afternoon.
The coverage of showers and isolated thunderstorms becomes scattered
as instability increases a tad this afternoon in the heat of the
day. The far north and far south are favored areas given available
moisture and ongoing activity. Peak convective coverage is expected
between 3pm and 7pm Sunday with a rapid decrease by sunset. Outflow
wind of 30 to 40 mph would be the primary hazard with any
convection. The upper-level flow then backs more to the west and
southwest overnight in response to a longwave trough along the
Pacific Northwest coast. This trough ejects a more vigorous
shortwave toward Wyoming and the Northern Rockies Monday and Monday
night. A healthy dose of precipitable water feeds eastward into the
western portion of the state 12-15Z/Monday and persists for about 9-
12 hours. QPF of one-third to two-thirds of an inch will be
widespread in the western mountains. The orographically-favored
Teton Range and southwest Yellowstone National Park have about a 60
percent chance of QPF exceeding one inch over the 24-hour period
ending 6pm Monday. Scattered showers spread east of the Continental
Divide Monday afternoon and evening, with the best chance coming
over the far north and the I-80 Corridor through southwest Wyoming.
Precipitation chances gradually diminish Monday evening as the
shortwave tracks eastward through Montana and drier air arrives from
the west. Showers do linger over the far north through much of
Monday night. As for temperatures, readings will be cooler Monday,
particularly across the far west given precipitation and cloud cover.

The shortwave will be near the Montana/Saskatchewan border by
12Z/Tuesday. Cyclonic flow lingers over the northern-third of
Wyoming on the backside of this system through Tuesday morning
allowing for a few showers to persist over the northern mountains.
Moisture and dynamics wane Tuesday afternoon with west-northwest
flow aloft leading to dry and and breezy conditions region-wide
as the shortwave tracks northeast. West to northwest surface
wind 15 to 30 mph will be common by mid afternoon. Tuesday night
sees a ridge building in from the southwest with a clear sky
and slightly above normal temperatures on tap.

Wednesday finds a dominant ridge setting up over the Great Basin
with the axis extending north to the Idaho Panhandle. Temperatures
rise to around 10F above normal Wednesday, a trend that continues
through at least Friday as the ridge remains in place. Models have
been consistent in advertising sub-tropical moisture emanating from
a weak low off the Baja coast rotating north around the backside of
the mid-level high by Friday. Blended model solutions have responded
by gradually increasing the chance of precipitation for Friday into
the weekend. This probably leads to another round of high-based
convection, potentially beginning as early as late Thursday across
southwest Wyoming.

A look at SNOTEL data indicates many mountain locations have lost 4
to 6 inches of snow water equivalent (SWE) over the past five days.
Given these trends and the ripening of the snowpack, the expectation
is that much of the mountain snowpack below 9,500 feet will melt by
the end of this period. The exception will be the far west where
even at 8K feet there was still 8 to 12 inches of SWE in the
northern Salt/Wyoming Ranges and the mountains of Teton County. The
relatively warm airmass and slug of Pacific moisture Monday could
also lead to rain-on-snow in the 8-10K range of the western
mountains. Smaller tributaries of the Snake are the most susceptible
to minor flooding over the coming several days. There is a good bit
of capacity remaining in waterways east of the Continental Divide,
but a loss of 2 to 3 inches of SWE a day can quickly change the
situation. Smaller tributaries of the west slope of the
northern Bighorn Range will be monitored given that
approximately 12 inches of SWE remains.

&&

.AVIATION /06Z TAFS THROUGH 06Z TUESDAY/...
Issued at 955 PM MDT Sun Jun 2 2024

Light winds, mostly dry conditions and VFR conditions will
remain in place through 12Z. SCT-BKN FL100-200 will also remain
in place across the forecast area, with the thicker (BKN-OVC)
clouds occurring over the western mountains. An isolated shower
could occur under these clouds. A weather system approaches
western Wyoming, with increasing precipitation chances for KJAC
starting after 12Z. KBPI and KPNA see rain chances increase
after 15Z, with the best chances by around 18Z. Chances spread
eastwards during the afternoon, however chances remain 20 to 40%
for locations east of the Divide. Have kept precipitation
chances out of the forecast for now as a result, but this may
change in future forecasts, if model trends increase.
Precipitation will gradually end across the area after 00Z,
becoming more isolated through the end of the TAF period.

Winds also increase ahead of this system, with gusts increasing
to 20 to 25 knots before 18Z at many sites. Windiest locations
will be KCPR and KRKS during the afternoon, with gusts over 30
knots. These winds will continue after 00Z for most sites, as a
cold front drops southward after 06Z Tuesday.

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

&&

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

&&

$$

DISCUSSION...Jones
AVIATION...LaVoie