458
FXUS65 KRIW 180405
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
1005 PM MDT Mon Jun 17 2024

.KEY MESSAGES...

- An abrupt wind shift and the arrival of cooler air follow
  passage of a cold front sweeping east across the region
  through Monday evening. Light snow falls in northwest Wyoming
  later this evening and overnight.

- The potential for strong to severe storms exists over mainly
  northern Johnson County between 3PM and 6PM MDT.

- Tuesday morning is chilly across the western valleys and
  basins as overnight lows fall to 27-32F.

- After an unseasonably cool Tuesday, temperatures begin to
  climb Wednesday through the end of the weekend. The best
  chance for widespread showers and storms is Friday.

&&

.DISCUSSION...
Issued at 125 PM MDT Mon Jun 17 2024

As of early Monday afternoon, satellite clearly shows an approaching
shortwave near the ID/MT border along the Bitterroot Range. A jet
streak has rounded the base of the associated trough and is making
its way toward southwest Wyoming. This SW-NE oriented jet streak
will aid precipitation production late this afternoon and evening as
it traverses the forecast area. Additionally, a surface cold front
moving through far west Wyoming continues east and works in concert
with the jet streak to boost coverage of showers and thunderstorms
east of the Continental Divide late Monday afternoon and evening.
Gusty west to southwest wind veers to the northwest behind the
cold front. This will led to persistent 20 to 35 mph northwest
wind through Monday evening in favored locations such as
northern portions of Johnson County and the Bighorn Basin, along
with the Wind River Basin.

A prevailing south-southeast surface wind has increased dew points
into the mid 40s across Johnson and Natrona Counties as of 1PM
Monday. CAPE is forecast to reach 500-800 J/kg between 2PM and
4PM and CAMS depict convection strengthening during this time as
CIN slowly erodes. The strongest storms are expected over
northern Johnson County ahead of the front, with the deepest
convection moving east and coming to a close between 6PM and 7PM
Monday. SPC has included a slight risk for this region. Large
hail will be the primary hazard with the strongest storms.

To the west, orographic northwest flow will favor a period of
precipitation this evening and overnight across northwest Wyoming.
The flow regime favors the heaviest snow in the higher elevations of
the Teton Range, while lower elevations above 7000 ft MSL in
Yellowstone could see snow accumulations up to around one inch. The
best chance for snow accumulation is from 8PM Monday to 2AM Tuesday.
As cooler air filters into the region, Tuesday morning lows will
fall to 27-32F. Cloud cover may elude locations south of an Alpine
to Bondurant to Pinedale line, where the coldest temperatures are
most likely. It would not be surprising to see overnight lows in the
mid 20s in a few locations, like Bondurant and south Lincoln County
if the westerly wind diminishes under a clear sky by sunrise Tuesday.

Tuesday keeps a moist westerly flow across the northern-third of the
forecast area. Precipitation lingers across this region through the
morning and becomes more widespread as instability increases Tuesday
afternoon. Temperatures will be unseasonably cool Tuesday with most
locations in the upper 50s to upper 60s, about 8 to 15 degrees below
normal. The trough departs to the northeast Wednesday and
temperatures begin to rebound and most areas return to dry
conditions. The exception could be the I-25 corridor late in the day.

A weak trough over California Thursday ejects eastward and crosses
the forecast area Friday. The Friday convection provides the
best chance for widespread showers and thunderstorms during the
Thursday to Sunday period. Temperatures warm Thursday and are
somewhat hindered by cloud cover and showers Friday.
Nonetheless, seasonal readings are anticipated both days. Zonal
flow in the wake of this wave allows for upward trending
temperatures and drier weather Saturday. Ridging then builds
over the Northern Rockies Sunday leading to above normal
temperatures and dry conditions to end the weekend.

&&

.AVIATION /06Z TAFS THROUGH 06Z WEDNESDAY/...
Issued at 1004 PM MDT Mon Jun 17 2024

Rain and even some snow showers are ongoing across much of the area
late this evening behind a cold front that has pushed east. This
activity will decrease over the next few hours, leaving VFR
conditions prevailing through most of Tuesday morning. This has also
brought gusty winds, which will linger at KCPR and KRKS all night.
By Tuesday afternoon showers will return to the northern half of the
area as moisture wraps back in to Wyoming. The best chance for
terminal impacts will be at KCOD, but certainly can`t rule out brief
impacts at KJAC and KWRL. This activity will end around sunset
Tuesday.

&&

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

&&

$$

DISCUSSION...
AVIATION...Myers