475
FXUS65 KRIW 171925
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
125 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 /18Z TAFS THROUGH 18Z TUESDAY/...
Issued at 1145 AM MDT Mon Jun 17 2024

A cold front associated with a shortwave trough will move through
the region today. This will bring increased chances for showers,
thunderstorms, and even snow showers to portions of the region. The
front will move northwest to southeast across the region during the
afternoon, with first chances for associated showers expected at
KJAC after 20Z. KCOD and KWRL will be the next terminals to see
chances for showers, with isolated thunderstorms also possible. KCPR
also is expected to see showers and thunderstorms associated with
the cold frontal passage, with best chances after 06Z. MVFR
conditions and mountain obscurations are expected with the shower
and thunderstorm activity, most notably at KJAC and KCOD. KJAC may
see occasional IFR conditions, and there is an outside chance (<20%)
that some snow flakes may mix in with rain showers after 06Z.
Otherwise, VFR will generally prevail through the period.

Gusty winds will occur both ahead of and behind the front. West of
the Divide, winds will generally remain westerly with a slight turn
to northwesterly at KBPI and KPNA. East of the Divide, southwesterly
winds early this afternoon will turn northwesterly behind the front.
This shift will be most notable at KRIW and KCPR. Gusts could reach
30 to 35 kts during the afternoon and evening at most terminals
ahead of and with the front, and winds will slowly decrease through
the late evening and overnight behind the front, but remain breezy.
KRKS has the best chance (40%) of occasional wind gusts reaching
40kts late this afternoon.

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...CNJ
AVIATION...Hensley