985
FXUS65 KRIW 170750
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
150 AM MDT Mon Jun 17 2024

.KEY MESSAGES...

- A cold front passes through today. As a result, gusty winds, shower
  and thunderstorm chances, mountain snow, and much cooler
  temperatures.

- A few strong to severe storms are possible for Johnson County today,
  mainly between 2PM and 8PM local time.

- Sub freezing temperatures occur for western valleys and basins tonight.
  Frost and freezing to outdoor vegetation is possible.

- Much cooler tomorrow (Tuesday), with temperatures climbing
  back above normal by Thursday.

&&

.DISCUSSION...
Issued at 148 AM MDT Mon Jun 17 2024

Current water vapor imagery shows an approaching cold front, the
associated upper-low currently over the Washington/Oregon/Idaho
border. This low tracks eastward into Montana today. The trough
swings down through Wyoming as a result, bringing the cold front
with it. Models show the main push of the front moving through
western Wyoming this afternoon, pushing east of the Continental
Divide this evening and overnight. The consequences of this frontal
push will be gusty winds, precipitation chances (including snow),
and cooler temperatures. These three things will be elaborated on
in the coming paragraphs.

Starting with winds, gusty southwest winds ahead of the front pick
up today late morning to early afternoon. Gusts 30 to 40 mph will be
common across central and southern areas, with gusts in the 20 to 30
mph range elsewhere. Stronger winds are possible with the frontal
passage. As the front passes through, winds shift more
northwesterly to northerly.

Cooler temperatures follow the front as well. Today`s highs west of
the Divide will range from the mid 70s in Sweetwater County to the
upper 50s and 60s for the western most basins and valleys. The cold
front passage will halt the diurnal temperature climb, so western
locations will be cooler today. East of the Divide, the front passes
in the afternoon and evening, so these locations should be able to
reach peak heating before the front passes. Highs here will be in
the mid 70s to mid 80s, warmest towards the Casper area where the
front passes latest.

Low temperatures Monday night/Tuesday morning are on the cool side.
Of note will be the western valleys and basins, where temperature
below freezing (32 degrees) are looking likely (75% to 100%
chances). The chance of less than 28 degrees (a hard freeze) are
around a 25% to 30% chance for the lower elevations of southern
Lincoln County, and for the upper Green River Basin, including Big
Piney and Pinedale. Star and Jackson Valleys are less than 10%
chance for less than 28 degrees. The one wrinkle in these
temperature forecasts will be cloud cover. Models have post-frontal
clouds. If clouds are more numerous than forecast, temperatures will
be warmer than forecasted. In any case, it may be a good idea to
prepare outdoor vegetation for sub-freezing temperatures.

Lastly, is precipitation. Starting with snow, snow levels look to
drop to around 6000 to 6500 feet at the lowest through sunrise
Tuesday, behind the front. Based on precipitation chance locations,
this puts snow over the western mountains (including Yellowstone
National Park) and the northern Bighorn Mountains. Most of the these
places are looking to receive somewhere on the order of 0.5 to 2
inches of snow. The higher elevations of the Tetons, Absarokas, and
Bighorns could see up to 6 inches. Notably, the Tetons have about a
60% chance to get more snow at the highest peaks based on the latest
model runs. Much of this snow falls Monday night, through about
sunrise Tuesday. Expect little to no impacts for Yellowstone, where
up to inch of snow is forecasted for the lower elevations.

For liquid (rain) chances, those will mostly be across the northern
half of Wyoming. Shower and thunderstorm chances occur this
afternoon, mainly east of the Divide, ahead of the incoming front.
Chances look isolated (15% to 30%) for most of these areas. Slightly
better chances (30% to 60%) occur with frontal passage for the
latter half of the afternoon and through the evening across the
eastern Bighorn Basin and for Johnson and Natrona Counties. Model
soundings are showing a favored environment for strong storms over
Johnson County between 20Z/Mon and 02Z/Tues (2pm and 8pm local).
CAPE values in the 1000 to 1500 J/kg range are possible based on a
few models, and given decent shear as well (bulk shear 50-60kts
based on latest RAP), some stronger storms may develop. Strong
outflow winds and hail are the main threats. The Storm Prediction
Center (SPC) has Johnson County in a marginal (category 1 of 5) risk
today, to account for the wind and hail threat.

Additional shortwaves behind the main trough keeps rain (and
mountain snow) chances through the day Tuesday, mainly across the
northern half of the area and mainly in the mountains. Dry slotting
early Tuesday and through the day will limit precipitation chances
across central areas. Highs Tuesday are noticeably cooler behind the
front. Most places will see highs in the 60s, which are 10 to 20
degrees below average for mid-June.

Temperatures begin to rebound Wednesday, with highs returning back
to seasonal normals as southwest flow dominates the state. Drier air
and lack of forcing will keep rain chances to near zero. The next
feature of interest is a trough in the late Thursday to Friday
timeframe. This looks to bring widespread rain chances, but
confidence is low at this range. High pressure builds in for the
latter half of next weekend, bringing warmer and drier conditions.
The Climate Predication Center (CPC) has Wyoming favored for warmer
than average temperatures in both the 6-10 and 8-14 day
outlooks.

&&

.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...Wittmann
AVIATION...Myers