229
FXUS65 KRIW 222346
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
546 PM MDT Wed May 22 2024

.KEY MESSAGES...

- A period of moderate to heavy snowfall is expected tonight
  across the Tetons and Gros Ventres, generally between 6 PM
  and midnight.

- Cooler and wetter conditions Thursday behind a cold front.
  Snow continues in the mountains but especially across the
  Bighorns. Windy for most locations Thursday.

- Warmer and drier on Friday. The next system will arrive this
  weekend bringing widespread precipitation chances.

&&

.DISCUSSION...
Issued at 400 PM MDT Wed May 22 2024

The next system is beginning to push into western Wyoming after a
short period of relatively tranquil conditions within transitory
ridging. Mid-level flow has become southwesterly in response to a
low pressure system that is dropping south into northeast Oregon.
Rain and snow showers will continue to increase across the west
through the afternoon as moisture is funneled up the Snake River
Plain. The rest of the area will remain on the drier side through
the afternoon, though a few showers will make their way east of the
Divide into the Bighorn Basin. Otherwise, the rest of the afternoon
will be breezy across the usual Wind Corridor as southwest wind
increases in response to a building pressure gradient ahead of a
cold front. Wind will quickly drop off after sunset.

Precipitation rates will ramp up further across western Wyoming this
evening with the arrival of increasing upper level divergence due to
a favorably placed ~100 knot jet. This should make for a fairly
heavy period of snowfall across the Tetons before midnight, perhaps
as much as an inch an hour for the higher elevations. While the
passes won`t get quite that much, it may still make for a difficult
drive this evening. Shower coverage east of the Divide will also
increase a bit, but most locations here will remain dry through
midnight.

There will be a relative lull across western Wyoming after midnight
as the low quickly approaches, turning the prevailing flow northerly
as a cold front surges east. This will shift the precipitation focus
to northern Wyoming by sunrise as moisture is wrapped back over the
state. Showers will spread to the rest of areas along and east of
the Divide through Thursday as the low skirts east across
central Wyoming. The cold front will result in temperatures much
cooler than today. Snow levels will drop to around 5,000 feet
in the morning, rising to around 7,000 feet in the afternoon.
With upslope favoring the Bighorns Thursday, a Winter Weather
Advisory remains in effect for generally 6 to 12 inches there.
Given the heavier period of snow tonight and continued lighter
snow tomorrow, we have issued a Winter Weather Advisory for the
Tetons/Gros Ventres as well. That said, impacts there may be
limited tomorrow due to spring temperatures and long daylight.

Another aspect of Thursday`s forecast will be wind as the front
passes, as well as when the northerly flow ramps up behind the low.
Accordingly, west wind will increase after sunrise across most of
the area. Then a northerly push will arrive in the Bighorn Basin by
early afternoon. Wind gusts generally look lower than needed for
highlights, but occasional gusts over 55 mph are possible (30
to 40 percent) in northern Johnson County Thursday afternoon.

Precipitation and wind will quickly decrease around sunset as the
system pulls away. Friday looks to see warmer and drier conditions
return as a weak ridge takes over. Guidance is then favoring the
next system arriving for the weekend. Details on this system are
uncertain as the pattern is not clearly organized. However, trends
remain favorable for a fairly widespread precipitation event
Saturday and Sunday before ridging returns on Monday. Currently,
this looks to be a warmer system than the current one, limiting snow
to higher mountain elevations.

&&

.AVIATION /00Z TAFS THROUGH 00Z FRIDAY/...
Issued at 545 PM MDT Wed May 22 2024

Another weather system continues to push into the region. Gusty
westerly winds will begin to decrease towards sunset. Winds will
continue to decrease and even become light and variable overnight
for most terminals. However, winds increase again starting by 10-12Z
and especially by 15-16Z Friday morning. Gusts 30 to even 40 kts
will be common for most terminals by 18Z Friday.

Precipitation chances will generally remain negligible to start the
period across the region, with the exception of KJAC, where -SHRA
will prevail through much of the evening (50% chance). Showers are
expected to become a steady light rain by around 02Z (70% chance).
KCOD will be the next terminal to see mentionable precip chances
(40% chance) with rain showers moving in after 06Z tonight and
prevailing through the rest of the period. KPNA may also see
some showers (30% chance) overnight. Another round of showers is
possible at KJAC by mid-morning Thursday, and likely (60%
chance) will be in the form of snow, rather than rain, with
temps dropping to freezing overnight. With any of these periods
of showers, expect prevailing VFR to occasional MVFR, degrading
to IFR during heavier rain showers and/or any snow showers.
Additional rain shower chances will spread across northern areas
east of the divide late in the period.

Another change to mention is a strong northerly push of wind
into the Bighorn Basin after 18Z/Thu which will move into
central WY (KRIW/KCPR) around 00Z/Fri, which should increase
precipitation chances Thursday evening and bring lower
ceilings.

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...
Winter Weather Advisory from 3 AM to 6 PM MDT Thursday for
WYZ008-009.

Winter Weather Advisory until 4 PM MDT Thursday for WYZ012.

&&

$$

DISCUSSION...Myers
AVIATION...Hensley/McDonald