393
FXUS65 KRIW 012202
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
402 PM MDT Sat Jun 1 2024

.KEY MESSAGES...

- High-based convection producing gusty outflow wind 40 to 50
  mph will be the primary hazard across southwest and central
  Wyoming through sunset Saturday. There is a 20 percent chance
  of wind gusts 50 to 55 mph.

- A ridge of high pressure boosts daytime high temperatures to
  much above normal for the period Wednesday through Saturday.

- The above normal temperatures allow for an active mountain
  snowmelt throughout much of the coming week. Foothill creeks
  and streams will see daily diurnal rises. The most notable
  rises are expected in smaller tributaries of the Snake River
  and the northern Bighorn Range.

&&

.DISCUSSION...
Issued at 124 PM MDT Sat Jun 1 2024

An active cumulus field stretches from a shortwave in northern
Nevada into southwest Wyoming early Saturday afternoon. Midday
webcams indicated generally virga from this weak convection
given a dry sub-cloud layer. In fact, the upstream KSLC 12Z
sounding showed a moist layer around 14K feet AGL. Soundings and
temperature-dew point spreads potentially approaching 50F lend
credence to the possibility of 45-55 mph convective outflow wind
gusts across southwest Wyoming later Saturday afternoon.
Thermodynamics and bulk shear become more favorable for
intensification after 3pm Saturday. After initial development
over southwest Wyoming, CAMs show some consistency in bringing
these showers into Fremont and Natrona Counties early Saturday
evening. Overall, high-resolution ensembles indicate a 20
percent chance of 50+ mph gusts during the 3pm to 9pm timeframe
within these regions of the forecast area. The mid-level
moisture plume lingers overnight as the upstream shortwave
traverses the forecast area. This combination leads to lingering
light showers over the aforementioned areas into Sunday
morning. Gusty outflow wind remains the primary hazard,
although lesser speeds of 30 to 40 mph would be most likely peak
gusts.

A secondary shortwave follows on the heels of this first wave,
reaching the western border Sunday morning. Instability looks weak
Sunday and mainly confined to central and southern sections through
early afternoon. As this shortwave crosses the region, further
stabilization comes to central Wyoming. In general, convection looks
to wane by early evening. Nonetheless, scattered showers are
expected across the region throughout the day. The upper-level flow
backs to the southwest Sunday night and Monday in response to the
next upstream shortwave. This provides a steady increase in mid-
level moisture across western Wyoming Monday morning. Dynamics and
deepening moisture favors more widespread convection across the
northwest-quarter of the state Monday afternoon and evening as the
shortwave approaches and treks just to our north. This track enables
lingering convection over the far north late Monday night into
Tuesday morning. One concern for Monday is the potential for rain-on-
snow in the western mountains, which would enhance snowmelt runoff
in the higher terrain feeding the Snake Basin. For the most part,
QPF in the far west mountains ranges from one-third to two-thirds of
an inch Monday and Monday evening. Higher totals could fall in the
mountains of the Teton Range and southwest Yellowstone National
Park. This situation bears watching given the rising water levels
associated with snow melt Saturday and Sunday.

Tuesday finds west-northwest flow aloft across the forecast area as
the shortwave moves into the Northern Plains. Breezy west to
northwest wind prevails into the evening hours. Any convection would
be confined to the far north. Daytime highs will be slightly above
normal. A building ridge over the Great Basin and West Coast allows
for these temperatures to climb beginning Wednesday. Daily high
temperatures will be approximately 10F above normal Wednesday
through Saturday leading to widespread 80s for most lower elevation
locales. Ensembles continue to depict a weak low in the Desert
Southwest circulating moisture north on the backside of the ridge
late in the week. Friday and Saturday appear to be the most likely
days for some of this sub-tropical moisture to reach Wyoming and
initiate late day convection.

Above normal temperatures, especially from Wednesday onward,
allow for active high-elevation snowmelt to continue. Much of
the remaining snowpack is above 9K feet, but there are certain
areas where lower snow still remains. As a result, the most
notable rises are expected in smaller tributaries of the Snake
River and the northern Bighorn Range. For the most part,
foothill creeks and streams see daily diurnal rises that will
need to be monitored.

&&

.AVIATION /00Z TAFS THROUGH 00Z MONDAY/...
Issued at 358 PM MDT Sat Jun 1 2024

VFR conditions prevail through the TAF period. At least isolated
(15% to 20%) shower and thunderstorm chances continue the first few
hours of the TAF period across central and southern Wyoming,
potentially impacting KBPI, KPNA, KRKS, KLND, KRIW, and KCPR.
Even if a shower/thunderstorm doesn`t impact a terminal
directly, gusty outflow winds around 40-45 kt are possible in
the vicinity of these showers.

Winds and rain chances decrease by late this evening, with
relatively light winds overnight. Rain chances (15% to 30%) move
into western Wyoming between 07Z and 14Z, with KJAC being the
potentially impacted terminal there. Isolated chances (15% to
25%) occur near the mountains Sunday afternoon. Chances then
remain too low to mention at any site, except KCPR, which has
the best chances (around 25% to 30%).

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...Wittmann/LaVoie