971
FXUS65 KRIW 042330
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
530 PM MDT Sat May 4 2024

.KEY MESSAGES...

- Breezy afternoon for some, but otherwise warm and sunny today.

- Impactful storm starts Sunday. Gusty winds 35 to 50 mph, with
  60+ possible for some locations. A High Wind Watch/Warning is
  in effect.

- Precipitation moves into western Wyoming Sunday. A Winter
  Storm Watch is in effect for the Tetons and Yellowstone.
  Precipitation becomes widespread across the area by Monday.

- Unsettled and windy workweek. Uncertainty in snow and rain
  amounts Tuesday and beyond.

&&

.DISCUSSION...
Issued at 142 PM MDT Sat May 4 2024

Not much to note about today (compared to what`s coming).
Temperatures are notably warmer today, with many places peaking in
the 60s; a few low 70s as well today. Besides some high clouds
passing through, the sky remains mostly clear as well. A breezy
southerly wind develops this afternoon for some locations, with
gusts 15 to 30 mph, nothing too unusual for a Wyoming spring day.

Current water vapor imagery shows the next approaching weather
system, a Pacific low centered just west of Oregon. This will be the
weather-maker over the next week.

The low makes landfall tonight, its center moving towards Wyoming.
As it does so, it tightens the pressure gradient, and brings a 100
knot jet. Southerly winds begin to increase after sunrise Sunday,
first impacting the higher elevations and southern Sweetwater
County. By mid-morning, gusts in excess of 50 mph are likely (70%)
for portions of Sweetwater County into southern Fremont County as
the southerly flow increases. By afternoon, gusts 30 to 50 mph will
be widespread for much of the area, including the Green River Basin,
the Wind River Basin, portions of Natrona and Johnson Counties, and
possibly into the Bighorn Basin to name a few. Gusts for southern
Sweetwater County are likely (80%) to be in excess of 58 mph. During
peak afternoon heating and mixing, high wind is possible across
portions of Sweetwater, Fremont, and Natorna Counties; a High Wind
Watch has been issued for the applicable locations. Sweetwater
County has been upgraded to a Warning given better confidence there.
Shift of Tails continues to highlight southern Sweetwater County in
the 90th percentile and a tongue of 80th+ percentiles for the
locations just mentioned. Of note is the Lander foothills, where
high winds are possible in the afternoon, as well as the evening, as
gusts begin to turn southwesterly. Another point of interest is
Thermopolis, where downsloping winds could reach up to 50 mph given
the favored direction. Expect travel impacts to I-80 and South
Pass/Red Canyon as strong crosswinds occur.

Winds decrease some Sunday night as mixing lessens. Winds also shift
southwesterly as the low progresses eastward. Monday morning, winds
begin to increase again. By late morning, high winds (gusts over 58
mph) are again possible (70%) for areas of Sweetwater County,
southern Fremont, and into Natrona County (Wind Corridor). These
winds should continue to around sunset Monday. The High Wind Watch
extends into Monday for this reason. As mentioned, it`s important to
note that high winds are not likely (greater than 90%) to occur
overnight Sunday into Monday. Winds pick up again for Tuesday.
Currently not seeing indication of high winds, as 700mb winds are a
bit lower (30 to 40 knots, rather than 50 to 60) and wind direction
is a bit more westerly (not as favored). Moderate confidence on this
at this time.

Taking a step back in time now, the other element to this system is
precipitation. Starting on Sunday, precipitation chances move into
far western Wyoming by 12Z (6am). Chances spread eastward through
the day, but should mainly impact locations west of the Continental
Divide Sunday. Not much mountain snow is expected through the
morning and afternoon as the best dynamics are slower to approach.
By later afternoon and evening, a 100 knot jet passes through,
bringing the left exit region dynamics to the area. This should
enhance snowfall for the mountains of western Wyoming Sunday night.
Through the day Monday, as the low tracks eastward, favored westerly
flow and ample moisture enhances snow further. Heaviest snow occurs
Monday morning and afternoon for the Tetons and southern
Yellowstone, and to a lesser extent, the Salt/Wyoming Ranges. Snow
continues into Tuesday. Wrap around moisture brings another better
wave of moisture to these same locations Tuesday morning and
afternoon. Confidence is lower in this given uncertainty in the
low`s track. A Winter Storm Watch is in effect for the Tetons and
Yellowstone Sunday evening through Tuesday night to cover this
prolonged snow and two waves of better snowfall. Through Tuesday
night, up to 2 feet of snow is possible for the higher elevations of
the Tetons. A foot to 20 inches for other Teton locations and
southwest Yellowstone. Up to 12 inches for other western mountain
locations. Again, it should be noted there is less confidence in
snow amounts for the latter half of this system, so totals are
subject to change.

Moving onto precipitation east of the Divide. Instability east of
the Divide could bring an isolated (15% or less) shower, or even
thunderstorm, Sunday afternoon. Better precipitation chances move in
Sunday night. Snow levels drop to around 6000 feet, so mainly rain
is expected for the lower elevations. Just before sunrise, however,
the cooling temperatures overnight may bring snow levels a bit
lower, so a few flakes could be seen early Monday morning.
Widespread rain chances through Monday, best over the foothills and
mountains where elevation will enhance the showers. Rain chances
continue Tuesday as the low is slow to move, stuck by a ridge over
the eastern United States. Moisture continues to get pumped in
through the start of the week. Favored locations will be over the
northern half of the state. The best place for moisture will be the
Bighorn mountains and into Johnson County due to the upper-level low
location. Models are still inconsistent in the placement of this
low, which will have great impacts on the total moisture into the
area, specifically the Bighorn Mountains and Johnson County. Dry
slot placement and proximity to the low will effect precipitation
totals. There is confidence in the Bighorns getting some good
moisture, at least initially, so have issued a Winter Storm Watch
for the snow in the initial round. Low confidence in the storm`s
track into Tuesday, so difficult to forecast snow amounts then and
beyond.

The low becomes almost stationary Tuesday and Wednesday. This is why
the storm`s track is so important, because it will determine where
the best moisture, and lack of, occurs these couple days. In any
case, the entire area sees cooler temperatures and unsettled
conditions through next week. There is some model indication for the
low finally moving eastward Thursday, but lots of variance in models
keeps confidence very low. Too much variance for Friday and beyond,
with the potential for either a continued unsettled pattern or high
pressure building in. Cluster analysis agrees with this assessment,
showing uncertainty in 500mb heights on Day 7 over the western
United States.

&&

.AVIATION /00Z TAFS THROUGH 00Z MONDAY/...
Issued at 510 PM MDT Sat May 4 2024

VFR conditions through the evening, with only high cirrus moving
in from the southwest. Shower activity begins to build in from
the west overnight. Impacts initially will be minimal, and
restricted to KJAC before 12Z with minimal vis and cigs
lowerings. As the system builds in during the day Sunday,
showers become more widespread for KJAC/KPNA/KBPI, with gradual
in crease in thunder threat in the afternoon. These showers will
build west late afternoon, impacting sites east of the divide
and KRKS between 21Z/03 and 03Z/04.

Other impact for Sunday will be strong south winds. All sites
except for KJAC will see south winds sustained 25kts, with
widespread gusts above 40kts during the afternoon. KRKS will be
at least a step above this, with sustained winds 35 to 35 knots,
which puts wind gusts in the 50 to 60kt range, and this may even
be a bit conservative.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
Winter Storm Watch from Sunday evening through late Tuesday
night for WYZ001-012.

Winter Storm Watch from Monday morning through Tuesday morning
for WYZ008-009.

High Wind Watch from Sunday morning through Sunday evening for
WYZ018.

High Wind Watch from Sunday morning through Monday evening for
WYZ019-020.

High Wind Warning from 9 AM Sunday to 9 PM MDT Monday for
WYZ028>030.

&&

$$

DISCUSSION...Wittmann
AVIATION...Straub