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791
FXUS65 KRIW 051911
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
111 PM MDT Sun May 5 2024

.KEY MESSAGES...

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- Strong winds today, with warm temperatures east of the
  Continental Divide.

- Snow and rain is spreading eastward today, with chances moving
  east of the Continental Divide by this evening as a cold
  front moves through.

- Cool and unsettled weak ahead, with mountain snow and lower
  elevation rain. Gusty winds through the start of the workweek.
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&&

.DISCUSSION...
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Issued at 110 PM MDT Sun May 5 2024

A messy forecast the next few days, from wind to snow to rain. The
driver of all of this? A potent Pacific low moving across the state
today and tomorrow, parking itself over eastern Montana a couple
days. Will divide the forecast into sections based on the differing
weather elements.

Snow/Rain West of the Continental Divide:

Precipitation has already begun across the west, slowly spreading
eastward as the upper low approaches. Snow levels generally above
8000 feet Sunday morning drop to 6000 feet Monday afternoon, then
down to near 5000 feet Monday night, before settling between 6000
and 7000 feet Monday afternoon. These snow levels will determine
that transition between snow and rain. In summary, snow occurs for
the mountains beginning this afternoon as those levels drop. The
lower elevations are favored for snow overnight as levels drop
further, before favoring rain once again Monday afternoon. This
system is fairly moist, so lower elevations may see a couple tenths
to around half an inch of liquid precipitation through Monday
afternoon.

For the mountains, winter highlights are in effect. Snow ramps up
this evening and overnight. Heaviest snow occurs over the Tetons and
southwest Yellowstone, with rates near 1 inch per hour possible
Monday morning and afternoon. Lesser rates look to occur Tuesday
night as the upper low moves up near the Montana/North Dakota
border, and slightly drier air advects in. Rates increase once again
Tuesday morning as the low retrogrades slightly, returning the
better moisture. A Winter Storm Warning over the Tetons and Gros
Ventres covers this snow threat, where 1 to 2 feet of snow is
forecast through Tuesday, with the higher Teton elevations possible
seeing up to 3 feet of snow. Being of Pacific origin, the snow
should be fairly wet, with snow ratios about 12:1 to 15:1. Winter
Weather Advisories are in effect for Yellowstone, the Wind River
Range, and the Salt/Wyoming Ranges. Snow totals to around a foot are
forecast for these areas through Tuesday. The western Absarokas are
also forecast for these amounts, but given the lesser extent of
these amounts, no highlight is expected for the Absarokas at this
time. The highest peaks, especially in the Wind River Range, the
Salt River Range, and southwest Yellowstone could see localized
totals higher than a foot. Of note, the northern half of Yellowstone
is only forecast to get 0.5 to 5 inches.

Strong Wind:

Strong winds occur today. The upper low brings a southerly flow, as
well as a 100+ knot jet. The main focus today is over Sweetwater
County, far southern Fremont County and southwestern Natrona County.
High Wind Warnings are in effect, as gusts over 58 mph are likely
(70%). Models, as well as shift of tails, continues to highlight
southern Sweetwater County, south of I-80, as the most favorable
location for strong gusts, up to 70 mph. Over much of the remaining
CWA (excluding northwest portions) gusts 40 to 50 mph are possible
through the afternoon today; Special Weather Statements (SPS) are in
effect for applicable locations today. Winds begin to decrease after
sunset today.

As the low moves through, winds also shift southwesterly. Winds
across portions of southern and central Wyoming continue to gust 20
to 30 mph overnight. Winds over these locations increase again
around sunrise Monday morning. Strong gusts over 50 mph occur for
the Wind Corridor (Sweetwater County through Natrona County) late
morning and through the afternoon. Winds gusting 20 to 40 mph occur
elsewhere. Given a more westerly flow, locations east of mountain
ranges are favored for some higher gusts, closer to that 40 mph
mark. Northern Johnson County will be another place to watch,as
northwest wind gusts 40 to 50 mph Monday afternoon. The current High
Wind Warning covers this threat Monday, with a High Wind Watch in
effect for Natrona County (including Casper and Outer Drive) for
Monday. Impact-wise, expect strong crosswinds for I-80 and South
Pass/Red Canyon today and Monday.

Gusty winds continue Tuesday. Still not seeing strong indications
for high winds, but gusts 35 to 45 mph are still forecast.
Widespread gusts 20 to 30 mph also occur for most of the area. Will
have to watch trends the next few model runs for better confidence.
Uncertainty when it gets to Wednesday, as there is lower confidence
in storm track. However, flow is looking to turn more northerly,
which would favor gusty winds in the Bighorn Basin and Johnson
County.

Precipitation East of Continental Divide:

Snow levels drop to around 6000 feet Sunday night, so most lower
elevation locations should receive rain with this upcoming system.
Rain chances begin to cross the Continental Divide mid-afternoon, but
the better chances don`t begin until after 00Z (6pm) as the jet
shifts east and the cold front advances through. Rain chances (20%
to 90%) spread through the area through Monday. That chance range is
large because there will be the initial wave of chances overnight
and into Monday morning, with scattered (30%-60%) chances through
the afternoon. All locations have at least 40% precipitation chances
through Monday. Total liquid at a location will depend on if you get
a scattered shower in the afternoon, but current forecast has
anywhere from 0.10 to 0.50 inches. Basins (like the Wind River and
Bighorn) are favored for lesser totals given unfavored, westerly
flow, with totals less than 0.10 through Monday. Inversely, the
eastern Bighorn Basin is favored for the upper of that range. With
favorable jet placement and low track Monday, Johnson and eastern
Natrona Counties could see a better push of moisture, and therefore
heavier rain, Monday morning to early afternoon. Heavier showers are
possible, with precipitable water values upwards of 200% of normal;
localized amounts to 1.50 inches are possible by late Monday
afternoon. The Weather Prediction Center (WPC) has these locations
in a marginal (at least 5% chance) excessive rainfall outlook for
Monday.

For the Bighorn Mountains, models have come into more alignment for
storm track Tuesday, favoring a slightly drier pattern. Moderate
confidence now exists for 8 to 10 inches across the northern range
through Monday, with only light snow into Tuesday. Lesser amounts (6
inches or less) for the southern portions of the range. Gusty winds
up to 60 mph are also possible for the passes. Another push of
moisture comes Wednesday as the low moves into a more favorable
position. Some uncertainty still exists with the storm track
Wednesday, however. A Winter Weather Advisory is in effect for the
Bighorn Range to cover this first round of snow Monday and the gusty
winds there.

Wednesday and Beyond:

As mentioned, models diverge Wednesday. This will determine the
placement of the upper low and how quickly it begins to move
eastward. Best estimate as of now is that precipitation chances
continue for at least the northern and eastern portions of the CWA
through at least Thursday. Given the focus on the next few days,
have not looked extensively at this period.
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&&

.AVIATION /18Z TAFS THROUGH 18Z MONDAY/...
Issued at 1225 PM MDT Sun May 5 2024

Scattered showers and thunderstorms will continue across western and
southern Wyoming through this evening, at times impacting KJAC,
KBPI, KPNA, and KRKS. This activity will also spill east of the
Divide this afternoon, though most terminals here will not see
impacts until around or after sunset. Precipitation chances will
then become fairly widespread through Monday morning as a low
pressure system passes over Wyoming. Confidence is low on specific
timing of impacts; MVFR to brief IFR conditions can be expected with
any precipitation impacting terminals. Otherwise, this system will
also bring strong wind to central and southern terminals this
afternoon, decreasing this evening. Wind will increase again by 12Z
Monday. Frequent mountain obscurations are expected across western
Wyoming through the period.

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...
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Winter Weather Advisory from 11 PM this evening to 6 PM MDT
Tuesday for WYZ001.

Winter Weather Advisory from 1 AM to 11 PM MDT Monday for
WYZ008-009.

Winter Storm Warning from 6 PM this evening to 11 PM MDT
Tuesday for WYZ012.

Winter Weather Advisory from 6 PM this evening to 6 PM MDT
Monday for WYZ014-015.

High Wind Warning until 9 PM MDT Monday for WYZ019-028>030.

High Wind Watch from Monday morning through Monday evening for
WYZ020.

Winter Weather Advisory from 11 PM this evening to 6 PM MDT
Monday for WYZ024.
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&&

$$

DISCUSSION...Wittmann
AVIATION...Myers