Area Forecast Discussion
Issued by NWS Riverton, WY

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000
FXUS65 KRIW 122302
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
502 PM MDT Tue Mar 12 2024

.KEY MESSAGES...

- A widespread and impactful winter weather event moves into
  the state this evening, with precipitation spreading east
  through the overnight.

- Showers will develop over central WY during the morning
  Wednesday and continue through the day. Daytime snow
  accumulations may be limited due to warm surfaces.

- Winds increase Wednesday, combining with moderate to heavy
  snowfall to create difficult travel conditions across much of
  Central WY for Wednesday evening into Thursday morning.

- A weak system may bring scattered showers to the state over
  the weekend but most will remain dry. Temperatures gradually
  return to near/above normal values for the start of next week.

&&

.DISCUSSION...
Issued at 253 PM MDT Tue Mar 12 2024

So begins the main event of this week and it looks to be quite the
humdinger. Impacts vary greatly across the state and
uncertainty in the forecast is still rather high. Currently
light snow showers are spreading across western WY and will
gradually dissipate by the evening. Western WY sees a brief lull
in precipitation this evening and overnight with snow returning
by Wednesday morning. Conditions remain mild and dry for areas
east of the Divide today. Natrona and Johnson Counties see near
elevated fire weather conditions again today. RH values are
forecast to be around 20% and winds will be gusty at times
ranging from 30 to 40 mph. Sweetwater County may see a few light
snow showers this evening as the trough moves closer to the
state. Impacts will be minimal with snow amounts on the light
side, with most surfaces seeing little to no accumulation.

Wednesday onward is looking to be eventful across the Cowboy State,
as a complex weather system begins to bring widespread impacts to
the state. Forecast models are indicating development of a
rather potent system, with the forecast being rather dubious.
The origin of this system is a deep trough that will move into
the PACNW from British Columbia during the day Tuesday. This
trough will gradually dig south aiding in lee cyclogenesis to
develop over Colorado. The closed low that develops due to this
will create northeasterly/easterly flow over the state,
producing upsloping in favored areas. By Wednesday night another
potent low barrels south over the Great Basin and aids in
pushing the low over Colorado eastward, towards the Great
Plains. The timing of this is crucial and is one of the many
aspects of this event that models have difficulty coming to a
consensus on. Depending on how long the low over Colorado stays
in place the impacts may vary greatly.

This system still has many unknowns, especially for areas east of
the Divide. Current forecasts show the greatest snowfall occurring
over the high elevations of western and central Wyoming. Winter
Weather Advisories continue for the Tetons and Gros Ventre Ranges as
well as Yellowstone National Park. Additional snowfall amounts in
these areas look to generally range from 2 to 5 inches. Areas in
Central Wyoming with favorable northeasterly/easterly upsloping flow
have the potential to see moderate to significant snowfall amounts,
such as the Lander Foothills and especially Casper Mountain. Models
have recently continued to increase snowfall amounts in these areas
but still have high uncertainty in the forecasted range of snow
amounts. An example of this would be Lander where the 10th and 90th
percentiles have large contrasts between each other. Current
forecasts show snowfall amounts ranging from 2 to 15 inches in
Lander, which encompass a wide range of impacts from minimal to
significant. The remainder of central basins will see light snow
accumulation, with amounts of 1 to 4 inches. However, amounts may
vary depending on the location of heavy snow banding that looks to
develop Wednesday night into Thursday. One thing to keep in mind
when it comes to snow accumulation is temperature of ground
surfaces. Due to the recent mild temperatures in places east of the
Divide, snow may have difficulty accumulating on surfaces during the
day Wednesday. Winds are forecast to increase Wednesday into
Thursday, with gusts of 30 to 40 mph in wind prone areas such as
South Pass and Sweetwater County, gusts may exceed 45 mph at times.
The combination of snow and gusty winds will likely lead to
difficult travel conditions over South Pass and I-80 for Wednesday
through Thursday afternoon.

Regarding winter highlights, Winter Storm Watches were issued
overnight for the eastern slopes of the Wind River Range and the
Lander Foothills. Today, these watches have been upgraded to
warnings, with the addition of the Rattlesnake/Green Mountain
Ranges and Casper Mountain. The combination of winds and
moderate to heavy snowfall will create difficult travel
conditions, especially overnight Wednesday through early
Thursday afternoon. Additionally, Winter Weather Advisories have
been issued for most of Sweetwater, Fremont, and Natrona
Counties due to light to moderate snowfall combined with gusty
winds. For more information regarding snowfall amounts and
probabilities, visit weather.gov/riw/winter.

The exact timing for the end of this system is still unknown as
models differ, with some having snow showers linger through most of
Thursday. Other models have showers quickly moving out of the
region Thursday. Overall, dry conditions return by Friday with
temperatures on the cool side, with highs near to below normal
values. Another weak disturbance may bring light snow to the
region for the weekend but impacts currently look to be low.
Scattered showers are what most models are showing for this
system, with little to no accumulation expected as a result.
Winds likely increase in parts of the Bighorn Basin and Johnson
County over the weekend, as a trough barrels across the Great
Plains. The start of next week is currently forecasted to see
the return of mild temperatures and dry conditions across the
Cowboy State.

&&

.AVIATION /00Z TAFS THROUGH 00Z WEDNESDAY/...
Issued at 502 PM MDT Tue Mar 12 2024

West of the Divide...KBPI/KJAC/KPNA/KRKS Terminals.

Low-end VFR conditions along with intermittent light snow to linger
at KJAC most of the overnight hours. Another round of moisture
arrives from the northwest late tonight and MVFR conditions return
for the morning. As low pressure takes shape to the south, the
expectation is for a more northerly wind to blow after
18Z/Wednesday. Light snow should taper off in the afternoon with
conditions improving to low-end VFR. Scattered snow showers around
southwest Wyoming fizzle around sunset, but additional moisture
streams into far southern Wyoming late this evening. It still looks
as though a period of light snow is possible at KRKS between 05Z-
10Z/Wednesday at KRKS. A persistent westerly wind 10-15kts at KRKS
increases to 15-25kts late Wednesday morning. In general, all
terminals to be VFR Wednesday afternoon. Mountain tops obscured.

East of the Divide...KCOD/KCPR/KLND/KRIW/KWRL Terminals.

All terminals to be VFR through at least 09Z/Wednesday before a cold
front begins to push south through the Bighorn Basin. Conditions
deteriorate to MVFR at KCOD between 10Z-15Z/Wednesday, with other
terminals about 2-4 hours behind. MVFR to become widespread between
15Z-18Z/Wednesday with a gusty north wind 12-25kts for most
terminals, KLND being the exception. KCPR and KLND see conditions
move to IFR around 18Z/Wednesday as upslope flow increases from the
east-northeast. Mountain tops become gradually obscured from north-
to-south between 10Z-18Z/Wednesday.

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 until 11 PM MDT this evening for
WYZ001-012.

Winter Weather Advisory from midnight tonight to 6 AM MDT
Thursday for WYZ008-009.

Winter Storm Warning from 6 AM Wednesday to noon MDT Thursday
for WYZ015-019-022.

Winter Weather Advisory from noon Wednesday to 6 AM MDT
Thursday for WYZ017-020.

Winter Storm Warning from noon Wednesday to noon MDT Thursday
for WYZ018.

Winter Weather Advisory from 6 PM Wednesday to noon MDT
Thursday for WYZ028>030.

&&

$$

DISCUSSION...Dziewaltowski
AVIATION...Jones


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