705
FXUS65 KCYS 162331
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
531 PM MDT Wed Apr 16 2025

.KEY MESSAGES...

- A potentially impactful winter storm is expected Thursday
  through Friday. Widespread accumulating snow is likely. Winter
  Storm Watches are in effect for portions of southeast Wyoming.

- Remaining unsettled during the weekend through early next week
  with chances of showers each day.

&&

.SHORT TERM /THROUGH FRIDAY NIGHT/...
Issued at 401 PM MDT Wed Apr 16 2025

Another warm day across the area this afternoon with temperatures in
the 60s and 70s. Clouds are gradually increasing across the area
thanks to a weak upper-level disturbance. This disturbance could
bring some light showers to the CWA this afternoon, and perhaps a
rumble of thunder or two as RAP soundings show steep lapse and weak
instability. Shower chances look a little better overnight and west
of the Laramie Range as that disturbance moves overhead.
Impacts from these showers will likely be minimal, however the
mountains could pick up an inch or two of snow.

Heading into Thursday, the weather will make a hard switch from
spring to winter. A potent positively tilted trough will drop
into northern Utah/western Wyoming on Thursday. With this trough
originating in Canada, cold air will drop into CONUS with it. A
strong cold front will gradually push across the CWA during the
day Thursday, as per deterministic GFS and ECMWF. 700 mb
temperatures behind this front will be as cold as -12C, which
puts this anomalously cold air in the 10th percentile for NAEFS
climatology. Ahead of the front in the warm sector, steep lapse
rates and moderate instability will exist across the Interstate
80 corridor from Laramie to Sidney. Some Hi-Res guidance has a
few showers and thunderstorms developing during the afternoon in
this area. While storms are not expected to be severe, cannot
rule out some small hail or strong winds. Given the strength of
the front, precipitation initially starting as rain will quickly
transition to snow. The front will move from northwest to
southeast, so Converse and Carbon Counties will be the first to
see flakes fly. The southern Nebraska panhandle will be the last
to switch over to snow, which will likely happen overnight on
Thursday.

Since this is a later season storm, snowfall amounts could be fairly
impressive. Of course, given the time of year, factors like sun
angle and how quickly the ground can cool off after a few days of
above average temperatures will play a role in snowfall totals.
However, given the strength of the front and that much of the snow
will fall overnight, accumulations are likely, especially across
southeast Wyoming. Went ahead and expanded the Winter Storm Watches
that were in place to the Interstate 80 and 25 corridors. The reason
being that there is still some uncertainty with this system based on
ensemble guidance and another round of models and the addition of Hi-
Res guidance will be useful. Many of the Watch locations are
teetering on the verge of Advisory/Warning criteria, so it is
possible many of these Watches could go either way. For now however,
it is safe to say that the biggest impacts and highest accumulations
will be in southeast Wyoming. The Nebraska panhandle will likely
still see snow, but accumulations should generally be under 2 inches
with the exception of the Pine Ridge.

Diving deeper into this system, a lot of the dynamics favor areas of
heavy, banded snow and orographic upslope flow. The GFS shows areas
of strong mid-level frontogenesis colocated with areas of QPF and
saturated DGZs. The main area of concern for this set-up will be the
Interstate 80 corridor between Laramie and Rawlins. This will likely
lead to heavy snow at times, especially late Thursday afternoon
through Friday morning. Snow accumulations in this area and north of
the Interstate 80 corridor could be anywhere between 4 and 8 inches,
with higher amounts possible in higher terrain. East of the Laramie
Range, orographics and upslope will play a larger role in snowfall
accumulation. North to northeast winds behind the cold front will be
favorable for orographic lift in both the North Laramie Range and
Pine Ridge. Will have to keep an eye on cities along the US-20
corridor, especially in Wyoming where 4+ inches will be possible.
The North Laramie Range will be in a prolonged period of moist
upslope flow, so this area is likely to see the highest snowfall
totals in the CWA for this event. Other areas to watch will include
the South Laramie Range and foothills as well as the Cheyenne Ridge.
The Cheyenne Ridge also favors upslope flow in northerly flow so
areas between Wheatland and Cheyenne could also see upward of 4+
inches of snow. Ensemble means from the GFS and ECMWF are fairly in
line with these totals.

Snow will continue through most of the day Friday, but will
gradually taper off throughout the day from north to south. Minimal
accumulations are expected after sunset on Friday, however there
could be a few lingering snow showers overnight. Behind the
front, temperatures will be much colder for Friday. High
temperatures will struggle to reach freezing for much of
southeast Wyoming, with low 40s expected for the panhandle. Cold
temperatures will continue into Friday night with teens and 20s
expected across the area.

&&

.LONG TERM /SATURDAY THROUGH WEDNESDAY/...
Issued at 304 AM MDT Wed Apr 16 2025

A more unsettled weather pattern looks likely in the medium to long
range. Models are in good agreement with the overall pattern across
the Intermountain West and the Front Range through the forecast
period as several chances for precipitation and cooler to near
average temperatures for this time of the year. It definitely looks
like a more typical spring pattern as we head into the last full
week of April.

As we head into the weekend, all models show a slow warming trend
even though temperatures will likely remain below average for mid
April. Northerly flow aloft will remain over the area through
Saturday night with the upper level flow backing into the west on
Sunday. Remaining chilly on Saturday with skies becoming partly
cloudy. However, low level instability and some daytime convection
will result in cloudy skies in the afternoon with scattered rain and
snow showers expected. Sunday looks somewhat drier with moderate
downslope westerly wind expected. It will be milder on Sunday with
temperatures close to average for April 20th (50s to low 60s).

Next week looks mostly unsettled with several upper level
disturbances, embedded in the mean westerly flow aloft, impacting
the forecast area nearly every day. The most potent disturbance and
associated jet max aloft looks to be later Monday and Tuesday, with
some respectable QPF amounts over portions of the forecast area.
Thankfully, precipitation type will be mostly rain outside of the
mountains as 700mb temperatures increase above -5c to 0c. The
mountains typically do well with spring snowfall in these patterns
which some much needed moisture for most of southeast Wyoming and
western Nebraska. Also added thunderstorms to the forecast Monday
and Wednesday with ensemble members and deterministic guidance
showing decent low level instability and increasing moisture/surface
dewpoints.

&&

.AVIATION /00Z TAFS THROUGH 00Z FRIDAY/...
Issued at 521 PM MDT Wed Apr 16 2025

While the flow aloft remains southwest, a weather disturbance
and cold front will move across the terminals on Thursday,
bringing increasing chances for rain and snow.

Wyoming TAFS...For Rawlins, ceilings will lower from 10000 feet
this evening to 5000 feet after 09Z, then to 2000 to 2500 feet
after 12Z, with occasional showers and visibilities of 4 miles
from 12Z to 15Z, with light snow reducing visibilities to
3 miles after 15Z. Winds will gust to 30 knots until 03Z, and
to 23 knots after 15Z Thursday.

For Laramie and Cheyenne, ceilings will lower from 10000 to
5000 feet after 10Z to 13Z, with a chance of showers after 15Z,
reducing visibilities to 4 miles and ceilings to 3000 feet.
Winds will gust to 30 knots at Laramie until 10Z, then to
30 knots after 15Z Thursday, with gusts to 22 knots at Cheyenne
until 03Z, and to 22 knots after 16Z Thursday.

Nebraska TAFS...For Chadron and Alliance, ceilings will lower to
10000 feet after 10Z, and to 1500 to 2500 feet after 14Z, with
showers reducing visibilities to 3 to 5 miles. Winds will gust
to 23 knots at Alliance until 10Z, and to 30 knots after
14Z Thursday, while winds will gust to 24 knots at Chadron
after 14Z Thursday.

For Scottsbluff and Sidney, ceilings will lower to 10000 to
15000 feet after 12Z, then to 3500 to 5000 feet after
15Z Thursday, with showers after 19Z at Scottsbluff reducing
visibilities to 4 miles. Winds will gust to 25 knots until 03Z,
and to 30 knots after 14Z Thursday.

&&

.CYS WATCHES/WARNINGS/ADVISORIES...
WY...Winter Storm Watch from Thursday afternoon through Friday
     morning for WYZ101-105.
     Winter Storm Watch from Thursday afternoon through Friday
     morning for WYZ102-109.
     Winter Storm Watch from midnight MDT tonight through Friday
     morning for WYZ103-112-114.
     Winter Storm Watch from 3 AM MDT Thursday through late
     Thursday night for WYZ104.
     Winter Storm Watch from Thursday evening through Friday
     afternoon for WYZ106-107-113-115.
     Winter Storm Watch from Thursday afternoon through Friday
     afternoon for WYZ110.
     Winter Storm Watch from Thursday morning through Friday
     morning for WYZ116.
     Winter Storm Watch from Thursday evening through Friday
     afternoon for WYZ117-118.
NE...None.

&&

$$

SHORT TERM...SF
LONG TERM...TJT
AVIATION...RUBIN