971
FXUS65 KCYS 012330
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
530 PM MDT Tue Apr 1 2025

.KEY MESSAGES...

- Scattered rain and snow showers are expected through tonight.
  A few isolated thunderstorms will have the potential to
  produce small hail through the evening.

- Snow showers may rotate back into the US-20 corridor through
  Wednesday morning, with up to three inches or so of slushy
  accumulation possible.

- Widespread gusty winds are expected Wednesday. There is
  approximately a 40% chance for high winds in the wind prone
  areas along I-80.

- Below average temperatures alongside a chance of light mixed
  precipitation moving into the weekend, with dry conditions and
  a warming trend to start next week.

&&

.SHORT TERM /THROUGH THURSDAY/...
Issued at 315 PM MDT Tue Apr 1 2025

A sprawling, disorganized upper level low continues to inch its way
across the western CONUS today, keeping unsettled weather in place
for our area through the forecast period. Current satellite imagery
shows a mess of clouds across the area, north of a powerful Pacific
jet stream racing cross the Four Corners states. Diffluence aloft
east of the Rockies has supported rapid lee cyclogenesis over east
central Colorado over the last several hours. The latest
mesoanalysis shows this surface low center developing on the far
southern end of the envelope of ensemble solutions which has led to
lighter winds than expected today. Still, breezy northwest winds
have encompassed most of the plains as the developing cyclone
strengthens and wraps in a cold front pushing southward. Steep lapse
rates and good low-level moisture have lead to modest convective
instability over southeast Wyoming once again today. However,
backing wind profiles and more narrow CAPE profiles will make it a
little more difficult to get storms as strong as what we saw
yesterday. We still may see a few isolated storms producing small
hail, but storms overall should be weaker than yesterday. Still,
expect to see scattered showers with embedded thunder increasing in
coverage over the next few hours thanks to a band of frontogenesis
swinging across the area along the upper level shortwave trough
axis. That forcing should end by about 9-10PM, leaving just
isolated rain and snow showers behind.

Tonight, the surface low pressure system will move northward across
the central to northern Plains, eventually forming a closed upper
level low by Wednesday morning. Models continue to show a broad
field of wrap-around moisture developing, but significant
differences remain concerning how far west this gets and whether it
will reach our area. ECMWF ensembles remain more conservative than
most GEFS members, and the official forecast leans towards the
ECMWF. The low pressure track is a bit further east than we would
want to see for good wrap-around snowfall into our area. In
addition, the TROWAL is not very impressive until the system has
passed our area. Still, on and off snow showers are expected along
the US-20 corridor overnight through Wednesday morning, with perhaps
a slushy dusting to 3" or so of accumulation. Probabilities for 1"
or more of snow are highest (around 60% per the HREF) around Lusk
and in the Pine Ridge south of Chadron. Due to the considerable
uncertainty remaining, no headlines have been issued, but there is
about a 20% chance for a higher end scenario producing 3-6" of
slushy snow along the US-20 corridor. Outside of that area, moist
northwest flow will produce some snow shower activity over much of
southeast Wyoming tonight (mostly west of the Laramie range), which
could lead to some light accumulations along portions of the I-80
corridor in its higher elevations, and of course in the mountains.
This activity is expected to remain mostly sub-advisory, but its
worth noting that the Snowy range in particular can be prone to over-
performing in northwest flow regimes.

Winds will also be on the increase as the surface low tightens up
and moves to the northeast. There is potential for a short period of
high winds along the wind prone areas of I-80. The GFS shows 700-mb
winds touching 50-knots early Wednesday morning, with GFS-based in-
house guidance depicting a 50% probability of high winds. Other
models are showing weaker winds aloft and gradients across the area
though, as the deterministic GFS is positioned near the 90th
percentile of the ensemble spread. As a result, decided not to issue
a High Wind Watch at this time, but expect a period of 50+ MPH gusts
in these areas late tonight into Wednesday morning. Gusty northwest
winds and cool temperatures will dominate the rest of the area
Wednesday, leading to a fairly wintry feel to the day despite the
calendar moving into April. Lingering snow showers will drop off
with the wind speeds Wednesday late afternoon and evening as the
storm system pushes further to the northeast and drier air starts to
work in. Thursday morning looks fairly quiet, but the next system
will begin to creep in from the south during the afternoon hours.
More on this system can be found in the long term section below.

&&

.LONG TERM /THURSDAY NIGHT THROUGH TUESDAY/...
Issued at 315 PM MDT Tue Apr 1 2025

Upper level trough across the Western US will continue to
consolidate with a cutoff low forming in the Desert Southwest on
Friday, with this system then transitioning eastwards through
the weekend allowing a quick transient ridge to move into the
region to start next week. The period should end with a fast
moving upper level shortwave that will briefly disrupt ridging,
but into the extended forecast just beyond the long term models
suggest a very deep ridging pattern and above normal
temperatures are likely. At the surface, a cold front will usher
in chances of precipitation and cooler than normal temperatures
to end the week and begin the weekend, but overall impacts
should remain minimal thanks to the more southward progression
of the low. For this update, did make some adjustments to the
NBM to include higher POP chances Thursday evening and again on
Friday to account for more convective induced possibilities, as
well as a slight bump to winds but still well below our
thresholds for high wind criteria. And with QPF amounts
generally less than a quarter of an inch, any precipitation that
does fall will be on the meager side, including any mixture
into snowfall. Although snow levels will fall to below 5000 feet
by Friday with colder temperatures, snow ratios will remain
around or under 10 to 1 outside of the mountains until Friday
afternoon, and by that time the bulk of the strongest QPF
should have already passed through the region. This alongside
spring warmth keeping ground temperatures on the warm side
should limit the majority of any significant accumulations once
again outside of the mountains.

Once this system passes into Saturday, benign weather should
overspread under ridging, with a steady warming trend to end the
forecast period. Widespread highs in the 40`s on Saturday will
give way to highs into the 60`s by Monday bringing the spring
warmth back as we move into next week. Overall there is moderate
to high confidence in the forecast (60-80%), particularly on the
back half of the weekend into next week where ensembles are in
solid agreement on our ridging and warming trend. Some
uncertainty still does remain on the system at the end of this
week, but with trends continuing to downplay our potential at
stronger conditions/higher precipitation, confidence is much
higher now in how this should evolve.

&&

.AVIATION /00Z TAFS THROUGH 00Z THURSDAY/...
Issued at 524 PM MDT Tue Apr 1 2025

The flow aloft will become northwest tonight.

Wyoming TAFS...Ceilings will range from 7000 to 10000 feet.
Winds will gust to 30 knots at Rawlins, to 32 knots at Laramie
until 02Z and after 12Z Wednesday, and to 35 knots at Cheyenne
until 01Z, and after 06Z.

Nebraska TAFS...Ceilings will range from 4000 to 8000 feet, with
occasional ceilings from 1800 to 3500 feet until 03Z at Chadron,
Alliance and Scottsbluff, with rain showers at Alliance and
Scottsbluff reducing visibilities to 4 miles. Winds will gust to
25 knots at Scottsbluff until 06Z, and gust to 30 knots at all
the terminals after 12Z Wednesday.

&&

.CYS WATCHES/WARNINGS/ADVISORIES...
WY...Winter Weather Advisory until 6 PM MDT this evening for WYZ112-
     114.
NE...None.

&&

$$

SHORT TERM...MN
LONG TERM...CG
AVIATION...RUBIN