Alliance, Nebraska 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

681
FXUS65 KCYS 041120
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
520 AM MDT Mon May 4 2026

.KEY MESSAGES...

- An powerful cold front arriving Monday will bring a chance
  for showers and thunderstorms, changing over to snow Monday
  night and continuing into Wednesday morning.

- Accumulating snowfall is expected over 5500 feet in elevation,
  with significant snowfall likely (70% chance) in the Snowy
  Range, southern Laramie Range, and their adjacent foothills.

- Travel impacts due to snowfall are expected along Interstate
  80 between Rawlins and Cheyenne, and Interstate 25 between
  Wheatland and the Colorado state line.

- Impacts may extend eastward on Interstate 80 towards Kimball,
  but confidence is lower for areas below 5500 feet in
  elevation.

- Snowfall is expected to be heavy and wet, which could damage
  tree limbs and produce power outages.

- Drier, warmer, and windier weather returns to the area for the
  second half of the work week.

&&

.SHORT TERM /THROUGH TUESDAY NIGHT/...
Issued at 445 AM MDT Mon May 4 2026

SYNOPTIC OVERVIEW: Conditions are fairly calm across the area this
morning, but big changes are around the corner as what could become
our most significant snow event of the season takes shape. The two
main synoptic players leading to the expected late season snow event
are clearly apparent on GOES water vapor imagery this morning.
First, we have the sprawling upper level low currently centered just
off the coast of central California. This features has become
elongated from east to west as expected, with a powerful subtropical
jet stream pushing across the desert southwest. Further north, mid
to high level cloud cover is visible along the northern branch jet
stream. This is pushing southward this morning as a series of potent
upper level shortwaves dive down from the north on the western flank
of the persistent upper level trough locked in over the Hudson Bay.
Deep Pacific moisture is stretched out from northern California to
southern Wyoming in the deformation zone between the two upper level
features. The northern branch system will bring an unseasonably cold
airmass into our area. As the cold air and forcing interacts with
the moisture, we will set the stage for a fairly prolonged
period of rain and snow across the area.

TODAY`S OUTLOOK: A surface high pressure system sliding down the
eastern edge of the Rockies this morning will push into our northern
zones a few hours after sunrise. This will be marked by a wind shift
to the north or northeast. The front will progress southward through
the area, and should reach the I-80 corridor by the early afternoon
hours. After this time, it will begin to slow down and/or stall as
it runs into the higher terrain of southern Wyoming and northern
Colorado. This process will lead to potent frontogenesis along the I-
80 corridor. Modest convective instability is expected to be present
in this area this afternoon, so we should see scattered showers and
isolated thunderstorms kick up during the afternoon into the
early evening. Recent guidance has trended weaker on the
instability, and a little slower on the arrival of the stronger
forcing for ascent. Thus, the precipitation potential in the
convective phase of this event has been trimmed back slightly.
Today`s activity will have the potential to produce gusty winds
and brief moderate to heavy rainfall.

TONIGHT THRU WEDNESDAY OUTLOOK: Around sunset, the front is expected
to break over the Laramie range and fill into Carbon and Albany
counties. This will initiate strong frontogenesis along I-80 along
with widespread isentropic lift over top the stalled frontal
boundary. Model guidance shows positive theta-e advection developing
which should support precipitation rapidly filling in across the
area by midnight. Snow levels will also be rapidly dropping during
this time. All but the highest peaks will start as rain this
evening, but expect snow levels to drop below the higher portions of
I-80 by the mid evening hours, then falling below 6000 ft between
midnight and 3am or so. Confidence has increased that for areas
above 5500 ft in elevation, the dominant precipitation type of this
event will be snow. The rain to snow transition is more uncertain
between about 4000 ft and 5500 ft. Here, precipitation type may be
determined by rates. In other words, during periods of heavier
precipitation expect to find snow, whereas lighter periods may
flip back over to rain. The best frontogenesis will slip to our
south into northern Colorado by mid morning or so Tuesday, but
models show the overrunning lift continuing. Precipitation
coverage may become a little more spotter with lesser rates
during this period. A reinforcing upper level shortwave and
accompanying surface cold front will push southward Tuesday
afternoon and evening, which should help provide a second surge
of frontogenesis and perhaps increased precipitation rates
(along with the coldest air of the event) Tuesday evening into
early Wednesday morning. With a very chilly airmass and
plentiful cloud cover expected Tuesday, high temperatures will
be considerably below average. Between Rawlins and Cheyenne,
temperatures will struggle to get above the low to mid 30s.
Further north and east, we won`t be quite as chilly, but expect
highs generally in the 40s. The reinforcing cold air Tuesday
night should push temperatures down into the 20s to around
freezing.

LIQUID TOTALS: So, with the overall evolution of the system
described, let`s get into amounts. Precipitation amounts have
remained remarkably consistent with this event over the last several
days. We have seen perhaps a slight southward shift in the overall
evolution of the system over the last 24 hours, but this has not
significantly changed the expected outcome for our area. The
deterministic NBM remains very aggressive on QPF, so the current
official forecast generally follows the ECMWF ensemble mean, which
is closer to the NBM 25th percentile. Confidence has also increased,
as the single driest LREF member still has over 0.5" of liquid
precipitation in Cheyenne! The favored area is also unchanged.
Current probabilities for over 0.5" of liquid precipitation are over
60% in an area south of a line from Walcott Junction to Chugwater to
Bridgeport. The highest QPF totals are expected to be in the Snowy
Range, southern Laramie Range, and their adjacent foothills.
Probabilities for over 1" of liquid are around 60% in Cheyenne, and
up to 80% around the I-80 summit and Medicine Bow Peak area.
Probabilities from global ensembles are similar in Laramie to what
they are in Cheyenne, but the large-scale models might be missing
the potential for shadowing off of the southern Laramie range, at
least in the city of Laramie proper. For the official forecast,
about 25% of the QPF was shaved off immediately around the city of
Laramie. The dominant low-level wind direction is expected to be
northeast or east, which is not a very good wind direction for
significant precipitation in Laramie. The frontogenesis and
overrunning can overcome this as the system is not a purely upslope
driven event, but this is likely to cut into totals somewhat. North
of the North Platte River, precipitation amounts will drop off
quickly. Unfortunately, this storm system is not expected to provide
much, if any, drought relief to the northern and eastern portions of
the forecast area.

SNOW EXPECTATIONS: As stated above, areas about 5500 ft are expected
to see most of their QPF falling as snow. However, very warm ground
temperatures and the very strong early May sun will lead to some
amount of melting on contact. Snow that falls during the day will be
much less likely to accumulate compared to snow that falls at night.
In addition, accumulations on warm surfaces such as roads and
sidewalks will be noticeably less than accumulations on grassy or
elevated surfaces. Snow ratios are expected to be quite low,
especially for the first half of the event. At the onset, we should
be seeing a very heavy, wet snowfall with ratios between 5:1 and
10:1 at best. Tuesday afternoon and overnight, ratios should improve
to around 11:1 to 13:1 as forecast sounding show colder air moving
in aloft, and even a brief period of overrunning lift in the
dendritic growth zone. That all being said, confidence in
significant snow accumulation causing widespread impact was high
enough to upgrade the previous Winter Storm Watches all to Warnings.
The two favored mountain ranges should easily see accumulations in
excess of 8". Both the Snowy range and southern Laramie range have a
30 to 40% chance to exceed 18" of snow. Travel along I-80 between
Laramie and Cheyenne may become difficult to impossible at times
during this event. Between Laramie and Rawlins, expectations are not
quite as high, but there is still a 50% chance for 8" at Arlington,
and 25% chance to exceed a foot.

SNOW ACCUMULATIONS: Confidence has also increased in some amount of
snow impacts in Laramie and Cheyenne. As discussed earlier, Laramie
is likely to struggle with some shadowing reducing snowfall rates.
Therefore, decided to issue a Winter Weather Advisory for the
Laramie Valley with the focus on the largest population center of
the zone, but with the caveat that Warning criteria amounts can be
expected on the western side of the valley closer to the Snowy
Range. This may need to be upgraded later if the shadowing fails to
materialize, but confidence was high enough that this will decrease
snow totals that we could leave the city in an Advisory for now.
Cheyenne is not expected to struggle with shadowing. In fact, the
east to northeast upslope flow is quite favorable for the city. With
the latest guidance showing a 60+% chance to exceed 6 inches,
decided to issue a Winter Storm Watch for Cheyenne. The most likely
scenario is currently for amounts to fall into the Winter Storm
Warning criteria, but confidence was not quite high enough to go
straight to a warning. The watch will allow the later shifts to
closely examine high resolution model trends as they become
available for the entire event later today, and make a decision
after that. The rain to snow transition is more uncertain between
about 4000 ft and 5500 ft. This includes areas such as Pine Bluffs,
Kimball, and Wheatland. The probability for warning criteria amounts
was too low here to include in the watch (20 to 40%). In addition,
impacts will also start a little later in these areas, perhaps not
until the daytime Tuesday or even Tuesday night. Therefore, there is
time to wait for more guidance. Further north and lower in
elevation, the most likely scenario is to see some falling snow
mixing in with the rain, but for snow to struggle to stick. We may
see a coating to an inch or two primarily on grassy surfaces.
Locally heavy banding may allow snow to stick and accumulate higher
totals in these areas, but this is not expected to be widespread.

IMPACTS: Another challenge with this system is the degree of impact.
On one hand, the time of year and warm ground temperatures suggest
lesser accumulation on roadways. However, that doesn`t mean zero,
especially for any snow that falls at night. Roadways will probably
be slushy and slick late tonight through Tuesday, but then there
could be some re-freezing and icing concerns Tuesday night into
Wednesday morning as colder air moves into the area. In addition,
the snow is expected to be very heavy and wet. Thanks to the
exceptionally warm spring weather so far, deciduous trees are
leaving out across the area. Therefore, the snow weight is expected
to strain or break tree limbs, which could lead to some downed
powerlines and power outages across the area. These concerns will
need to be closely watched over the next 24 hours as the storm
system gets going.

&&

.LONG TERM /WEDNESDAY THROUGH SUNDAY/...
Issued at 445 AM MDT Mon May 4 2026

The axis of the strong upper level shortwave trough is expected to
pass over the area Wednesday morning, which should abruptly conclude
the widespread lift, and also begin to advect in drier air aloft.
While some snow shower activity may linger into Wednesday afternoon,
we should be mostly done with the system by midday. The strong sun
will help to melt snow and improve travel conditions quickly,
especially as clouds clear in the afternoon hours. Wednesday will
still be quite chilly across the area, with highs expected in the
upper 40s to low 40s in areas receiving accumulating snow from this
system, and upper 40s to mid 50s elsewhere. Ridging will try to
build in across the western US by Thursday. Our area will remain on
the periphery of the ridge. Initially, expect a period of increased
winds as warm air advection takes over. Probabilities for high winds
remain around 20 to 40% in the wind prone areas on Thursday. Still,
a windy day can be expected for much of the area as temperatures
recover to near or slightly above seasonal averages.

The area is expected to remain under a fairly unsettled northwest
flow pattern through the end of the operational forecast period.
While temperatures will most likely recover to near normal, global
model guidance keeps a parade of upper level shortwaves moving
through the area every day or two. There are significant disparities
in timing and strength of each shortwave, which makes it difficult
to get into the details. The make take away is that the unsettled
weather pattern will keep chances for showers in the forecast for
much of the long term period. There is potential for another system
to tap into some cold air by the upcoming weekend, but only a
handful of ensemble members are cold enough for snow levels to drop
to the level of our populated areas.

&&

.AVIATION /12Z TAFS THROUGH 12Z TUESDAY/...
Issued at 515 AM MDT Mon May 4 2026

A cold front is approaching the area will bring more difficult
aviation conditions for the next few days. The front will arrive
first at CDR in about 2-3 hours, and then reach CYS and SNY
around 18z-20z. Expect winds to shift to the north or northeast,
wind gusts of 25 to 30 knots expected along and behind the
front.

Scattered showers and isolated thunderstorms will develop this
afternoon mainly along the I-80 corridor. These will have the
potential to bring gusty and erratic winds, lightning, and brief
VIS drops due to moderate rainfall during the afternoon and
early evening hours.

After sunset, rain showers will rapidly increase in coverage
with most seeing precipitation by midnight. CIGs will drop late
this evening towards early Tuesday morning, particularly in
Wyoming. Rain will also mix with and change over to snow at
Wyoming terminals. IFR conditions are expected along the I-80
corridor towards the end of the TAF period.

&&

.CYS WATCHES/WARNINGS/ADVISORIES...
WY...Winter Weather Advisory from 9 PM this evening to 9 AM MDT
     Wednesday for WYZ103.
     Winter Weather Advisory from midnight tonight to 9 AM MDT
     Wednesday for WYZ105-106-115.
     Winter Storm Warning from 9 PM this evening to noon MDT
     Wednesday for WYZ110-114-116-117.
     Winter Storm Watch from late tonight through Wednesday morning
     for WYZ118.
NE...None.

&&

$$

SHORT TERM...MN
LONG TERM...MN
AVIATION...MN