570
FXUS65 KCYS 260500
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
1055 PM MDT Fri Apr 25 2025

.KEY MESSAGES...

- Cold, gloomy conditions expected to continue through the mid- morning
  hours tomorrow before slowly clearing out and warming into
  the 50s and 60s.

- Isolated to scattered showers possible Saturday with breezy,
 southerly to southeasterly winds.

- There is a Marginal Risk (Level 1 of 5) for severe weather
  across western Nebraska on Sunday.

&&

.SHORT TERM /THROUGH SATURDAY NIGHT/...
Issued at 225 PM MDT Fri Apr 25 2025

Gloomy and cold day east of the Laramie Range today where
temperatures struggle to reach the 40F mark with very low clouds and
light mist and drizzle. West of the Laramie Range is the place to be
today with some clearing across Carbon and Albany counties allowing
temperatures to increase into the mid-40s to 50s this afternoon.
Expecting some additional warming west of the Laramie Range as skies
remain partly cloudy this afternoon into the early evening hours.
Locations east will likely not see much to any sun today as low-
level cloudy keep a firm grasp over the region due to moist,
easterly upslope flow. Upslope flow will continue through tomorrow
morning ahead of a stationary front across the Laramie Range. Gloomy
conditions expected to continue throughout the evening.

Fairly active weekend ahead with predominantly southwesterly flow
aloft through Sunday. This evening will continue to be quite gloomy
for locations east of the Laramie Range as moist, upslope flow
continues with easterly winds. Dense fog has been ongoing along the
I-80 Summit since the overnight and does not look to let up anytime
soon, despite flow increasing this evening. As a result, extended
the Dense Fog Advisory to 8PM Friday, but this may need to be
further extended by the evening crew. Temperatures have struggled
under the low cloud deck, with many locations not reaching the 40s
today, except for locations west of the Laramie Range. Overnight
lows will be fairly mild as low clouds continue overnight. Most
areas will remain in the mid-30s to low-40s tonight.

Saturday will continue with the low clouds and fog potential as a
stationary front stalls out along the Laramie Range. With upper-
level flow remaining southwesterly ahead of an approaching low just
off the coast of California, an embedded shortwave will traverse to
the northwest of the CWA, potentially acting to trigger additional
showers across the region. Surface flow turns southerly to south-
southeasterly east of the front, leading to continued forcing along
the stalled boundary. With 500mb vorticity maxima ejecting out ahead
of the approaching low and continued low-level support, isolated to
scattered showers are possible throughout the day, especially east
of the Laramie Range. These showers may produce a brief clap of
thunder, but widespread thunderstorms are not expected due to the
lack of low-level instability and cooler, stable air early to mid-
morning. However, ceilings should begin to increase away from the
500ft AGL as winds increase at the surface due to a strengthening
high pressure system just north of Minnesota. This same
strengthening high pressure system will lead to winds gusting to
around 40kts across the CWA due to the strong reverse gradient. The
southerly winds associated with the strengthening high will also
advect in additional low-level moisture, leading to dewpoints
approaching the mid- to upper-40F range across western Nebraska.
These dewpoints will further support precipitation chances east of
the Laramie Range, but severe weather is not expected.

&&

.LONG TERM /SUNDAY THROUGH THURSDAY/...
Issued at 225 PM MDT Fri Apr 25 2025

Sunday looks like an isolated severe thunderstorm
day for the Intermountain West. At 500mb the jetstream is southeast
of the upper level low allowing it to lift through the Rocky
mountain region. There looks to be sufficient divergence at the
500mb level to sustain any cells that develop. At the surface there
is a tight Theta-E gradient along the Wyoming/Nebraska border for
storms to develop along. The winds are perpendicular to this
gradient suggesting a discrete storm mode however further east and
later in the day the winds shift to the south. At first thought, the
storms will start off in a more discrete mode in the afternoon
however, the storms will eventually transition to a more of a
linear/QLCS mode in the evening. Both the NAM and Global models have
between 1500 to 2500 j/kg in the afternoon right along the border as
well. With the latest 12z run, soundings point to a wind threat as
the inverted-V signal popped up with 1,000 Joules of DCAPE.
Depending on what storm mode is occurring will create the type of
threat. There is enough energy for a hail threat if the discrete
storm mode develops but a wind threat could develop with both. So
Sunday afternoon could be have a little excitement as the signal for
severe is definitely prevalent.

Monday, another shortwave moves through the Intermountain West to
continue our precipitation chances in the afternoon. There is some
strong isentropic lift on the tail end of the trough of the system
that moved through Sunday. There looks to be some warm air advection
as well to provide some extra lift for some stratified rain.

Mid to late in the week, another Low pressure system comes up
through the central plains to give us another slight chance of rain
before an upper level ridge moves across the Intermountain West to
give us a break in precipitation.

&&

.AVIATION /06Z TAFS THROUGH 06Z SUNDAY/...
Issued at 1055 PM MDT Fri Apr 25 2025

Made a few tweaks to the previous TAF package (00z) with lower
chances for IFR or lower conditions over KRWL and KLAR compared to 6
hours ago. Fog and low clouds will remain over the eastern plains
tonight due to persistent east to southeast winds.

HAZARDS/WEATHER TRENDS: Low CIGS and IFR conditions will continue
for all western Nebraska terminals and KCYS, so not many changes
there with LIFR conditions expected to continue through 15z Saturday
and maybe later further east. KCYS will likely see lifting CIGS
between 15z and 18z.

Further west, KLAR and KRWL are trending away from fog and low
clouds with gusty south to southeast winds. Kept a few TEMPO groups
for some low CIGS. Otherwise, VFR expected with gusts between 20 to
30 knots possible.

&&

.CYS WATCHES/WARNINGS/ADVISORIES...
WY...Dense Fog Advisory until 8 PM MDT this evening for WYZ116.
NE...None.

&&

$$

SHORT TERM...AM
LONG TERM...MM
AVIATION...TJT