280
FXUS65 KCYS 200448
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
1048 PM MDT Sat Apr 19 2025

.KEY MESSAGES...

- Warmer and slightly drier Sunday through Tuesday. Windy
  conditions possible Sunday night and Monday.

- Another period of  unsettled weather expected mid to late
  week with numerous showers and widely scattered thunderstorms.

&&

.SHORT TERM /THROUGH MONDAY NIGHT/...
Issued at 237 PM MDT Sat Apr 19 2025

Tonight will be rather cool as mostly clear skies will allow for
radiational cooling to be efficiently "cool" the environment at
night. Looking at satellite our 40`s and 50`s temperatures are
slowly chipping away at our snowpack throughout the day. Even with
the mountainous towns of Laramie and Rawlins the slightly dwindling
snowpack is visible with the true color RGB product. These areas
with little to no snowpack will have warmer overnight temps closer
to the thirties compared to their more snowy neighbors with
temperatures in the upper teens to low 20`s.

Sunday, the upper level trough will be pushing off to the east into
the Northern Plains as a weak ridge settles over the Intermountain
west through Monday. However, a weak shortwave will flatten the
ridge slightly keeping temperatures in the 50`s and 60`s east of the
Laramie range and 40`s/50`s west of the Laramie range. There is a
stream of vorticity that will cause the mountains to receive a tiny
bit of snow but, no precipitation is expected east of I-25 due to
the westerly downsloping flow to reinforce the dry air at the lower
levels of the atmosphere. Sunday night will be warmer as a 700mb
begins to develop as the shortwave begins to tighten the pressure
gradient. The winds will be a little gusty throughout the night with
the progged northwest to westerly flow. This downsloping flow should
keep the overnight temperatures in the 30`s.

As this shortwave pushes through the Intermountain West, some of the
12z higher-res models depict a surface low developing off the
mountains and creating a Wyoming low. This surface low Monday
increases precipitation chances for the mountains and maybe a 25 or
lower percent chance of precipitation for the lower elevations. This
is mainly due to the stout dry layer at the portion of the
atmosphere that may prevent most if not all the precipitation from
reaching the ground. The elevated to potentially high winds on
Monday will also help keep the stout dry layer in place. Monday`s
elevated winds look to peak in the afternoon as the subsident flow
becomes the strongest creating a mountain wave signature in the GFS
omega field. This mountain wave signature adds confidence to the
elevated to high wind forecast as subsidence looks to "bottom out"
with strong but thin fields of rising motion next to them. The 700mb
jet creating this wave looks to achieve peak wind speed of 40 to 50
knots with some isolated 60 knot area. The next few model runs
should provide a trend to base our decision on needing any wind
headlines for Monday. By Monday night the winds should slowly start
to die as we disconnect from the upper boundary layer and stop
diurnal mixing that was helping to bring down those higher wind
speeds. However, winds will still likely remain a little breezy
Monday night keeping us relatively warm with temperatures in the
30`s and 40`s.

&&

.LONG TERM /TUESDAY THROUGH SATURDAY/...
Issued at 237 PM MDT Sat Apr 19 2025

Unsettled weather will return for the middle to later part of the
week with a late spring-like weather pattern setting up. A series of
weak shortwave troughs moving across the Rockies will keep
precipitation chances in the picture through the period. Picking up
on Tuesday, one shortwave trough will be moving out to the
northeast. While some moisture will be present in the mid-levels,
the boundary layer looks very dry. As a result, expect to see
isolated to widely scattered convective showers developing during
the afternoon, but precipitation will have a hard time reaching the
ground. By Wednesday, a surface high pressure system settling in
over the northern Plains combined with weak lee cyclogenesis ahead
of the next shortwave trough will turn the surface winds east to
southeast. Expect to see good low-level moisture advection pushing
dewpoints into the 30s to low 40s by the end of the day Wednesday.
While there may be fluctuations with weak passing fronts and
localized storminess, a general southeast low-level flow regime
looks to last through at least Friday and possibly Saturday.
Ensembles remain in fairly good agreement showing dewpoints
remaining high (for this time of year) during this period, generally
in the 30s to mid 40s. The moist low-levels will set the stage for
convective activity each day, with the coverage and intensity of
each day`s storms determined by the timing of shortwave troughs
aloft adding a little synoptic lift. It is too early to get into the
details, but expect to see scattered to numerous showers and
thunderstorms each afternoon and evening Wednesday through Saturday.
Temperatures during this period will overall be fairly close to
average for this time of year.

&&

.AVIATION /06Z TAFS THROUGH 06Z MONDAY/...
Issued at 1035 PM MDT Sat Apr 19 2025

The upper level disturbance is expected to push east across the
Nebraska panhandle and extreme southeast Wyoming overnight. This
disturbance will allow for the wind to shift to the northwest
towards 15Z at the KCYS/KLAR TAF sites at the Nebraska TAF sites
by 21Z. Mid and high clouds will then spread over the area
tomorrow, but with limited low level moisture would not expect
much if any precipitation except for the mountains. The clouds
should depart the area tomorrow evening as weak upper level
ridging moves into the forecast area.


&&

.CYS WATCHES/WARNINGS/ADVISORIES...
WY...None.
NE...None.

&&

$$

SHORT TERM...MM
LONG TERM...MN
AVIATION...REC