706
FXUS65 KCYS 201108
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
508 AM MDT Sun Apr 20 2025

.KEY MESSAGES...

- Warmer and slightly drier Sunday through Tuesday. A few
  thundershowers possible Monday.

- Very windy conditions possible across southeast Wyoming Sunday
  night and Monday. Gusts may approach 55 mph in a few areas
  along and west of Interstate 25.

- Active long term pattern returns to the region with the
  possibility of strong, to maybe severe, thunderstorms on
  Wednesday.

&&

.SHORT TERM /THROUGH MONDAY NIGHT/...
Issued at 305 AM MDT Sun Apr 20 2025

Current IR Satellite loop shows mostly clear skies over southeast
Wyoming and western Nebraska early this morning as the slow
moving upper level trough axis remains over the area. This
trough axis will continue to weaken and drift eastward into the
high plains today. There should be enough low level instability
on the western fringe of this trough to help produce some rain
showers today, mainly this afternoon, as a weak
disturbance/vorticity max aloft moves southeast across the
region behind the trough. High temperatures are forecast to be
about 10 degrees warmer compared to Saturday with readings in
the 50s to low 60s along and east of the I-25 corridor, and
closer to 50 degrees west of the Laramie Range. Expect mainly
rain shower activity in the lower elevations with some light
snow above 7500 feet. Otherwise, low temperatures early this
morning are not nearly as cold as last night`s and this should
generally be the case through Monday night as most of the snow
pack from the last storm system has likely melted.

Main forecast concern early this week will be the potential for
very windy conditions across southeast Wyoming and some locally
strong winds along and west of the I-25 corridor on Monday. In
addition, some thunder shower activity is expected to develop on
Monday ahead of the next Pacific cold front. Models in good
agreement with the timing of the cold front and associated upper
level trough axis as it quickly swings through the area Monday
afternoon. Deterministic models and ensembles show ample low
level instability, at least enough to initiate shallow convection
across the area, although vertical extent will likely be
limited due to CAPE values of 200 to 400 j/kg and forcing
arriving later in the day. In addition, the boundary layer will
be pretty dry with dewpoints lowering into low to mid 20s into
the afternoon under westerly downslope winds. High resolution
CAMs do show widely scattered showers and a few thunderstorms
quickly moving eastward over the area, so kept POP between 15 to
25 percent for most of the lower elevations...mainly for widely
scattered showers and an isolated thunderstorms or two. As for
winds, difficult wind forecast shaping up for Sunday night and
Monday. In-house wind guidance has been slow to highlight any
risk across the area with the most favorable location(s) along
Interstate 25 from around Wheatland northward to Douglas.
Further southwest, in- house guidance shows marginal conditions
for Arlington and the I-80 Summit. Models do show a developing
mountain wave along the Laramie Range Monday with 600mb winds
approaching 50 knots. However, low level pressure gradients are
not impressive. There is also the uncertainty of how convective
development during the day will influence the overall wind
pattern across southeast Wyoming. Increased winds near high wind
criteria, especially along the I-25 corridor, but kept them
just below High Wind criteria for now. This may be a case where
outflow winds from daytime convection may dominate the
environmental winds...especially with models showing inverted V
soundings across a bulk of the area. Typically, these strong
wind gusts are very brief and handled by a Special Weather
Statement.

Surface cold front moves across the rest of the area Monday
night as daytime convection diminishes in the evening. The bulk
of the cooler air behind the front will slip by to our north,
with only slightly cooler temperatures Monday night into
Tuesday.

&&

.LONG TERM /TUESDAY THROUGH SATURDAY/...
Issued at 304 AM MDT Sun Apr 20 2025

Active weather returns this week with multiple chances for
precipitation and the possibility of some strong to borderline
severe thunderstorms by midweek. Zonal upper-level flow will be
present Tuesday after a shortwave traverses overhead Monday,
increasing winds. Zonal flow remains overhead throughout the day as
the next system pushes into the Pacific Northwest. As this system
slowly pushes further inland late Tuesday night, southwesterly flow
will develop across much of the region. Some lobes of 500mb
vorticity will advect through the zonal flow to provide a brief
period of rising motion in the afternoon, before even more lobes
eject out ahead of the incoming upper-level trough over the Pacific
Northwest. The initial vorticity maxima progressing overhead will
provide enough support for isolated to scattered showers Tuesday
afternoon into the evening hours. PoPs remain meager, only around 20-
30% for these disturbances aloft. Despite this, mostly clear skies
turn partly cloudy in the afternoon hours as showers develop.
Temperatures will warm into the 50s and 60s area wide, with the
Panhandle nearly approaching 70F.

More activity is expected on Wednesday, some of which may be fairly
impactful. Zonal to slightly southwesterly flow aloft will turn more
amplified with a stronger southwesterly component to the winds as
the upper-level trough pushes into the Intermountain West. As the
flow turns more southwesterly, a secondary Canadian trough pusses
southward, increasing height gradients at 250mb and aiding in the
amplification of the overall flow. As a result, the upper-level jet
strengthens to the north, placing the region broadly within the
dynamically-favored, right entrance region of the upper-level jet.
The jet will be a little too far north for the best jet dynamics to
be overhead, but synoptic lift is still expected under this
quadrant. Several lobes of 500mb vorticity maxima will eject out
ahead of the approaching trough and further support synoptic lift
across the region. In addition to this, the 500mb jet will be
slightly further to the east in relation to the 250mb jet above,
leads to slight westward displacement with height, favoring a
stronger system, despite the slight positive tilt of the 2550mb
trough. The 700mb trough is a bit more difficult to pick out of the
flow looking at long range models without significant smoothing, but
appears to be fairly well aligned with the 500mb jet with a very
slight eastward displacement between the two features. A fairly week
700mb jet develops across western South Dakota by Wednesday
afternoon in between this system and the Canadian trough dropping
south out of Canada over northern portions of the Midwest. As a
result, locations east of the Laramie Range will be broadly within
the right entrance region of the weak, 700mb jet, favoring enhanced
lift closer to the surface. Strong 700mb warm air advection will be
ongoing throughout the day as the trough pulls in the warmer, more
moist air over the southern Planes and southern New Mexico. Fairly
strong surface WAA will also be ongoing in sync with the 700mb WAA,
leading to surface temperatures rising quickly into the 60s to near
70s east of I-25. Additionally, a strong cold front will push into
the region late in the afternoon and evening hours Wednesday,
leading to some concentrated enhanced, surface forcing along the
leading edge of the front, further supporting ongoing rising motion
from surface and 700mb WAA. Ahead of this front, dewpoints across
portions of the Panhandle are currently progged to increase into the
low- to mid-40s, which is enough moisture for convection in this
area. Precipitable water values will climb into the 0.5 to 0.6 range
in the Panhandle, with surface-based CAPE values increasing towards
800 J/kg in some deterministic, long range models. Forecast sounding
from the GFS across the southern Panhandle suggest fairly dry low-
levels with a small saturated layer between about 700 and 600mb
before drier air returns above 500mb. Despite the drier levels, good
low-level veering along with modest helicity values within these
soundings. Based on this information, some strong storms appear to
be possible Wednesday across mostly western Nebraska, though some
severe storms cannot completely be ruled out at this time. This even
is still several days out, however, so some values and placements of
jet streams will likely change over the next few days. Wednesday
appears to be a day to watch as far as thunderstorms are concerned.

The remaining long term pattern remains active with multiple chances
for precipitation across the region. Thursday will be cooler than
Wednesday in the post-frontal environment with highs in the 50s
expected area wide. Residual precipitation will be present across
the CWA Thursday as a backdoor cold front pushes into the region,
leading to surface forcing and the potential for ongoing showers and
maybe a thunderstorm or two. Significant model disagreement starts
with Friday`s high temperatures, with long range models likely being
too warm behind the backdoor cold front. Trended temperatures on
Friday down from the NBM, due to colder temperatures likely
occurring than is forecast in the models at this time. Precipitation
chances remain across the region throughout the remainder of the
long term forecast.

&&

.AVIATION /12Z TAFS THROUGH 12Z MONDAY/...
Issued at 504 AM MDT Sun Apr 20 2025

Fairly quiet today today, except for some isolated rain showers
that may develop around the terminals and gusty winds. Clear
skies ongoing this morning across all sites, except KRWL that is
under some low stratus. Gusty winds are expected at all
terminals by the afternoon hours, with most sites gusting
between 25 and 30kts. Mostly sunny conditions become partly
cloudy this afternoon, but ceilings should remain VFR.
Confidence is low on any of the afternoon isolated rain showers
impacting the terminals, so did not include rain for any
terminals. Winds decrease by the evening with mostly clear skies
overnight.

&&

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

&&

$$

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