330
FXUS65 KCYS 191403
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
803 AM MDT Wed Mar 19 2025

.KEY MESSAGES...

- Much colder and breezy conditions are expected today with a
  few snow showers and lingering blowing snow issues.

- Marginally high winds are possible for some of the wind prone
  areas of southeast Wyoming tonight into Thursday.

- Windy weekend and work week expected, but temperatures will be
  spring-like for the work week.

&&

.UPDATE...
Issued at 756 AM MDT Wed Mar 19 2025

We are issuing a quick update for the I-80 summit zone. Latest
webcams were showing quite a bit of snow and blowing snow across
this area with the visibility ranging from one quarter to one
half mile. The strong wind gusts of 30 to 40 mph are expected to
persist through the day with limited sunshine. As a result,
temperatures will struggle to reach freezing. This combined
with the gusty wind speeds will most likely reduce the
visibility from blowing snow. Occasional snow showers will also
be possible at times. Therefore, we went ahead and issued a
Winter Weather Advisory through 00z.

&&

.SHORT TERM /THROUGH THURSDAY NIGHT/...
Issued at 430 AM MDT Wed Mar 19 2025

Radar trends show snow showers beginning to wind down this morning
across the area, but observations still indicate areas with
visibility under one mile generally along the I-80 corridor. GOES
satellite imagery shows the powerful upper level low now over
central Kansas and moving quickly eastward. Most synoptic forcing
for ascent has come to an end, leaving mainly just the low-level
upslope flow on northerly slopes to support light snow. While the
snow is winding down, winds are holding strong with widespread gusts
of 30 to 40 mph continuing. The recent snowfall was fairly light and
low density, and seems to be blowing quite easily. So, while falling
snow impacts will wane, we may see blowing snow impacts continue
through the day today for all areas that received snowfall. Most of
the area should see periods of sunshine today, so while temperatures
will be stuck in the 20s and 30s, the sun may be enough to crust
over the snow surface and bring blowing snow concerns to an end, but
there is quite a bit of uncertainty with that.

Otherwise, we may see a brief mid to late morning lull in snow
showers, but expect another round to kick up around noon. A band of
elevated 500-mb vorticity is expected to push over the area this
afternoon on the back edge of the longwave trough, concurrent with
the coldest temperatures aloft. Despite the cold surface
temperatures, the very cold temperatures aloft will produce quite
steep lapse rates by early afternoon, with fairly good moisture
(average RH > 70%) below 500-mb in southeast Wyoming. As a result,
expect scattered snow showers to develop mainly over the higher
terrain and in adjacent valleys of southern Carbon, Albany, and
western Laramie counties. This convective snow shower activity will
be fairly shallow, and thus unlikely to deliver much more than a
dusting, but we may need to watch for pockets of reduced visibility
as these showers move through. This should wane quickly after 00z as
dry air and negative differential vorticity advection kick in on the
back edge of the longwave trough.

Transient ridging shifts over the area this evening into Thursday
morning, with southwest flow aloft returning by tomorrow morning
ahead of the next shortwave trough. The next trough will be more
disorganized and cross the Rockies much further north, supporting
lee cyclogenesis over southern Alberta and into eastern Montana.
While the surface cyclone will be well to the north, a trailing
pressure trough to the south will increase the cross-barrier
pressure gradient tonight into the day Thursday. 700-mb gradients
are generally just below what we`d want to see for high winds.
Modeled MSLP gradients are more impressive, but there is come
concern the GFS may be overestimating this. The NAM and various Hi-
res modes show more marginal MSLP gradients likely insufficient to
produce high winds. For now, decided to add a Watch for the
Arlington and Bordeaux areas which can do well in WSW flow, but this
is fairly low confidence. Surface driven wind events tend to have a
harder time reaching high wind criteria as we head into the spring
season. Otherwise, look for a warmer but still breezy day Thursday
with increasing mid to high level cloud cover.

&&

.LONG TERM /FRIDAY THROUGH TUESDAY/...
Issued at 430 AM MDT Wed Mar 19 2025

Relatively quiet weather for this time of year Friday onward on the
backside of the departing upper-level trough. Behind this feature,
northwesterly flow will slowly turn westerly throughout the day
Friday as a weak ridge attempts to develop over the Intermountain
West. The 700mb trough will slowly push off to the southeast across
southern Canada towards the Great Lakes Region. 700mb height
gradients will be elevated as this trough slowly moves away from the
region, leading to a 45 to 50kt northwesterly jet overhead
throughout much of the morning into the early afternoon hours.
Currently looks like elevated to near high wind criteria winds
especially along the I-80 Summit, with the downsloping flow off the
Laramie Range keeping temperatures fairly mild in the mid-40s to low-
50s range for Friday afternoon. With westerly to southwesterly flow
over the western portions of the CWA, mountain snow is expected to
continue through the day Friday and through most of the weekend.

Saturday will be the best day in the long term for snow chances
across the region as an upper-level trough drops into Montana during
the day and strong 500mb cyclonic vorticity advection develops and
advects through the CWA. An associated 700mb trough will dive into
the Dakotas from southern Canada, leading to yet another day with
increased 700mb height gradients across the region and gusty winds
near the surface. Winds are expected to be stronger on Saturday as
compared to Friday due to a 55 to 60kt 700mb jet setting up overhead
and a much stronger surface pressure gradient due to the developing
surface low over eastern South Dakota. An attendant cold front will
drag from north to south across the CWA, enabling the better chances
for snow fall. 700mb temperatures fall into the -10 to -12C range,
leading to a quick decrease in temperatures throughout the afternoon
and evening hours. Due to the progressive nature of this system,
significant snow accumulation is not expected at this time with only
the mountains seeing a good shot at significant accumulations.

A warming trend is expected Sunday onward as snow chances slowly
come to an end as northwesterly flow develops ahead of a slowly
approaching upper-level ridge over the western CONUS. With
northwesterly flow aloft, the door remains open for multiple
disturbances to pass through, which will be the case for the
upcoming week. As of now, these disturbances are expected to
increase cloud cover, but no significant precipitation is expected
due to the drier air advecting in from Canada. As these disturbances
push through, strong winds will develop as 700mb height gradients
strengthen then weaken, nearly on a diurnal cycle. Currently looks
to be calm winds at night, increasing during the day, and decreasing
into the overnight hours from Sunday onward. No disturbance stands
out as being the most impactful at this time, but daily high wind
watches or warnings may be necessary for the week ahead, especially
along the I-80 Summit due to these periodic disturbances. With the
winds increasing during the peak heating hours, downslope flow will
aid in much warmer than average temperatures throughout the week,
but also potential fire weather concerns due to the dry nature of
downsloping winds. Tuesday and Wednesday are the warmest days as of
now, with highs in the 60s and 70s area-wide, including west of the
Laramie Range. Should feel like spring this week, though breezy
conditions will be expected daily.

&&

.AVIATION /12Z TAFS THROUGH 12Z THURSDAY/...
Issued at 506 AM MDT Wed Mar 19 2025

Complex forecast this morning as lingering snow showers meander
in and around most terminals. Snow showers at KCYS ended around
an hour ago, but blowing snow remains with little to no
reduction in visibility. KLAR and KRWL will continue to see snow
showers over the terminals for the next 2 to 4 hours. All
Wyoming terminals are expected to see MVFR to near-MVFR ceilings
this morning, with ceilings rising by afternoon. Gusty
conditions across all Wyoming terminals early this morning.

Snow showers have cleared out of the Nebraska terminals this
morning, but lingering blowing snow and low ceilings remain.
KCDR, KSNY, and KAIA may see additional lingering snow showers
later this morning, but confidence in these showers impacting
the terminal is low. Included PROB30 groups for these sites.
Snow may remain just to the east of these terminals throughout
the morning hours. Gusty conditions ongoing and expected to
continue throughout much of the 12Z TAF period.

&&

.CYS WATCHES/WARNINGS/ADVISORIES...
WY...High Wind Watch from late tonight through Thursday evening for
     WYZ106-110.
NE...None.

&&

$$

UPDATE...REC
SHORT TERM...MN
LONG TERM...AM
AVIATION...AM