Area Forecast Discussion
Issued by NWS Cheyenne, WY

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FXUS65 KCYS 231150
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
550 AM MDT Sat Mar 23 2024

.KEY MESSAGES...

- Mild conditions are expected to persist today.

- Increasing chances for rain and mountain snow showers this
  afternoon and evening. Minor snowfall accumulations possible
  above 9000 feet. Showers may produce 45+ MPH wind gusts. A
  couple of lightning strikes are also possible.

- A winter storm will bring much colder temperatures, strong
  winds, and the potential for accumulating snowfall to portions
  of the area from Sunday through Tuesday. The most likely areas
  for impactful snow and blowing snow will be over the Pine
  Ridge in Niobrara, Sioux, and Dawes counties, as well as the
  Cheyenne Ridge in far southeast Wyoming and the southern
  Nebraska Panhandle. A Winter Storm Watch has been posted for
  Dawes and northern Sioux counties.

&&

.SHORT TERM /THROUGH SUNDAY NIGHT/...
Issued at 302 AM MDT Sat Mar 23 2024

A challenging, low-confidence forecast persists w/ regard to the
upcoming potential for accumulating snowfall & strong winds over
portions of the CWA from Sunday afternoon through Monday morning
with wrap-around moisture on the back side of an intense surface
low over the central plains. One more pleasant day ahead for all
of southeast Wyoming and the western Nebraska Panhandle w/ upper
level ridging and resultant 700-millibar temperatures from +2 to
+4 deg C. Daytime high temperatures should reach the 60s for the
majority of areas along/east of I-25 today. Showers are expected
to become more numerous across the region during the afternoon &
evening in response to mid-level vort energy & increasing upper-
level difluence in advance of a deepening western US trough. May
see a few lightning strikes w/ forecast soundings indicating the
potential for upwards of 200 J/kg of CAPE in response to cooling
mid-level profiles.

Significant surface cyclogenesis is still expected to take place
over eastern Colorado on Sunday with the GFS/GEM/ECMWF in decent
agreement with a ~980 millibar surface low near Pueblo around 18
to 21z. Despite the position of the surface low being very close
to the benchmark for a significant snow storm across much of the
region, global deterministic models and their ensembles continue
to suggest extremely modest QPF values and thus minimal possible
impacts from this system. This appears to be the result of a dry
push associated with the northern stream short wave & cold front
blasting across the CWA on Sunday morning, which prevents a more
organized and expansive precipitation shield from engulfing much
of the CWA in the deformation zone on the back side of a rapidly
deepening cyclone. Nonetheless, showers are expected to increase
in coverage through the day on Sunday w/ daytime highs being met
by late morning for the majority of locations. Northerly surface
flow in the wake of the cold frontal passage will strongly favor
the Pine Ridge from Niobrara county eastward into northern Sioux
and Dawes counties in the Neb Panhandle, and perhaps areas along
the Cheyenne Ridge further south as well. The 00z high-res model
suite featured substantial support for localized pockets of high
QPF in the vicinity of these terrain features. However, this has
continued to be largely absent in the global models w/ the small
exception of the 00z ECMWF ensembles which hinted at a ribbon of
40-60 percent probabilities of at least 0.5 inch of QPF over the
northern NE Panhandle from Sunday afternoon through Monday AM on
the nose of the TROWAL. This is where overall confidence appears
to be highest for more substantial snow amounts, but significant
uncertainty remains regarding the western or southwestern extent
of impactful snow w/ substantial model spread. The mountains are
also not a sure bet, even for advisory-level snow as the dynamic
forcing and adequate moisture may be focused to the south & east
of the Snowys and Sierra Madres.

The rapidly deepening cyclone will likely give way to impressive
pressure gradients across the CWA, while rapidly cooling 700-hpa
temperature profiles maintain steep lapse in the presence of 50+
knot 700 to 800 hpa flow along the I-80 corridor from the Summit
eastward. Wind gusts up to 55 MPH appear likely with a few rogue
gusts in excess of 60 MPH certainly possible near the CO line in
far southeast WY and the southern NE Panhandle. These winds with
any notable snowfall would certainly be a problem with potential
for near-blizzard conditions. While the prospects of this do not
seem particularly high, it is still a very real possibility over
portions of the area. Winds along the Pine Ridge may still be on
the gusty side, but will likely not be as strong courtesy of the
wake downstream of the Black Hills. This should mitigate chances
for a true blizzard, but substantial blowing & drifting snow may
still be a good possibility even if lower amounts are realized.

Despite significant model discrepancies, in response to the most
recent trends in guidance as well as coordination w/ neighboring
WFOs including LBF/UNR, have opted to add northern Sioux as well
as Dawes county to a Winter Storm Watch valid from 18z Sunday to
18z Monday as this area has the best combined support from high-
res and global guidance w/ the greatest confidence in the higher
impacts from snow and blowing snow. Some guidance is hitting the
eastern portion of Niobrara county pretty hard as well, but over
all there is more variability in the guidance as you head to the
west. Headlines may be expanded in the future, especially across
the Cheyenne Ridge and perhaps the Snowys/Sierra Madres as well.

&&

.LONG TERM /MONDAY THROUGH FRIDAY/...
Issued at 302 AM MDT Sat Mar 23 2024

A fairly unsettled pattern will exist throughout the long term even
after this Sunday`s winter storm is through. The winter storm will
largely be over by daybreak Monday as models agree on the surface
low being somewhere in eastern Nebraska. Lingering snow showers will
still be possible throughout the day as wrap around moisture looks
possible in the Nebraska panhandle. Snow accumulations throughout
the day will be minimal. The other concern during the day Monday
will be the winds. A rather impressive pressure gradient across the
state of Nebraska on the backside of the low will lead to strong
northerly winds. Gusts up to 45 MPH are possible, especially across
the panhandle. Depending on a number of factors, including snow
amount and time of year, blowing snow could be an issue. Cannot rule
out any visibility reductions with the blowing snow. The northerly
winds will keep temperatures well below average on Monday with many
locations not even making it above freezing. These cold temperatures
will increase the likelihood of any blowing snow. Winds will die
down Monday evening, leading to a cold and quiet overnight.

After Monday, temperatures will gradually warm up through Thursday.
During this time, there`s near daily chances for precipitation as
the CWA still sits under the upper-level trough responsible for the
winter storm. Luckily, impacts during this time look minimal with
only light precipitation expected. High temperatures peak on
Thursday as brief ridging builds in. Highs will climb above average
with many locations east of the Laramie Range making it into the
60s. However, this warmth may be short lived as both the GFS and
ECMWF show a substantial trough moving into the CWA by the end of
the week. Of course this system is still about a week out so things
are likely to change, but it at least looks like temperatures will
get colder and precipitation will be possible.

&&

.AVIATION /12Z TAFS THROUGH 12Z SUNDAY/...
Issued at 524 AM MDT Sat Mar 23 2024

VFR expected for the southeast WY terminals through the forecast
period. There is a slight chance for VCSH to transition to -SHRA
for the southeast WY terminals near 0z Sunday. Wind gusts up to
25-35 knots can also be expected for southeast WY terminals. The
NE Panhandle terminals will experience MVFR/IFR fluctuations
this morning, and then gradually lift to low-end VFR by 18z.
Wind gusts for the NE Panhandle terminals will also be 20-35
knots at times today. Please see individual terminals for
further information.

&&

.CYS WATCHES/WARNINGS/ADVISORIES...
WY...None.
NE...Winter Storm Watch from Sunday afternoon through Monday
     morning for NEZ002-095.

&&

$$

SHORT TERM...CLH
LONG TERM...SF
AVIATION...BW


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