Area Forecast Discussion
Issued by NWS Cheyenne, WY

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000
FXUS65 KCYS 242124
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
324 PM MDT Sun Mar 24 2024

.KEY MESSAGES...

- Light to moderate snow is expected along the I-80 corridor
  from Laramie to Cheyenne through early this evening. Localized
  heavy banding is possible. Winter Weather Advisories are in
  effect until 6 AM MDT Monday.

- Wind gusts 45-55 MPH are expected along the I-80 corridor
  through tonight. Areas of blowing and drifting snow are
  possible, especially west of Cheyenne.

- Much colder temperatures will arrive by late tonight, with
  overnight lows 10 to 15 degrees. Sub-zero wind chills are
  expected.

&&

.SHORT TERM /THROUGH MONDAY NIGHT/...
Issued at 216 PM MDT Sun Mar 24 2024

The main forecast challenge over the next 6 to 12 hours revolves
around the potential for banded snow over portions of south east
Wyoming, particularly from the Southern Laramie Range and points
southeastward into the I-25 corridor near Cheyenne. The high-res
model suite continues to show significant spread in QPF amounts,
with the potential for convective elements leading to an extreme
level of uncertainty at only ~3 hours lead time. A large area of
clearing was observed from 16-18z today, roughly along and south
of a line from Rock River to Pine Bluffs. This supported rapidly
increasing temperatures with daytime highs briefly reaching 55 F
at KCYS. This has supported rapidly steepening lapse rates, with
the help of strong 700-mb CAA overhead. Low-level instability is
plentiful with the latest SPC Mesoscale Analysis indicating 500+
J/kg SBCAPEs (50-100 J/kg 0-3 km CAPE) in this corridor. Intense
low-level convergence/frontogenesis along the frontal zone could
support banded snowfall along the I-80 corridor mainly from near
Elk Mountain eastward into Cheyenne/I-25. The past few HRRR runs
have suggested potential for 1-2 inch per hour snow rates across
the South Laramie Range/I-80 Summit area between 21z and 0z this
afternoon w/ total amounts well in excess of 6 inches using a 10
to 12 to 1 ratio. Some initial concerns regarding how quickly we
could see a change over from rain to snow with potential for the
cold air to lag a bit behind the initial frontal passage, but w/
the help of convective activity & resultant wet bulbing, surface
temperatures have plummeted from the middle 50s to middle 30s at
KCYS over just the past couple of hours. As winds increase post-
fropa, widespread gusts 45-55 MPH are anticipated this evening &
overnight. Fortunately, most of the snow should precede the high
wind gusts, but blowing and drifting snow impacts may linger for
a few hours into the night depending on the overall snow type as
temperatures continue to fall, and whether or not the fresh snow
pack will be particularly blowable. Even so there are sufficient
concerns for dangerous travel conditions due to black ice with a
potential flash freeze. In an earlier update, decided to issue a
Winter Weather Advisory for these aforementioned areas for total
accumulations possibly in excess of 3 inches beneath some of the
heaviest bands through the early evening. Admittedly though, our
confidence is quite low.

The rapidly deepening SE Colorado surface low will should bottom
out near 980 millibars late this afternoon before ejecting north
and east across central KS into eastern NE through 12z Mon. This
would normally be a favorable storm track for a significant snow
event for our CWA, but given the prior dry slotting which likely
resulted from the yesterdays northern short wave, this helped to
shunt the better moisture and dynamics further to the east. Wind
gusts will still become quite strong this evening and overnight,
especially along the I-80 corridor from Cheyenne to Sidney given
the very strong pressure gradients. Wind gusts up to 55 MPH will
be a good bet, though forecast soundings would suggest we should
be just shy of warning criteria. As such, we opted to cancel the
inherited High Wind Watch although a rogue 60 MPH gust still can
not be entirely ruled out. Concerning snow accumulations for the
western Nebraska Panhandle, still believe the best chance for 2-
5 inch accumulations will be found over the Pine Ridge in Dawes/
North Sioux counties & areas along the Cheyenne Ridge across the
southern Neb Panhandle. Turning much colder across all areas for
tonight, with lows 10-15 degrees and wind chills as low as -10 F
by sunrise Monday.

&&

.LONG TERM /TUESDAY THROUGH SATURDAY/...
Issued at 216 PM MDT Sun Mar 24 2024

Minimal changes made from the previous forecast package. The primary
forecast concern in the long term forecast period will be the
generally unsettled pattern as shortwave passages and brief ridging
move through the forecast area, resulting in limited to low impacts.

Due to the brief shortwave passage Tuesday, isolated, weak rain/snow
showers cannot be ruled out across southeast Wyoming and western
Nebraska. The mountains have chances for light snow showers near
daily Tuesday through Thursday, but overall accumulations will be
minimal. As brief ridging takes over Wednesday into Thursday, CAG to
CPR pressure gradients jump up to near 50 meters, so gusty to strong
winds are possible, mainly in wind prone areas of southeast Wyoming.
During this time frame, temperatures will gradually increase each
day starting in the 30s to 40s Tuesday, increasing Wednesday to 40s
and 50s, then 50s and 60s Thursday and Friday. The next system is
forecast to start making its way into southeast Wyoming and western
Nebraska Friday into Saturday, so cooler temperatures and increasing
precipitation chances are likely. Long range guidance remains quite
variable from run to run, so confidence in impacts and timing
remains low.

&&

.AVIATION /12Z TAFS THROUGH 12Z MONDAY/...
Issued at 538 AM MDT Sun Mar 24 2024

A tough aviation forecast to pin down the next 24 hours as a
strong low pressure moves into the region. VFR for KLAR and KCYS
will transition to MVFR/IFR by this afternoon and evening. KRWL
will fluctuate between MVFR/VFR through the forecast period.
The NE Panhandle terminals will see large fluctuations between
IFR/LIFR/MVFR this morning through Monday morning due to BLSN
and -SN. Wind gusts will ramp up this afternoon for all
terminals, and continue to be gusty into the overnight hours.
Wind gusts of 30-45 knots are anticipated at most terminals
during this time. Please see individual TAFs for further
information.

&&

.CYS WATCHES/WARNINGS/ADVISORIES...
WY...Winter Weather Advisory until 6 AM MDT Monday for WYZ110-
     115>118.
NE...Winter Weather Advisory until noon MDT Monday for NEZ002-095.
     Winter Weather Advisory from 6 PM this evening to noon MDT
     Monday for NEZ003-021-055.

&&

$$

SHORT TERM...CLH
LONG TERM...LEG
AVIATION...BW


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