Area Forecast Discussion
Issued by NWS Cheyenne, WY

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FXUS65 KCYS 241236
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
636 AM MDT Sat Oct 24 2020

.SHORT TERM...(Today - Sunday Night)
Issued at 435 AM MDT Sat Oct 24 2020

An active 24-48 hours ahead across southeast Wyoming & the western
Nebraska Panhandle. High winds have developed in the Arlington and
Elk Mountain areas, as well as the Interstate 80 Summit early this
morning courtesy of strong low-level height gradients & H7-H8 flow
on the order of 55 knots. Occasional gusts up to 60 MPH are likely
through at least mid-day before gradients weaken w/ the passage of
an intense cold front. 700 mb CAG-CPR gradients could remain above
50 meters until at least 21z this afternoon, so Arlington will see
higher gusts a little later than the I-80 Summit. Only change this
AM was to start the High Wind Warning immediately for the Southern
Laramie Range, as a few WYDOT observations are indicating gusts in
the 50-55 MPH range already. These winds will only get stronger in
the hours after sunrise.

A strong winter storm is still poised to bring widespread snow and
bitterly cold temperatures to much of the region, starting tonight
and continuing through late Sunday night. The accompanying intense
cold front will blast southward across the CWA today, clearing all
zones by 6 PM MDT. Temperatures are expected to fall quite rapidly
with the frontal passage. Periods of rain and/or snow will develop
this afternoon with the initial frontal surge. It does appear that
snow squalls could be possibility immediately along the front. The
main event will be this evening and overnight with snow increasing
in coverage and intensity from north to south across the CWA. Very
strong jet energy associated with a vigorous mid/upper-level short
wave digging into the Great Basin & Four Corners & very impressive
isentropic upglide will support snow growth across all zones. Snow
rates could initially be quite intense with a deep dendritic layer
seen on forecast soundings. This combined w/ excellent orographics
will support heavy snow accumulations over the higher terrain. Our
confidence is highest here, so the Winter Storm Watch was upgraded
to a Warning for the Snowy/Sierra Ranges & the South Laramie Range
and adjacent foothills. We also transition ARL to a Warning at 00z
as the combination of snow and wind will create problems for those
areas.

Confidence remains very low for other zones. Model blends continue
to show extremely high (8+ inch snowfall totals) over a large part
of the CWA. Modeled QPF supports this, but one major problem could
be the rapidly cooling thermal profiles. GFS profiles for Cheyenne
as early as 12z Sunday show the majority of the sounding under -15
deg C, leading to questions with snow type and whether needles and
and column crystal structures will limit accumulations. Models are
generally in better agreement than they were 24 hours ago, and now
have a general consensus focused more on the I-80 corridor for the
heaviest snowfall. However, a lot of guidance is pointing to quite
a bit of mid-level drying after 12z Sunday as what appears to be a
substantial dry slot begins to wrap around the system. Not exactly
buying the heavy precipitation rates into Sunday afternoon, so the
official forecast cuts snowfall amounts in model blends by roughly
30 percent. It is certainly still possible to get widespread 8-12"
amounts, but feel much more comfortable with a general forecast of
5 to 8 inches given the uncertainties & potential failure modes of
current guidance. A challenging snow forecast indeed.

One thing is for sure: Dangerously cold temperatures are likely w/
this system. Depending on observed snowfall totals and how quickly
skies clear on Sunday night, temperatures may plunge well below 0F
with wind chills potentially as low as 30 to 40 degrees below zero
for the morning commute.

.LONG TERM...(Monday - Friday)
Issued at 435 AM MDT Sat Oct 24 2020

A general transition to warmer & drier weather can be expected for
the extended forecast period. In areas where significant snow does
fall, temperatures will likely remain quite chilly through Tuesday
at the earliest. The GFS & ECM continue to develop a closed upper-
low over the southern Plains through mid-week, but at this time it
appears this will remain too far south to cause any impacts to our
region. Broad upper-level ridging is expected to take shape by the
latter half of the week.

&&

.AVIATION...(For the 12Z TAFS through 12Z Sunday morning)
Issued at 628 AM MDT Sat Oct 24 2020

Currently IFR ceilings KSNY and KAIA look to continue through
mid-morning as GOES-16 Nighttime RGB showed low clouds develop
over the NE Panhandle before upper level clouds moved overhead.
Soundings suggest low-level cloud break could occur by 15z,
however the presence of upper level clouds could stall the
breaking of the low-level inversion an extra hour or two. Gusty
winds across KLAR and KRWL early this morning will continue and
increase to around 35 kts later this morning before a cold front
arrives later today with northerly winds, lower ceilings, and
reduced visibility due to snowfall for all area terminals. The
front and associated impacts will sweep north to south across the
area passing through KCDR as early as 18z. By Saturday evening,
expecting most area terminals to be impacted by IFR visibility due
to snowfall lasting through much of the night.

&&

.FIRE WEATHER...
Issued at 435 AM MDT Sat Oct 24 2020

No fire weather concerns. Windy conditions this morning across the
southeast Wyoming wind corridors. A winter storm & associated cold
front will bring widespread snow and bitterly cold temperatures to
the entire region tonight trough Monday.

&&

.CYS WATCHES/WARNINGS/ADVISORIES...
WY...Winter Weather Advisory from 3 PM this afternoon to 6 PM MDT
     Sunday for WYZ101>103-105>108.

     High Wind Warning until 6 PM MDT this evening for WYZ110-116.

     Winter Storm Warning from 6 PM this evening to 6 AM MDT Monday
     for WYZ110-112-114-116-117.

     Winter Weather Advisory from 6 PM this evening to 6 AM MDT
     Monday for WYZ104-109-111-113-115-118-119.

NE...Winter Weather Advisory from 3 PM this afternoon to 6 PM MDT
     Sunday for NEZ002-003-019-095-096.

     Winter Weather Advisory from 6 PM this evening to 6 AM MDT
     Monday for NEZ020-021-054-055.

&&

$$

SHORT TERM...CLH
LONG TERM...CLH
AVIATION...MB
FIRE WEATHER...CLH



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