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000
FXUS65 KCYS 191747
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
1147 AM MDT Fri Apr 19 2024

.KEY MESSAGES...

- Dense Fog Advisory remains in effect for the South Laramie
  Range through Friday morning. Visibility below one-quarter
  mile is expected. Some patchy fog is also possible in Laramie
  County Friday morning.

- A weak system will move into the CWA Friday night into
  Saturday enhancing upslope flow. Snow accumulations of 1 to 3
  inches will be possible for the Interstate 80 corridor between
  the South Laramie Range and Sidney.

- Much warmer temperatures are expected for Sunday through
  Thursday.

&&

.UPDATE...
Issued at 1126 AM MDT Fri Apr 19 2024

The main concern this afternoon is the potential for freezing
drizzle/fog to linger between the I-80 Summit and Cheyenne.
Latest radar loop was showing the freezing drizzle was starting
to diminish slightly on the northern edges of Laramie county,
due our low level warm advection become weaker. However, the
visibility was still quite limited across the I-80 summit with
freezing drizzle still occurring between Cheyenne and Buford.
This is going to be a tricky forecast this afternoon especially
leading into this evening and overnight. The latest HREF/HRRR
has things improving this afternoon over Cheyenne and points
west except for the I-80 summit which keeps the visibility
limited. However, this evening and overnight conditions may
deteoriate once again as the low level warm air advection
strengthens again with the potential for more freezing
drizzle/fog and some snow mixed in which could make things
interesting tomorrow morning. In the meantime, we are going to
nudge temperatures down a tad this afternoon mainly along the
I-80 corridor where the thick cloud cover is in place.


&&

.SHORT TERM /THROUGH SUNDAY/...
Issued at 138 AM MDT Fri Apr 19 2024

Quiet across the CWA early this morning with calm winds and no
precipitation. Radar returns across the area do show snow, but it
remains far south of the Wyoming/Colorado border. Weak south to
south-southeast winds have allowed low stratus to persist around the
Cheyenne area and foothills. Dense fog has also been persistent
in the South Laramie Range, so did end up extending the Dense
Fog Advisory through noon Friday. Winds will turn more easterly
as the morning progresses which will allow the South Laramie
Range to be socked in fog through the morning and potentially
into the afternoon. Fog may also develop in Laramie County
during the morning with the favorable upslope flow.

Did lower PoPs from the previous forecast during the day today. Hi-
Res guidance has become less excitable with precipitation. Still
cannot rule out some light snow showers and flurries, mainly across
the Interstate 80 corridor from Laramie to Sidney. Accumulations are
not likely, but a dusting of snow could be possible.

Heading into Friday night, snow chances will increase. The
stationary boundary in central Colorado will gradually shift
northward, pushing moister and slightly warmer 700 mb air into
the CWA. This will create a bit of an overrunning set-up where
warmer, moister air will move up and over the colder air at the
surface. This kind of set-up could be an efficient snow
producer, especially since most of the precipitation will fall
Friday night into Saturday morning when temperatures will be
cold enough snow. Models are in great agreement regarding
location of precipitation as well as snowfall amounts. The most
likely area of highest snowfall amounts looks to be along the
Wyoming/Colorado border and just south of it. For our area,
highest accumulations will be along the Interstate 80 corridor
from the South Laramie Range to Sidney. Here, 1 to 3 inches of
accumulation will be possible between Friday night and Saturday.
Snow accumulations drop off north of the North Platte River
Valley as the warm, moist air does not make it that far north.
Little to no accumulation is expected in these areas. Areas west
of the Laramie Range will also not see much in terms of
precipitation either. As previously mentioned, models and
ensembles are in agreement with these totals, albeit Hi-Res
guidance is on the lower side of this range. Decided to forgo
any winter headlines with this package. Some areas may see low-
end Winter Weather Advisory criteria, however, given the time of
year, not really expecting snow to accumulate on roads or
pavement. An area to watch would be the South Laramie Range and
foothills as the higher elevation and snow could lead to slick
roads, especially late Friday night into Saturday morning. Snow
will taper off by Saturday evening with clearing skies
overnight.

&&

.LONG TERM /SUNDAY NIGHT THROUGH THURSDAY/...
Issued at 138 AM MDT Fri Apr 19 2024

Exiting Clipper shortwave moves through Sunday morning. Not much
with this system except maybe some brief colder air behind the
system. But 700mb temperatures rise pretty quickly through the
day Sunday as upper level ridge builds in behind the low. By
00Z Monday...GFS forecast 700mb temperatures up near +6C across
our southern zones...up from -2C Sunday morning.

Shortwave ridge axis shifts east Sunday night with the approach
of another northern low. Looking at elevated winds Sunday night
into Monday as the low tracks east into the Dakotas. Associated
surface cold front tracks through Monday morning. Hangs up along
the Laramie Range or slightly west for the upcoming future with
east to southeast winds east of the front and southwest winds
west.

Due to the southeast winds...we do see a lot of low level
moisture from the Gulf of Mexico riding up from the southeast.
GFS 850mb humidity well into the upper 70 to 90 percent across
the CWA. Weather becomes unsettled Wednesday afternoon as upper
level vort max rides over the stalled surface front.

Front waffles back and forth across our eastern zones with that
low level moisture. Looks like showers will continue along this
front through at least Saturday when winds shift back to the
west.

&&

.AVIATION /18Z TAFS THROUGH 18Z SATURDAY/...
Issued at 1123 AM MDT Fri Apr 19 2024

IFR conditions persist at KCYS with freezing drizzle continuing
for the next few hours before lifting with MVFR to VFR
conditions briefly anticipated before dropping once again to IFR
with snow showers late tonight into early tomorrow. KLAR and
KSNY may also experience light rain/snow showers late tonight
into early tomorrow. Elsewhere across southeast Wyoming and
western Nebraska will remain VFR with CIGS near 5kft
sticking around until later this afternoon/early evening.

&&

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

&&

$$

UPDATE...REC
SHORT TERM...SF
LONG TERM...GCC
AVIATION...LEG