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-- Discussion containing changed information from previous version are highlighted. --
000
FXUS65 KCYS 181021
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
421 AM MDT Thu Apr 18 2024

.KEY MESSAGES...

-- Changed Discussion --
- Potential exists for areas along I-80 from the Summit to Pine
  Bluffs to receive modest snow accumulations Friday night
  through Saturday morning.

- A cold front will bring cold and unsettled weather for
  Thursday through Saturday, along with a chance for rain and
  snow showers.

- Much warmer temperatures are expected for Sunday through
  Wednesday.
-- End Changed Discussion --

&&

.SHORT TERM /THROUGH FRIDAY/...
-- Changed Discussion --
Issued at 225 AM MDT Thu Apr 182024

Post frontal conditions continue across the region as the
surface cold front has pushed just over the border into
Colorado. This front was associated with a brief burst of snow
showers across portions of the I-80 corridor last evening. A
persistent NW flow aloft a will keep the area under the
influence of steady CAA. As we head into late Thursday
night/Friday morning, models keep cyclonic flow across the
region associated with an elongated trough extending across much
of central Canada. A weak trough sitting off the California
coast begins to move inland and this will shift the flow across
Colorado to the SW. This is what will drive the cold front to
transition into a weak warm front and push it back north close
to the I-80 corridor where it will meet the CAA continuing to
spill into the region. This will create a stationary boundary to
develop somewhere across SE Wyoming and the Nebraska Panhandle
during the day on Friday.

Models and ensemble members seem to be in fairly good agreement,
with ensemble members begin to shift toward higher snowfall
accumulations for areas along the I-80 corridor and the I-80
Summit. HI-RES models are also suggesting when the shortwave
trough breaks off from the main trough axis and passes through
the region, the flow shifts toward a warm, moist, easterly
upslope flow which would overrun the the stationary boundary and
naturally over the higher terrain to the west. The concern is
for a window where moderate snow could fall and bring advisory
level snow accumulations with it. The biggest question mark
regarding snow potential will be the quality of the moisture as
QPF values don`t look too impressive, but with strong lifting
mechanisms in place this may counteract that concern. Areas from
the I-80 Summit to Pine Bluffs will be watched for the potential
for some winter weather highlights to be issued.
-- End Changed Discussion --

&&

.LONG TERM /FRIDAY NIGHT THROUGH WEDNESDAY/...
-- Changed Discussion --
Issued at 225 AM MDT Thu Apr 18 2024

The long term remains active, with multiple chances for
precipitation. However, a warm up will lead to above average
temperatures for most locations.

Still some lingering snow showers expected on Saturday as the broad
trough that was over much of northern CONUS finally breaks down and
pushes further eastward. Most of the snow looks to fall Friday night
into early Saturday morning, but some minor accumulations will still
be possible during the day Saturday with well below average
temperatures expected. Models are still in good agreement with the
highest accumulations in the South Laramie Range and points eastward
along the Interstate 80 corridor. The Cheyenne Ridge could also see
some enhancement of snowfall totals as some models point to weak
easterly flow at the surface Saturday morning. Overall, most models
are in pretty good agreement over snowfall totals for this event. By
Saturday evening, snowfall totals from Cheyenne to Sidney could be
in the 1 to 3 inch range with higher totals in the South Laramie
Range, but lower totals everywhere else. Most models including the
ECMWF ensemble mean, NAM, and NBM are in agreement with these
totals. The one outlier continues to be the GFS, showing totals
almost double what other models are showing. Even the GEFS members
show quite a large spread, with nearly a third of them showing 6 or
more inches for Cheyenne. Based on GFS soundings, it appears the GFS
shows an overrunning event, with warm, moist air moving up and over
the colder airmass at the surface. This kind up set-up could be an
efficient snow producer depending on the time of day it falls
and the snowfall rates. Being that it is late April, sun angle
will be a determining factor in snowfall amounts, so any snow
Friday night and Saturday morning will have the best chance for
sticking and accumulating. Again, although the GFS is an
outlier, it is a solution that should be considered.

A rapid warm up is expected on Sunday with strong warm air advection
from a weak upper-level ridge. 700 mb temperatures will be above
freezing, allowing highs to jump over 20 degrees from Saturday`s
highs in some locations! Temperatures will actually be above normal
on Sunday. A weak disturbance moving over the CWA Sunday night could
lead to some light showers or sprinkles, but nothing impactful
in terms of precipitation.

Another weak shortwave will drop down from Canada on Monday, bring
with it a cold front and another chance for precipitation. Luckily,
this does not look like a strong front, but it will be enough to
lower temperatures closer to average on both Monday and Tuesday.
Again, precipitation with this front will likely be light and not
cause impacts.

Headed into Wednesday, weak ridging will lead to above average
temperatures and drier conditions throughout the day. Yet another
weak disturbance aloft could spark some showers over the CWA
Wednesday night.
-- End Changed Discussion --


&&

.AVIATION /18Z TAFS THROUGH 18Z FRIDAY/...
Issued at 457 PM MDT Wed Apr 17 2024

A cold front has moved through the region, and light rain and
snow showers will impact VIS and CIGs the next 24 hours. Expect
gusty winds behind this cold front as it moves south, with gusts
to 20-32 knots across the forecast area until approximately
6z-8z, when winds begin to dissipate. Fluctuations between
MVFR/IFR/LIFR will be expected as the evening and overnight
hours progress. Please see individual TAFs for further
information.

&&

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

&&

$$

SHORT TERM...AW
LONG TERM...SF
AVIATION...BW