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-- Discussion containing changed information from previous version are highlighted. --
000
FXUS65 KCYS 191124
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
524 AM MDT Mon Apr 19 2021

.SHORT TERM...
-- Changed Discussion --
(Today through Wednesday)
Issued at 330 AM MDT Mon Apr 19 2021

Primary forecast concern will be snow accumulations and associated
impacts over the next 24 hours across southeast Wyoming and
western Nebraska. Strong Canadian cold front is currently
positioned near Rawlins and stretches east to just north of
Cheyenne at this hour. Whitaker, about 15 miles north of Cheyenne,
is reporting north winds and temperatures lowering into the 20s
while Cheyenne is still hanging on to a gusty west wind and a
temperature of 42 degrees. The front has pushed south through the
Nebraska panhandle with Sidney and Kimball reporting northeast
winds and lowering temperatures. Snow has developed north of the
North Platte River Valley, but accumulations so far have been
light from near Douglas over towards Chadron Nebraska. This front
will continue to move southwest through Cheyenne and eventually
Laramie over the next 3 hours, with snow developing along the I-80
corridor sometime between 800 AM and noon local time. Added some
fog across the Pine Ridge and Cheyenne Ridge due to the potential
for advection fog behind the front.

No major changes to the forecast during this winter weather event.
Although the system remains on the progressive side, moving south
of the area by late this evening, there are some impressive
dynamics including strong frontogenesis, upper level diffluence,
WAA aloft, and upslope winds/convergence at the surface. 0.5 to
1.0 inch snowfall rates look like a good bet along and west of the
I-25 corridor for a few hours this afternoon and into this
evening. Current headlines look good, with maybe a slight
adjustment in Advisories needed for the central and southern
Nebraska panhandle. East winds do not appear quite as strong
compared to 24 to 48 hours ago, so do not expect blowing snow to
be a major issue. Drifting snow will likely be the primary concern
besides any falling snow, with drifting possibly on area roadways
across southeast Wyoming, especially the higher elevation
mountain passes. Snow accumulations and road impacts will be
tricky with this system, mainly because of the time of the day
most of the snow occurs. In theory, snow would accumulate more
efficiently at night this time of the year compared to the
daytime. On the other hand, pavement temperatures should remain
relatively cold due to the much colder than normal temperatures
the last 7 days. Snowfall rates as little as a half inch per hour
may be enough to overcome pavement temperatures during the day.
Kept snowfall accumulations somewhat below consensus and ensemble
guidance, but even then, still coming up with a solid 3 to 5
inches across the Advisory area, and 5 to 9 inches in the
mountains including the I-80 Summit.

For Tuesday and Wednesday, overnight lows will be very cold for
this time of the year and generally in the teens to low 20s. Some
single digits are likely west of I-25 where skies clear out and a
decent snowpack remains. Longwave trough will remain over the
northern/central plains and the Great Lakes region through the
middle of the week, resulting in cold northerly flow for the Front
Range. Daytime high temperatures will struggle to increase into
the upper 30s to low 40s as 700mb temperatures remain near -10
to -15c across most of the region. A secondary disturbance will
dig south on the backside of the main upper level low across the
Great Lakes Region. This disturbance will likely enhance snow
shower activity across the eastern plains and in the mountains
on Tuesday into Tuesday evening. Lower chances for snow showers
expected on Wednesday as the low to midlevels stabilize. However,
a weak shortwave trough will approach from the west, which may
result in an increasing threat of snowfall near the Colorado
border through the afternoon hours.

.LONG TERM...(Wednesday Night through next Monday)
Issued at 243 AM MDT Mon Apr 19 2021

Deepening trough over Nevada and developing closed 500 mb low
pressure will continue moving eastward with vorticity advection
ahead of the centered pressure and coupled moisture advection
bringing in another round of precipitation to mainly along the I-80
corridor east of Rawlins into the NE Panhandle. Accumulations are
fairly lacking in both the GFS and EURO 00z runs to be too excited
for but certainly any moisture right now is appreciated. Trended a
touch more to the EURO for the WED-Thu system after 00z Thu as GFS
limits moisture more on the CO side. With some of the precipitation
occurring in the overnight in sub-freezing temperatures, snowfall
accumulation possible but given the liquid amounts available, any
accumulation will remain around a tenth to a dusting especially on
grassy surfaces, roadways likely to be only wet though
some minor glazing could be a concern.

With the main trough axis moving eastward through the region Friday,
another ejected pocket of vorticity and mositure via another
shortwave for a better chance of some additional moisture to more of
the forecast region. Model solutions showing upwards of around 0.10"
of liquid for the event for many with southern mountains seeing it
mainly in snow. Ridging sets in ahead of shortwave Saturday night
into Sunday with precipitation chances extremely limited based on
model solutions as main ridging again persists ahead of our next
system that will approach CA coast Monday afternoon with following
round of precipitation middle of next week.

Otherwise, temperatures and conditions will return to more of late
April with daytime highs into the 50s and 60s by Friday and into
the 70s for the Panhandle and North Platte River Valley on Sunday.
Overnight lows Thursday through Saturday will remain near to below
freezing for many.
-- End Changed Discussion --

&&

.AVIATION...
-- Changed Discussion --
(For the 12Z TAFS through 12Z Tuesday morning)
Issued at 515 AM MDT Mon Apr 19 2021

Latest satellite and surface observations have captured the
movement of the cold front through the region this morning with
advection freezing fog in the wake of the front. Contrary to some
of the observations such as KCYS, the wind and freezing fog are
contributing to light riming and not really freezing rain as shown
by area sensors. Further deteriorating conditions for all sites
today as snowfall chances pickup later this morning through the
afternoon being heavy at times with best chance for accumulating
in afternoon and evening. Flight conditions will remain at least
at MVFR to LIFR for all sites into the evening as conditions lift
slowly back to VFR as snow ends from north to south.
-- End Changed Discussion --

&&

.FIRE WEATHER...
Issued at 234 AM MDT Mon Apr 19 2021

No fire weather concerns this week due to widespread snowfall today,
and cold temperatures through early Thursday morning. Models are
indicating a warming and drying trend for next weekend, which may
result in increasing fire weather concerns across portions of the
region.

&&

.CYS WATCHES/WARNINGS/ADVISORIES...
WY...Winter Weather Advisory until 6 PM MDT this evening for WYZ101-
     102.

     Winter Weather Advisory from 9 AM this morning to 11 PM MDT this
     evening for WYZ105>108-110-114-115-117>119.

     Winter Weather Advisory until 11 PM MDT this evening for WYZ103.

     Winter Storm Warning from 9 AM this morning to 3 AM MDT Tuesday
     for WYZ116.

NE...Winter Weather Advisory until 6 PM MDT this evening for NEZ002-
     095.

     Winter Weather Advisory from 9 AM this morning to 11 PM MDT this
     evening for NEZ020-054.

&&

$$

SHORT TERM...TJT
LONG TERM...WM
AVIATION...WM
FIRE WEATHER...TJT