661
FXUS65 KCYS 071107
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
407 AM MST Thu Nov 7 2024

.KEY MESSAGES...

- Areas of freezing fog and/or freezing drizzle are possible
  along the I-80 corridor from the summit eastward Thursday
  night into Friday morning.

- The storm system will return to the area from the south late
  Friday into Saturday, bringing a better chance of rain and wet
  snow. Exact accumulations are still uncertain.

- Sunday night into Monday another upper level disturbance is
  expected to transverse across the area on Sunday night and
  Monday and bring another round of strong winds especially
  across the wind prone areas.

&&

.SHORT TERM /THROUGH FRIDAY NIGHT/...
Issued at 124 AM MST Thu Nov 7 2024

The upper-level trough remains split between the northern stem and
southern stem. The northern stem continues to push off to the east
over the Great lakes region based on GOES16 mid-level water vapor
imagery. The southern flank remains over the Four Corners region as
of 09Z with an ample plume of moisture ejecting out ahead of the
trough axis over eastern Colorado and New Mexico. Snow remains
ongoing across Colorado at this time, with little to no snow showers
currently over southeast Wyoming and western Nebraska. In fact,
clear skies remain over the majority of the CWA, except the far
southern Panhandle in Kimball and Cheyenne counties. With the clear
skies and very cold air behind the departing cold front,
temperatures are cold this morning. Current temperatures are in the
single digits to teens across southeast Wyoming and western
Nebraska, except the southern Panhandle that is relatively warm in
the upper-20s. Temperatures are expected to continue dropping
overnight as radiational cooling continues.

The upper-level trough is not expected to evolve or move much
throughout the day, with models placing the upper-level low over
northern New Mexico by 09Z Friday, with current placement directly
over the Four Corners region. The lack of propagation of this upper-
level low is due, in part, to the relative lack of any significant
cyclonic vorticity advection at 500mb. With the upper-level low
remaining essentially stationary to the south, mid- and upper-level
easterly flow is expected. A weak ridge axis across western Nebraska
southeast through Missouri and Arkansas at 700mb will promote moist,
southerly flow across. Precipitable water values increase into the
0.5 to 0.75in range throughout the day, with the highest values over
the Nebraska Panhandle. The moisture will continue to advect in
throughout the day as the 700mb low slowly migrates easterly and
pushes the weak ridge further to the east. Weak to modest isentropic
lift will develop over the far southern Panhandle by 21Z, leading to
increased precipitation chances from 21Z onward. With temperatures
warming into the low-40s to low-50s across the Panhandle today,
precipitation across Cheyenne County will likely start as rain
before transitioning to a rain/snow mix.

After the sunsets, isentropic lift remains in place, and ample moisture
remains over southern portions of the CWA, there precipitation is
expected to become more wide-spread along the I-80 corridor. 700mb
temperatures are progged to remain below 0C overnight Thursday into
the early morning hours Friday. Forecast soundings suggest a
saturated low-level profile with temperatures at and below 0C
throughout the entire saturated layer. This suggests the development
of supercooled water droplets that may lead to ice formation at the
surface. Therefore, included freezing drizzle/rain into the forecast
along the I-80s corridor from the Summit eastward through the
southern Panhandle. Forecast sounding profiles from the HRRR suggest
that the Summit to Cheyenne has the best chances of producing this
freezing drizzle, but cooling temperatures and increased moisture in
the southern Panhandle suggests the freezing drizzle may extend to
the east through the southern Panhandle. Ice accumulation is
expected to remain less than a 10th, but roads may become slick with
black ice Friday morning. Additionally, forecast HRRR soundings
suggests the profile in the southern Panhandle can support both snow
and freezing drizzle, with some of the particles remaining
supercooled to the surface and other remaining completely frozen in
snowflakes. This could lead to the development of ice and snow along
I-80 in the southern Panhandle with the snow potentially covering
icy spots and potentially keeping roads icy longer. With low Friday
morning in the teens and 20s area-wide, roads will likely drop to
below freezing temperatures overnight, promoting any freezing rain
and/or snow to stick to the road surfaces.

Models have come into significantly better agreement on the track of
the southern flank of the upper-level trough and 700mb system
associated with it. Models suggest the 700mb low moving from eastern
New Mexico northerly through western Kansas before advecting more to
the northeast into central Nebraska by Saturday morning. The track
is further west than previous model runs, suggesting the continued
chance for wrap around precipitation from the surface low. With
moisture streaming into the region throughout the day Friday and
temperatures remaining fairly cold Friday night into Saturday
morning, snow looks likely along and west if I-25 while rain
transitioning to a rain/snow mix looks most likely east of I-25.
Could see some accumulation out of this system before it becomes
vertically stacked and weaker by Saturday morning. Accumulation
between 0.5 and 1.5 is possible along I-80 from Cheyenne to Laramie
and Rawlins. Minimal accumulation is expected in the Panhandle due
to the later switch over to a rain/snow mix. Overall, impacts will
likely be fairly minimal, though some accumulation is possible.
Temperatures Friday night into early Saturday morning will drop into
the mid-20s to low-30s area-wide.

&&

.LONG TERM /SATURDAY THROUGH WEDNESDAY/...
Issued at 230 AM MST Thu Nov 7 2024

Confidence increasing on the Saturday event as the GFS has come
in line with what the ECMWF has been advertising the past
several days. Both are now showing the closed/vertically stacked
low over northeast Colorado/northwestern Kansas with wrap around
moisture on the north side of the low across the Panhandle and
eastern half of southeast Wyoming. Did increase PoPs to 80 to 90
percent in the Panhandle and 60 to 70 percent in the southeast
Wyoming plains east of the Laramie Range. Still some question on
snow amounts as this low still showing fairly warm 700mb
temperatures at -2 to -3C over the Panhandle. Tds above freezing
as well out there...so wet bulbing temperatures not looking
likely. So where the heaviest QPF is (over the Panhandle) it
may just stay a cold rain. If guidance starts coming in
colder...we may need to consider winter headlines. Think we are
okay right now with just rain. May be a little more messier over
here by Cheyenne and over the Summit where freezing levels are
forecast to be down near 6K feet. Could see advisories being
issued for the eastern foothills and the I-80 Summit zones.
Still a little early for any headlines as of yet.

Low tracks east northeast Saturday afternoon and evening into
south central Nebraska with precipitation slowly coming to an
end from west to east. Upper ridge still on track for Sunday to
move across the CWA. GFS 700mb temperatures climb to +4 to +5C
so still expecting near 60 highs in the Panhandle and mid to
upper 40s across Carbon and Albany Counties west of the Laramie
Range. Warm conditions continue into Monday as ridge axis shifts
to our east Monday afternoon.

Need to be on the lookout for strong winds Monday night into
Tuesday. Craig to Casper 850/700mb height gradients up to
59/40mtrs Monday evening and GFS 700mb winds up near 50kts out
by Arlington. Random Forest guidance showing a 50 percent chance
of strong winds at Arlington and 40 percent chance at Bordeaux.
Did increase winds to warning criteria at Arlington for Tuesday
morning. Held off on Bordeaux as wind direction from the
southwest not a favorable direction for strong winds at
Bordeaux.

Next cold front moves in Tuesday night into Wednesday morning
with a fast moving upper shortwave moving through. Given the
speed at which this system is moving through...weather impacts
should be minimal.

&&

.AVIATION /12Z TAFS THROUGH 12Z FRIDAY/...
Issued at 403 AM MST Thu Nov 7 2024

Uneventful TAFs this morning with clear to scattered high clouds
over southeast Wyoming and most of the Nebraska Panhandle.
Things begin to change this afternoon as low level moisture
increases from the south and southeast. Latest HRRR and HREF
guidance showing flight conditions lowering first at KSNY and
eventually KIBM and KCYS. Look for IFR/LIFR conditions for these
airports by this evening...continuing into Friday.

&&

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

&&

$$

SHORT TERM...AM
LONG TERM...GCC
AVIATION...GCC