929
FXUS65 KCYS 222340
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
440 PM MST Fri Nov 22 2024

.KEY MESSAGES...

- Elevated to critical fire weather conditions are expected
  Saturday for portions of Southeast Wyoming, with above
  average temperatures, low humidity, and gusty winds.

- Colder temperatures will return Sunday and last through next
  week. Chances for snow will be greatest on Sunday and
  Wednesday.

&&

.SHORT TERM /THROUGH SUNDAY/...
Issued at 300 PM MST Fri Nov 22 2024


A ridge of high pressure remains in control of the weather this
afternoon over southeast Wyoming and western Nebraska. Warming
temperatures at the mid-levels and decent surface mixing have lead
to afternoon temperatures running around 15 to as much as 20 degrees
above average, especially east of the Laramie Range. A few high
cirrus clouds continue to drift from west to east over the region
but other than this, expect a tranquil evening and overnight with
lows running around 5-8 degrees above average.

On Saturday, the ridge of high pressure will begin to break down as
a strong Pacific jet moves in from the west during the afternoon
hours. Temperatures at the surface will continue to run around 15
degrees above average despite some height falls aloft through the
day. A stronger downslope wind regime will in part help to mitigate
the lowering heights, resulting in similar temperatures to Friday
especially in areas east of the Laramie Range in the high plains and
into western Nebraska.

The other forecast concern on Saturday into Saturday evening will be
with regard to Fire Weather. The downslope wind pattern will help to
assist with both warming and drying at the surface. Gusty afternoon
winds especially in areas along/near the Laramie Range could exceed
40 mph for a few hours. In forecast zones where fuels remain
critical, a Fire Weather Watch has been issued from 10am through 6pm
on Saturday afternoon and evening. RH values should recover above 20
percent by sunset, however in areas where winds remain elevated for
longer periods, RH may take a bit longer to recover. This would
likely be especially true in the higher elevations and warm belt
regions of the Laramie Range. Expect an increase in high and mid-
level clouds overnight which will help keep overnight lows a bit
warmer ahead of a cold front expected to arrive on Sunday morning.
Overall wind speeds should remain below high wind criteria, even in
the typically windy corridor of I-80 in the Snowy Range foothills.
In-house guidance supports wind gusts in the 45-55 mph range, so we
will need to watch the wind forecast closely in case it does inch
upward. Forecast confidence is quite high through the short-term
with regard to temperatures. The main question is whether or not Red
Flag criteria will be exceeded for enough hours on Saturday to
warrant a Red Flag warning. Future forecast cycles will address
these concerns.

&&

.LONG TERM /SUNDAY NIGHT THROUGH FRIDAY/...
Issued at 300 PM MST Fri Nov 22 2024

An active weather pattern will carry through the Thanksgiving week
for our area. A trough passing to our north on Sunday will push the
ridge eastward and bring our streak of mild temperatures to an end.
A cold front will pass through Sunday as a surface high pressure
slides down the eastern side of the Rockies. Another subtle
shortwave trailing behind the main trough to the north will ride
along the frontal boundary, and pass through the area Sunday and
Sunday night. This secondary system will bring a chance for
snowfall, but forecast totals look pretty modest at this time. While
good cloud level moisture embedded in westerly flow will promote
accumulating snow in the higher terrain, the rest of the lifting
profile is pretty muddled, and probably mainly due to frontogenesis.
The signal for overrunning is not really clearly present, but some
confluent flow in the lower to middle atmosphere may support a brief
period of banded precipitation Sunday afternoon or evening. As of
now, this looks like a sub-advisory to low-end advisory event for
the mountains and Carbon county, with just a few flurries up to an
inch or so for the High Plains. Totals closer to 3" could be
possible in the plains if banded snowfall manages to develop, but
that is still uncertain at this time.

Drier air moves in quickly behind this system, with below average
precipitable water values over the area by daybreak Monday. Look for
dry conditions and more sunshine as highs end up about 5F cooler
than average for this time of year. A more complex period of
activity sets up beginning on Tuesday as a strong, zonally oriented
jet sets up over the western half of the CONUS. A good fetch of
moisture off of the Pacific ocean will likely return snowfall to the
mountains Monday night or Tuesday morning, but an elongated surface
trough developing in the lee of the Rockies from Montana down to
central Colorado will keep the colder air off to our northeast on
Tuesday. A weak vort-max passing by to our north while a stronger
shortwave moves along the zonally oriented jet will set up a period
of more active weather into Wednesday. The main uncertainties at
this time will be how far south the main shortwave tracks. A more
northerly track would help stall the frontal boundary further north
and thus lead to snowfall developing over the High Plains during the
big travel day on Wednesday. Ensembles are split on whether this
will occur, or if the wave will pass just south and thus keep most
of the snowfall confined to the I-80 corridor and southward as shown
by the deterministic GFS/ECMWF. With big travel implications, we`ll
need to keep a close eye on this system. Regardless of Wednesday`s
outcome, there is pretty good confidence (80% probability) that the
system will be out of the area by Thursday morning, leaving drier
but quite chilly conditions in place for Thanksgiving Day.

&&

.AVIATION /00Z TAFS THROUGH 00Z SUNDAY/...
Issued at 437 PM MST Fri Nov 22 2024

West flow aloft tonight will become southwest on Saturday.
Ceilings will remain around 20000 feet. Winds will gust to
23 knots at Rawlins until 15Z, then gust to 38 knots at Rawlins
and Laramie after 15Z Saturday.

&&

.CYS WATCHES/WARNINGS/ADVISORIES...
WY...Fire Weather Watch from Saturday morning through Saturday
     afternoon for WYZ418>420.
NE...None.

&&

$$

SHORT TERM...MAC
LONG TERM...MN
AVIATION...RUBIN