-- Discussion containing changed information from previous version are highlighted. --
367
FXUS65 KCYS 202307
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
507 PM MDT Fri Sep 20 2024

.KEY MESSAGES...

- Gusty winds and low humidity will lead to fire weather
  concerns today. Red Flag Warnings are in effect across
  portions of southeast Wyoming from late morning into the early
  evening.

- Well below normal temperatures expected this weekend as we see
  the autumnal equinox occur, with some areas seeing a higher
  chance of rain showers.

&&

.SHORT TERM /THROUGH SATURDAY NIGHT/...
Issued at 207 PM MDT Fri Sep 20 2024

Latest upper air analysis depicts the persistent upper level ridge
remaining firmly in place across the south central CONUS with the
next Pacific System beginning to make landfall across the Western
Seaboard. Observations throughout the morning have been mild
temperatures, gusty winds, and dry conditions across the CWA. As a
result, fire weather has been the main concern throughout the
morning and into the afternoon hours, with Red Flag Warnings in
effect for Converse, Niobrara, North Platte, North Goshen, and
Northeast Carbon County through 8pm tonight. In addition, went ahead
and added the Arlington area into the Red Flag Warning with minimum
humidity values in the low teens and stronger winds across the zone.
Should begin to see excellent humidity recovery in the overnight
period, with the advance of a cold front expected to dig across the
zone through tomorrow.

Forecast remains on track over the next 36 hours, as the upper level
closed low begins its journey across the Intermountain West and
centering directly over the Four Corners tomorrow afternoon. Should
be little impacts during the day associated with this system, with
much of the moisture remaining south of the WY/CO boarder as PWATs
remain on the dry side. Regardless, will see a decent cooldown
tomorrow with daytime highs a good 20F lower than today and
overnight lows in the mid to high 30s across the lower terrains for
southeast Wyoming, in the low to mid 40s for the Nebraska Panhandle,
and at or below freezing in the higher terrains. Some chances for
orographically induced precipitation across our higher peaks with
some of the first mentions of snow for the season. Increased
confidence for some light snow tomorrow evening and into the
overnight hours with forecast soundings indicating cold temperatures
above 9000ft, and decent moisture profiles in the dendritic growth
zone above mountain tops. Could even see rain snow mix across above
7000 ft where temperatures however at or below freezing across the
summit. Precipitation is possible to spread northward into the lower
counties in southeast Wyoming and the Nebraska Panhandle as the
closed low scraps across the northern fringes of upper level ridge
and progresses a bit further north. With this movement, some
enhanced moisture advection into the zone is expected and PWAT
anomalies around 1.5 sigma above climatology through late Saturday
night and into Sunday morning. As a result, went ahead and increased
PoPs to around 50% from the South Laramie Range to Sidney Nebraska
with likely rain across those zones with temperatures remaining a
bit too warm.

&&

.LONG TERM /SUNDAY THROUGH FRIDAY/...
Issued at 315 AM MDT Fri Sep 20 2024

While surface high pressure over the central plains will continue to
slide eastward with the upper level wave, the associated cold air
mass will still be lingering early Monday morning. 700mb temps
around 2-4C lingering in the NE panhandle could lead to morning lows
in the mid-30s. Latest NBM MinT probabilities below 32F degrees
ranges from 10-30%. However, tight mslp gradients with the approach
of the next shortwave passage from the northwest could lead to gusty
overnight winds keeping the low-levels mixed with warmer
temperatures. Gusty winds will persist throughout the day, but local
in-house guidance is not too excited for any high winds with this
system. Scattered showers will be possible with the frontal passage,
especially around the Sierra Madre/Snowy Ranges through Monday night.

Starting with the middle of next week, 500mb height rises across
much of the Intermountain-West are likely as temperatures gradually
warm up slightly above average for late September. However, forecast
uncertainty begins to grow with interactions of a cutoff low over
the southern CA coast as well as the eventually upper level trough
approaching the PacNW. Latest cluster analysis shows the majority of
GEFS members on the warmer and drier side while ~1/3 of EC
membership brings the closed low northwest into the Great Basin with
cooler temps and chances of precipitation to the CO Rockies into
southern WY on Wednesday. Will need to continue to monitor the
origination of this upper low and model trends over the next few
days for potential changes to the latest forecast.

&&

.AVIATION /00Z TAFS THROUGH 00Z SATURDAY/...
-- Changed Discussion --
Issued at 505 PM MDT Fri Sep 20 2024

VFR conditions are expected to continue for the 00Z TAF period.
Clear skies and mostly calm winds are expected overnight.
Attention turns to an approaching cold front from the northwest
that will pass across the region between 08 and 11Z. Winds will
turn northerly and become very gusty with the passage of this
cold front. Several terminals will gust between 25 and 30kts,
with locally higher gusts possible. Winds are expected to
remain gusty throughout the day after the cold front passes
early this morning.
-- End Changed Discussion --

&&

.CYS WATCHES/WARNINGS/ADVISORIES...
WY...Red Flag Warning until 8 PM MDT this evening for WYZ417>422-
     427.
NE...None.

&&

$$

SHORT TERM...MRD
LONG TERM...MB
AVIATION...AM