-- Discussion containing changed information from previous version are highlighted. --
775
FXUS65 KCYS 212326
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
526 PM MDT Sat Sep 21 2024

.KEY MESSAGES...

- Near freezing temperatures are possible in some low-lying and
  wind sheltered areas tonight and again Sunday night. A Frost
  Advisory is effect for Converse and Niobrara counties
  tonight.

- Above average temperatures and dry conditions expected for the
  middle of the work week, with increased chances for critical
  to near-critical fire weather conditions Wednesday onward.

&&

.SHORT TERM /THROUGH MONDAY NIGHT/...
Issued at 300 PM MDT Sat Sep 21 2024

It certainly feels more like fall across the area just in time for
the autumnal equinox which occurs at 6:43AM Sunday. While mostly
clear skies have been in place through the day today, a large cloud
shield is visible on satellite imagery approaching from the
southwest as a potent closed low spins across the Four Corners
region. Meanwhile, the surface high pressure behind last night`s
cold front is currently in eastern Montana on its way into the
Dakotas. As this shifts east, low level winds will continue to shift
from northerly to northeasterly, and then easterly. As pressure
builds to the east and falls over western Colorado, we will see a
few hours of a tight pressure gradient over Carbon county, producing
some gusty northeasterly winds through the evening. This won`t last
long though as the surface high drifts southward and settles in over
northeast Colorado by morning. Easterly flow overnight will bring in
slightly better low-level moisture, but recent guidance has
continued to trend south with the precipitation shield. So much so,
that it is unlikely any part of the area aside from perhaps the
higher peaks will see any appreciable precipitation. Still, expect
mid to high level cloud cover to pull in across most of the area
while low clouds will make it to the I-80 corridor. Some areas of
fog will be possible where this cloud deck intersects the terrain,
mainly around the I-80 summit. While we have a fairly chilly airmass
in place today, the increasing cloud cover will prevent a widespread
freeze. Still, some valleys especially in the northern part of the
forecast area where cloud cover is more uncertain will see light
winds with the cool airmass in place. As a result, some patchy frost
may show up in the low-lying and wind-sheltered areas around
Converse and Niobrara counties, plus the Shirley Basin. While this
shouldn`t be too widespread, it does look like enough to warrant a
Frost Advisory through Sunday morning. Sensitive vegetation could
become damaged or killed, but this doesn`t look like a widespread
freeze event just yet.

Cloud cover will clear progressively through the day Sunday as the
storm system quickly ejects off to the east. Temperatures will still
remain on the cool side for this time of year with highs in the 60s
to low 70s for most. Behind this system, transient ridging will
start to build in ahead of another shortwave diving down from the
north set to arrive late Monday. The rapid warm air advection aloft
will make it a little bit more difficult to get a freeze Monday
morning, but clear skies behind a cold snap this time of year can
often get close. A pressure trough working in just east of the
Laramie range will also keep a light breeze going for most areas,
which will again help prevent a widespread freeze. However, low-
lying and wind-sheltered areas may once again see some frost Monday
morning. The most likely areas then will probably be the low valleys
and creek beds of Laramie, Kimball, and Cheyenne counties. After
that, expect temperatures to return to above normal values before
cloud cover increases late in the day Monday ahead of the next cold
front.

&&

.LONG TERM /TUESDAY THROUGH SATURDAY/...
Issued at 300 PM MDT Sat Sep 21 2024

No major changes to the long term forecast, with a broad upper level
ridge building across the Intermountain West and further into the
High Plains by the middle of the week. A decent warmup is expected
during this time period, with 700mb temperatures climbing to around
from +4c upwards to +16c through Thursday. As a result, will begin
to see daytime highs from the high-60s to low-70s on Tuesday and up
to the low to high-80s on Thursday, and Thursday being the warmest
day of the week. Decided to keep temperatures close to the NBM, with
any model guidance still a bit too far out to make any significant
changes. However, over the next couple of days will need to start
considering blending in higher NBM percentiles to the current
blends, with the NAEFS mean 700mb temperatures in the climatological
90th percentile for the majority of our areas in southeast Wyoming
and the Nebraska Panhandle on Wednesday and into the 99th percentile
on Thursday.

All in all, relatively dry forecast associated with the upper level
ridge remaining firmly in place throughout the week, which will
inevitably raise fire weather concerns as the minimum RH values drop
to the mid to low-teens for much of southeast Wyoming and the
Nebraska Panhandle. Winds do not appear to reach fire weather
critical thresholds during the times of critical level humidity
values through Wednesday. However, will need to monitor the
development of an upper level closed low that will retrograde
towards eastern Colorado by late week, which could induce some
stronger gradients for the CWA and some possible shortwave
disturbances to pulse across the zone. Will also need to monitor the
progression of the upper level closed low, with the positioning
given the region continued dry and windy weather or possible
precipitation with some wrap-around moisture components.

&&

.AVIATION /00Z TAFS THROUGH 00Z MONDAY/...
-- Changed Discussion --
Issued at 521 PM MDT Sat Sep 21 2024

A trough aloft over Colorado will move slowly eastward into the
western High Plains states on Sunday.

Scattered to broken clouds from 9000 to 15000 feet will prevail
until mid morning Sunday, with ceilings near 2500 feet at
Cheyenne from 10Z to 16Z, then skies will be clear after mid
morning Sunday. Winds will gust to 29 knots at all terminals
except Scottsbluff until 03Z, then gust to 23 knots at Cheyenne
after 16Z Sunday.
-- End Changed Discussion --

&&

.CYS WATCHES/WARNINGS/ADVISORIES...
WY...Frost Advisory from 1 AM to 8 AM MDT Sunday for WYZ101-102.
NE...None.

&&

$$

SHORT TERM...MN
LONG TERM...MRD
AVIATION...RUBIN