-- Discussion containing changed information from previous version are highlighted. --
166
FXUS65 KCYS 200424
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
1024 PM MDT Thu Sep 19 2024

.KEY MESSAGES...

- Gusty winds and low humidity will lead to fire weather
  concerns tomorrow. Red Flag Warnings are in effect across
  portions of southeast Wyoming.

- Dry conditions continue through the remainder of the week
  before precipitation chances return over the weekend.

&&

.SHORT TERM /THROUGH FRIDAY NIGHT/...
Issued at 1238 PM MDT Thu Sep 19 2024

Latest surface analysis depicts high pressure building from the
Western Seaboard through to the Intermountain West, with the cold
front that swung through our area days prior continues to dig
towards the Great Lakes. Observations throughout the morning
have seen milder conditions with clear overhead skies,
temperatures in the low to mid 60s, and some gusty winds across
our wind prone areas.

Short term forecast remains on track, with the upper level closed
low continuing its track eastward, and a new storm system developing
across the Pacific, which will likely be coming onshore across
southern California beginning this afternoon. Tightening gradients
is expected once this system begins traversing across the
Intermountain west, somewhat amplifying the blocking pattern to the
south, extending it slightly northward and into the central CONUS.
Between the increased gradients and shortwave disturbances
propagating within the flow, began trending up wind speeds for the
northern and western zones in southeast Wyoming. In addition,
expecting much drier conditions to continue tomorrow associated with
the upper level ridging. As a result, will likely see a return to
fire weather tomorrow, with RH minimums at or below critical
thresholds and an increase to wind speeds through tomorrow
afternoon. With the latest model trends, increased confidence exists
for Red Flag Warnings for Converse, Niobrara, North Platte, North
Goshen, and Northeast Carbon County. Red Flag Warnings are in effect
for those counties from 11AM to 8PM MDT tomorrow.

&&

.LONG TERM /SATURDAY THROUGH THURSDAY/...
Issued at 214 AM MDT Thu Sep 19 2024

Little changes for the forecast package during the medium range to
extended outlook. Model guidance depicts multiple impulses of cooler
Canadian air advecting toward our cwa from this weekend into early
next week before we gradually begin to see an improvement in daytime
highs. Deterministic model guidance remains for appreciable rainfall
on Saturday and Sunday for the I-80 corridor east of the Laramie
Range to help some areas with drought concerns.

It will feel like fall this weekend as deterministic and ensemble
model data remain in modest agreement for an impactful weather
system to arrive. Lee cyclogenesis is anticipated as a mid-latitude
cyclone/Colorado Low scenario evolves. A longwave trough is
anticipated to have a slower trajectory over the Intermountain West
into the High Plains, as a split-flow weather regime at H5 aloft
occurs. The first measurable snowfall of the season is still modeled
for elevations above 10,000 feet in the Snowy Range Mountains on
Sunday. Temperatures will be chilly, especially on the eastern
foothills and lower elevations of southeast WY. An upslope regime is
anticipated to set up, translating to daytime highs in the lower 50s
to low 60s from cloud cover and rain showers. Looked up the
climatology for the day of 22 September for Cheyenne, and the record
low-high maximum for that day is 40 degrees. It`s doubtful we will
stay that cool, but it will most certainly be a day to have an extra
layer on while outdoors. Depending on that surface low track,
several drought-stricken areas could see beneficial rainfall between
0.5 and 1.0+ inches. Highest confidence for rainfall to persist is
across the I-80 corridor east of the Laramie Range and east of the I-
25 corridor.

The high plains will continue to see below normal temperatures early
next week. The cut-off Upper level low (ULL) will slowly become
absorbed into the mean steering flow again by Monday, with another
weak cool front on its way towards our cwa. The cool air will spill
down the spine of the Colorado Rockies via northwest flow aloft by
Tuesday, keeping the region below normal for daytime highs. By the
middle of next week however, upper level ridging will begin to
amplify over the western CONUS. The upper level ridge will begin its
trek towards the High Plains, and bring another opportunity for
slightly warmer temperatures and dry conditions.

&&

.AVIATION /00Z TAFS THROUGH 00Z SATURDAY/...
-- Changed Discussion --
Issued at 1022 PM MDT Thu Sep 19 2024

VFR conditions for the 06Z TAF period with clear skies and
mostly calm winds overnight. Primary aviation concern for
tomorrow will be increasing winds at KLAR, KRWL, and KAIA. Winds
may gust up to 25kts. Aside from gusty winds, expecting clear
skies throughout the day.
-- End Changed Discussion --

&&

.CYS WATCHES/WARNINGS/ADVISORIES...
WY...Red Flag Warning from 11 AM to 8 PM MDT Friday for WYZ417-418-
     421-422.
NE...None.

&&

$$

SHORT TERM...MRD
LONG TERM...BW
AVIATION...AM