645
FXUS65 KCYS 181727
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
1127 AM MDT Wed Sep 18 2024

.KEY MESSAGES...

- Strong gusty winds continue in the wind prone areas through
  midday Wednesday. A High Wind Warning is in effect.

&&

.SHORT TERM /THROUGH THURSDAY NIGHT/...
Issued at 350 AM MDT Wed Sep 18 2024

Current IR Satellite loop shows the strong storm system from
yesterday pushing northeast and stalling across Montana early
this morning with a textbook comma cloud pattern noted on
Satellite. This storm is producing heavy rainfall across the
lower elevations with heavy mountain snowfall over 9500 feet.
For southeast Wyoming and western Nebraska, mainly a wind
forecast today with some very dry air advecting eastward on the
backside of the storm. Current High Wind Warnings look good at
this time with a few locations reaching gusts of 45 to 50 MPH.
Winds are expected to strengthen around sunrise. Will keep the
High Wind Watch going for eastern Platte county due to lower
confidence and a conditional risk for strong gusty winds
associated with mountain wave activity and low level mixing.
Otherwise, cooler temperatures expected today but it will remain
near normal for this time of the year. Observations near
Alliance Nebraska do show some patchy fog early this morning,
but expect this fog to dissipate by sunrise with increasing cold
air advection and northwest winds at the surface.

All models show a shortwave ridge axis building into the Front
Range and Rocky Mountain region Through late Thursday. Expect a
gradual warming trend on Thursday with dry conditions
continuing through early Friday morning.

&&

.LONG TERM /FRIDAY THROUGH TUESDAY/...
Issued at 355 AM MDT Wed Sep 18 2024

An active weather pattern remains well advertised for the extended
forecast due to multiple weather disturbances impacting our cwa. A
large dome of high pressure will be spread across the Mexican
Plateau into Texas. The associated upper level ridge at H5 will
amplify northward on Friday, bringing WAA and southerly flow to the
high plains. Further upstream, a pronounced upper level longwave
trough will be carving its way out of the Southern CA and Four
Corners region. This will assist with a 75-85 knot jet streak max
propagating toward the Intermountain West by Friday afternoon. Model
soundings depict a dry air mass being present for most of our
forecast zones, and a well-mixed lower boundary layer should provide
breezy winds during the afternoon hours. 700mb temperatures are
progged to have a spread of +10C to +14C by 0Z Saturday. The dry air
mass and breezy winds will create elevated fire weather concerns for
our cwa, especially east of the I-25 corridor. Daytime highs are
progged to be approximately 5-12 degrees F above average for SE WY
and western NE. Highs in the 80s for areas east of the I-25
corridor, and 70s west of that line for the lower elevations.
Slightly cooler temperatures for the mountain zones of mid 50s to
low 60s.

Our primary focus then turns to this weekend as deterministic and
ensemble model data are coming into better agreement for an
impactful weather system to arrive. The autumnal equinox will occur
this coming Sunday, and based off the current weather data, it will
certainly feel like it outdoors. At this time of inspection, lee
cyclogenesis is advertised via a mid-latitude cyclone/Colorado Low
scenario. The aforementioned longwave trough is anticipated to
become cut off from the mean steering flow at 500mb between Saturday
afternoon and late Sunday. The timing and location of the surface
low still needs some work to have a higher confidence, but have
taken the initial steps of bumping up PoPs and lowering MaxT for
Sunday. The first measurable snowfall of the season is possible for
elevations above 10,000 feet in the Snowy Range Mountains on Sunday.
For lower elevations, 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
upper 40s to low 60s from cloud cover and rain showers. Depending on
that surface low track, several drought-stricken areas could see
beneficial rainfall between 0.5 and 1.5 inches. The GFS is the most
aggressive, with the Euro and Canadian model hugging the I-80
corridor and areas south into CO with the highest potential rainfall
accumulation.

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 /18Z TAFS THROUGH 18Z THURSDAY/...
Issued at 1125 AM MDT Wed Sep 18 2024

VFR conditions expected across area terminals through the
forecast period with gusty west to southwest winds continuing
into this afternoon. Most terminals can expected gusts 25-35 kt
before winds begin to calm headed into this evening with SKC
prevailing.

&&

.CYS WATCHES/WARNINGS/ADVISORIES...
WY...High Wind Warning until 3 PM MDT this afternoon for WYZ101-
     103>105-107-110.
NE...None.

&&

$$

SHORT TERM...TJT
LONG TERM...BW
AVIATION...MB