-- Discussion containing changed information from previous version are highlighted. --
739
FXUS65 KCYS 201738
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
1138 AM 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 315 AM MDT Fri Sep 20 2024

We will be on the periphery of an amplifying upper level ridge
across the Central Plains today. WAA will ensue, and we will see
above average temperatures across the high plains and mountain
zones. An approaching cold front will steepen the surface
pressure gradients, and elevated to gusty winds will become
prevalent by this afternoon. The dry air mass, downslope winds,
and cured fuels will thread the needle to critical for fire
weather concerns this afternoon. Have added a couple of fire
weather zones to the inherited Red Flag Warning, so that all of
our fire weather zones on the border of the cwa for UNR/RIW have
fire weather headlines. Daytime highs in the low 80s to near 90
degrees are expected for areas east of the Laramie Range, with
the warmest temperatures expected in the North Platte River
Valley and northern Sioux/Dawes County this afternoon.
Temperatures in the 60s and 70s are expected for areas west of
the Laramie Range, outside of the highest elevations of the
Snowy/Sierra Madre Mtn Ranges.

A quickly advancing cold front will bring cooler conditions the
area on Saturday. The cold front should have pushed south into
CO and the Central Plains by daybreak for most locations. We
will see some elevated winds as this occurs, but very little in
terms of chances of precipitation. Expect it to be a dry cold
front overall. The daytime highs will be 10-20 degrees cooler on
Saturday, with most areas seeing the 60s in the lower
elevations. It will be a pleasant day for those that plan on
being outdoors to catch up on some yardwork. A secondary surge
of cooler air will arrive Saturday night as a longwave trough
and upper level low create surface lee cyclogenesis along the
Continental Divide/Front Range of CO. High resolution model
guidance continues to nudge the main swath of precipitation
slightly further to the south, mainly along the CO/WY/NE border.
This is about 30-50 miles further south than the previous 24-48
hours of model data analysis. Highest confidence of a wetting
rainfall resides now between the State border and I-80 corridor
from Cheyenne to Sidney. Other areas will see rain showers, but
accumulation looks minimal. As previously mentioned, elevations
above 10,000 feet in the Snowy Range and Sierra Madre Range
could see light snowfall accumulations. Will continue to monitor
this, but winter headlines are not anticipated. Daytime
temperatures will start off in the upper 30s to low 40s in the
lower elevations on Sunday morning, and will really struggle to
climb until there is afternoon sunshine. The areas expected to
see the warmest daytime highs are in our northern forecast zones
where cloud cover will erode more quickly as the departing
weather system propagated towards the Central Plains by Sunday
afternoon. Expect the coolest temperatures of the low to mid 50s
hugging the state border east of the Laramie Range toward
Sidney along the I-80 corridor, with areas further north and
west seeing the highest potential for daytime maximums in the
low to middle 60s.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.

&&

.LONG TERM /SUNDAY THROUGH THURSDAY/...
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 /18Z TAFS THROUGH 18Z SATURDAY/...
-- Changed Discussion --
Issued at 1135 AM MDT Fri Sep 20 2024

VFR conditions expected for the forecast period. Some gusty west
to southwest winds are possible this afternoon, mainly in
Wyoming but also briefly in the Nebraska panhandle. A strong
cold front will sweep through the area late tonight, bringing a
wind shift and gusty northerly winds for all High Plains
terminals, and gusty northeast winds at KRWL towards Saturday
morning.
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&&

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

&&

$$

SHORT TERM...BW
LONG TERM...MB
AVIATION...MN