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882
FXUS65 KCYS 291745
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
1145 AM MDT Mon Apr 29 2024

.KEY MESSAGES...

- High Wind Watches and Warnings have been issued for strong
  winds early Tuesday morning through Tuesday afternoon across
  the wind prone corridors of southeast Wyoming and areas
  immediately east of the Laramie Range.

- A Fire Weather Watch has been issued for portions of the North
  Platte River Valley from a line of Torrington, WY to
  Bridgeport, NE on Tuesday afternoon due to gusty winds and low
  relative humidity values.

- Active extended forecast period with multiple rounds of rain
  and snow showers with occasional thunder possible daily
  through the weekend.

&&

.SHORT TERM /THROUGH TUESDAY NIGHT/...
Issued at 330 AM MDT Mon Apr 29 2024

Overnight GOES-16 satellite imagery for the water vapor channel
depicts an occluded upper level low over the Northern Plains,
and a potent upper level disturbance also over the Pacific
Northwest. Between these two weather features, we have a
stationary frontal boundary analyzed as of 9Z nearly parallel to
the WY and SD/NE state lines. Additionally, frontal analysis
depicts the next cold front draped from north-central MT to the
southwest across northern CA. A few areas across the northern
tier of the NE Panhandle confined to the Pine Ridge, and
extending further west along the HWY 20 corridor into Niobrara
County are observing low visibility from patchy, dense fog as of
9Z this morning. This should clear up quickly after sunrise.

We will be in the warm sector today as a weak upper level ridge
amplifies across the Intermountain West. WAA will ensue, and we
can expect a nice recovery in temperatures. Daytime highs in the
60s and 70s are forecast today, with mostly sunny skies from
this morning through the early afternoon hours. The
aforementioned cold front will trek towards our cwa throughout
today, and we will see a gradual building of cloud cover from
west to east by the late afternoon hours. The chance of rain
showers with an isolated rumble of thunder should hold off until
the evening and overnight hours for areas west of the Laramie
Range. A few isolated showers may make it over the higher
terrain and spill out into the high plains late this evening,
but weather impacts remain minimal.

The main focus for the short term forecast package is the short
period of strong, gusty winds for Tuesday. Upper level winds
will approach 40-50 knots at 700mb between 6z and 12z Tuesday.
This will cause the higher terrain to see elevated to
potentially strong wind gusts of 50-60mph shortly after sunrise
for several areas. Once the nighttime inversion erodes on
Tuesday morning, expect atmospheric mixing to take place where
subsidence is modeled to occur. Highest confidence exists for
the Central and South Laramie Range, Arlington/Elk Mountain
areas, and portions of eastern Platte County along the I-25
corridor. Blowing snow may become an issue across the Elk
Mountain/Arlington forecast area, specifically the I-80
corridor. However, increased sun angles should limit this. The
last area of focus for high wind headlines is central Laramie
County. Between 15z-21z, model guidance depicts elevated to
strong winds pushing east off the foothills for a short time.
Went with a High Wind Watch for the I-25 corridor to reflect
this.

Additionally, the adiabatic warming and downslope effects from
west to northwest winds could have an impact to 1 hour grass
fuels along the North Platte River Valley on Tuesday. Despite
temperatures being slightly cooler by a couple of degrees across
the majority of the cwa on Tuesday, the wind gusts of 30-40mph
could assist with any potential fire spread quickly. Have also
gone with a Fire Weather Watch for areas along the North Platte
River Valley from approximately a line of Torrington, WY to
Bridgeport, NE. The recent rainfall should limit the potential
for fire concerns that have 10-100 hour fuels in other regions
of the NE Panhandle where RH minimums should stay slightly more
elevated near 20-25 percent. Wind gusts should slowly subside
across the cwa by sunset on Tuesday evening, along with our next
chance for light precipitation for Tuesday night and beyond.
More information on this can be read in the extended forecast
discussion.

&&

.LONG TERM /WEDNESDAY THROUGH SUNDAY/...
Issued at 330 AM MDT Mon Apr 29 2024

Primary forecast concern will be in the medium range as the next
Pacific upper level trough and associated cold front push
across the forecast area late Wednesday and Thursday. The 00z
GFS has backed off on the aggressive solution from yesterday and
is now trending towards the ECMWF with a more progressive
storm system, and overall, less precipitation across southeast
Wyoming and western Nebraska. GEFS and ENS ensemble forecasts
are also in better agreement and show much less overall spread
in temperatures, precip, and potential snowfall. Although most
of southeast Wyoming should see rain changing to snow Wednesday
night and colder temperatures, expect these conditions to be
temporary with little or no impactful snow accumulations. For
most of Wednesday, high temperatures should be seasonable and
in the 50s to mid 60s before the cold front pushes into the
region later in the day. Bumped temperatures a few degrees
warmer, mainly along the I-80 corridor which should be well

Quiet overnight expected with only a few lingering showers around.
These showers will likely not impact any terminals. VFR conditions
are expected at all terminals. However, there is a 30 percent chance
of MVFR CIGs at KCDR overnight, but confidence in this occurring is
low at this time. Breezy conditions are likely throughout the day
Monday at southeast Wyoming terminals. Increasing clouds and showers
will also be possible at these terminals during the evening hours
due to an incoming disturbance.
ahead of the leading edge of the cold front.

For Wednesday night and Thursday: kept POP between 50 to 70 percent
with lower values as you head north of I-80 Wednesday night. Trended
POP lower for Thursday due to the more progressive nature of the
system with the steady precipitation likely ending before sunrise
Thursday. Expect additional rain/snow shower activity during the day
Thursday as the trough axis moves through eastern Wyoming and into
western Nebraska and the Dakotas, but additional precip amounts
should be minimal as northwest flow aloft develops. Models are no
longer trending colder with this quick hitting system, so kept highs
in the 40s to mid 50s across southeast Wyoming...and in the low to
mid 50s across western Nebraska. Will continue to monitor this
system early this week, but it doesn`t look nearly as impactful
compared to 1 to 3 days ago.

For later this week and into next weekend, models show a return
to quasi-zonal flow across the northern and central Rocky
Mountain Region and adjacent Front Range as we head into the
weekend. Although we are getting to the time of the year where
showers and thunderstorms are possible each afternoon in/near
the mountains, a pleasant weekend is expected overall with highs
returning to the mid 60s to mid 70s by Sunday. Kept a mention
of showers and thunderstorms, mainly in the mountains, with POP
around 10 to 15 percent in the lower elevations due to low
confidence on day 7.

&&

.AVIATION /18Z TAFS THROUGH 18Z TUESDAY/...
-- Changed Discussion --
Issued at 1139 AM MDT Mon Apr 29 2024

VFR conditions currently across area terminals with latest GOES
visible imagery showing SCT cumulus developing across the area
late this morning. Expecting gusty west winds through much of the
afternoon. A cold front will pass through the region tonight
into early Tuesday morning with chances for precipitation,
likely rain, for area terminals with potential for reduced
visibility and lower ceilings. Latest HREF guidance suggests
KLAR and KRWL are the most likely terminals to see conditions
drop to MVFR levels starting around 06z tonight. As the front
continues to move west to east early in the morning, strong west
to northwest winds are expected to develop and continue through
much of Tuesday.
-- End Changed Discussion --

&&

.CYS WATCHES/WARNINGS/ADVISORIES...
-- Changed Discussion --
WY...Fire Weather Watch from Tuesday afternoon through Tuesday
     evening for WYZ433.
     High Wind Warning from 6 AM to 6 PM MDT Tuesday for WYZ106-107-
     116-117.
     High Wind Warning from 3 AM to 6 PM MDT Tuesday for WYZ110.
     High Wind Watch from Tuesday morning through Tuesday afternoon
     for WYZ118.
NE...Fire Weather Watch from Tuesday afternoon through Tuesday
     evening for NEZ435-436.
-- End Changed Discussion --

&&

$$

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