509
FXUS65 KCYS 161146
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
550 AM MDT Mon Sep 16 2024

.KEY MESSAGES...

- Isolated to scattered showers and thunderstorms are possible
  this afternoon and evening, mainly along and west of I-25.

- A powerful storm system will move through the area Tuesday,
  bringing widespread showers and thunderstorms with the
  potential for strong to damaging winds.

- Strong winds may continue in the wind prone areas through
  Wednesday morning. A High Wind Watch has been issued.

&&

.SHORT TERM /THROUGH TUESDAY NIGHT/...
Issued at 420 AM MDT Mon Sep 16 2024

After an active day of thunderstorms yesterday, fairly tranquil
conditions have taken over this morning as last evening`s shortwave
pushes northward across the Dakotas. The break will be short lived
though, as active weather will continue through the next few
days. The next player is visible on GOES water vapor channels as
an area of higher mid to upper level moisture content over
Colorado and New Mexico drifting northward this morning.
Meanwhile, the next major weather maker is a powerful upper
level low pressure system currently centered over northern
California. As this digs southward into the western US, the
shortwaves ejecting out ahead of it will help to amplify the
downstream ridge today. Today`s wave will bring with it more
abundant moisture pulled out of the lingering monsoonal
moisture. Models show decent isentropic lift developing mid to
late afternoon, mainly over the western part of the forecast
area and continuing through the evening hours. This should
support the development of scattered showers and a few
thunderstorms thanks to some marginal convective instability
present. A few of these may be capable of producing strong
winds. While most of the lift supporting this activity will dry
up around midnight, a potent moisture plume will continue
streaming into southwest Carbon county, likely keeping showers
around through Tuesday morning.

Tuesday will be a very dynamic day across the area. The closed low
over California today will start to race to the northeast on
Tuesday, reaching northwest Wyoming by the afternoon or evening
hours. Strong southerly flow ahead of the storms system will
continue to pull in deep moisture. 700-mb meridional wind
component and integrated water vapor transport will both be
approaching climatological maximums Tuesday afternoon according
to the NAEFS mean. As the upper level low approaches, it will
start to run into the stubborn Rex Block to the east and gain a
negative tilt. Meanwhile, warm temperatures and decent low level
moisture will still be present over our area Tuesday morning
and early afternoon ahead of the storms system. All of these
pieces will come together for what is increasingly looking like
a very active afternoon and evening. The cold front is expected
to begin pushing into Carbon county around noon, leading to the
development of an impressive pressure boundary. A meso-high
behind this will run into lee- troughing ahead, likely
supporting some strong surface winds along the front. In
addition, extremely strong frontogenesis and modest positive
theta-e advection will support widespread showers along the
frontal boundary. Instability near the frontal boundary will
also support thunderstorms embedded along. Current best
estimates on timing are for this to move through Rawlins around
noon, Laramie around 2-3PM, Cheyenne, Wheatland, and Douglas
around 3-4PM, Scottsbluff, Torrington, and Lusk around 4-5PM,
and Chadron, Alliance, and Sidney around 5-6PM, but this is
certainly subject to change. Due to the impressive dynamics
associated with this system, it is possible we could see strong
to severe winds along nearly the entirety of a line of
thunderstorms that stretches across the entire forecast area.
Scattered additional thunderstorms may also linger behind the
frontal boundary. Models are currently showing the mid- level
frontal boundary (i.e. the end of the moisture) running about 3
hours behind the surface pressure trough that is likely to be
roughly the front edge of thunderstorm activity. In addition to
the rainfall and wind, expect a very rapid temperature drop of
20-30F within perhaps just a few minutes. While for most areas
this will mean a drop to the upper 40s to low 60s behind the
front, those in the high mountains should be prepared for a
rapid drop into the 30s along with rain quickly changing to
snow. While snow accumulation will will not be significant,
those expecting a summer day in the mountains may be taken off
guard by the front in the afternoon hours.

Dry air advection will occur very quickly behind the secondary
front, clearing out skies quickly late Tuesday evening. However, the
upper level low will then eject out across northeast Wyoming,
bringing strong westerly flow overhead as the main trough axis
clears through the area. Strong surface pressure and low level
height gradients along with WSW winds at 700-mb of 50 to 60 knots
will support a second round of high winds in the typical gap wind
areas. With the slightly southerly component still remaining to the
flow, the areas most likely in play will be Arlington/Elk Mountain
and the I-25 corridor between Bordeaux and Casper. The period of
strongest winds look to between midnight and 9AM Wednesday. For now,
decided to issue a High Wind Watch just for the Arlington/Elk
Mountain area starting Tuesday at noon to capture the initial
frontal boundary and the strong post-frontal winds. This area is the
least likely to see a break, which motivated the single headline.
Elsewhere, confidence is lower in winds remaining strong between the
initial thunderstorm-associated strong winds and the second
overnight round. The first round could be handled with short-fused
Severe Thunderstorm Warnings, with a High Wind Watch starting much
later in the evening to handle the second round overnight. Will let
a few more model cycles come in before making that decision
though.

&&

.LONG TERM /WEDNESDAY THROUGH SUNDAY/...
Issued at 420 AM MDT Mon Sep 16 2024

Long range models are in reasonable agreement through Friday
night, but are now showing significant discrepancies for the
weekend regarding the potential for prolonged rainy and cool
weather for the weekend.

For the middle of this week, models in good agreement with the
timing of a pretty strong cold front pushing across the area by
early Wednesday morning. High temperatures on Wednesday will
likely be 10 to 15 degrees cooler compared to Tuesday, but still
should be around average for this time of the year. It will be
pretty windy behind the cold front with 700mb winds over 55
knots for central and east central Wyoming, with some evidence
of mountain wave activity and stronger winds on the lee-side of
the Snowy Range and Laramie Range. Increased winds a bit more
with in- house wind models showing the wind prone areas flirting
with high wind criteria, including areas near the I-25
corridor.

Temperatures will rebound slightly on Thursday as the next
Pacific trough moves across the southern Great Basin region and
moderate to strong southwesterly flow aloft develops along the
Front Range. Models are in better agreement for Thursday and
Thursday night as the GFS has backed off on its more
progressive solution...and now is more in line with the Canadian
and ECMWF solutions with the Pacific upper level trough slower
and much further west compared to yesterday`s 00z run. Most of
Friday is looking drier and warmer in comparison to what some
models were showing a few days ago, mainly due to the Pacific
trough taking its time moving eastward and still over the far
eastern Great Basin region, so increased high temperatures into
the 70s for most locations with POP generally below 30 percent
through the day.

For Friday night and the weekend, this is when models start to
diverge considerably with the 00z ECMWF and 00z GFS showing
completely different solutions compared to yesterday`s 00z
cycle. This is to be expected with blocking patterns as models
struggle to resolve them in the medium to extended forecast
range. Regardless, it looks like unsettled and cool weather for
next weekend. The potent upper level trough is forecast to
cross into the central Front Range and slow down...potentially
forming a closed low around the Colorado/Wyoming/Nebraska border
on Saturday. The ECMWF is now showing a prolonged moderate to
heavy rain event with embedded thunderstorms Saturday afternoon
through Sunday evening...mainly because of low to midlevel
convergence associated with strong frontogenesis and
cyclogenesis across the region. The GFS on the other hand is now
more progressive at the onset of the event, mainly because it
delays the Rex Block formation across the eastern U.S. until
later in the weekend. Therefore, not as cool and somewhat drier
weather for southeast Wyoming. The Canadian maintains its
aggressive solution, similar to the ECMWF, but is 18 to 24 hours
earlier...showing moderate to heavy rainfall developing as
early as Friday evening. Still expect a gradual cool-down this
weekend with highs in the 60s and nighttime lows in the 30s and
40s. There is still lower than average confidence in POP and
timing of rainfall and mountain snowfall. All models have
trended warmer in the 700mb level, so heavy mountain snow
accumulations look less likely at this time. Kept POP between 30
to 50 percent, but increased the length of time through
Saturday night. Unsettled weather could linger well into Sunday,
as the 00z ECMWF is suggesting.

&&

.AVIATION /12Z TAFS THROUGH 12Z TUESDAY/...
Issued at 550 AM MDT Mon Sep 16 2024

Southwest flow aloft will continue today ahead of the next Pacific
low pressure system pushing across California. Relatively quiet
weather expected with low impacts to Aviation.

HAZARDS/WEATHER TRENDS: VFR conditions will prevail with occasional
south to southwesterly wind gusts between 20 to 25 knots. Widely
scattered rain showers and some embedded thunder possible near
KLAR and KRWL this afternoon.

&&

.CYS WATCHES/WARNINGS/ADVISORIES...
WY...High Wind Watch from Tuesday afternoon through Wednesday
     morning for WYZ110.
NE...None.

&&

$$

SHORT TERM...MN
LONG TERM...TJT
AVIATION...TJT