-- Discussion containing changed information from previous version are highlighted. --
498
FXUS65 KCYS 172329
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
529 PM MDT Tue Sep 17 2024

.KEY MESSAGES...

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

- Strong winds may continue in the wind prone areas through
  Wednesday morning. A High Wind Watch is in effect.

&&

.MESOSCALE...
Issued at 121 PM MDT Tue Sep 17 2024

Latest GOES imagery shows storms developing across western WY/CO
associated with the large scale upper level low centered along the
UT/ID border. As this system continues to take on a negative tilt,
strong mid/upper level flow continues to round the base of the
trough with support for large scale forcing extending into our area.
Instability is rather limited west of the Laramie Range with more
gradual vertical development, however deep unidirectional flow has
led to showers/tstorms with a few wind gust over 50 mph being
reported. Early morning storms across Sweetwater Co have appeared to
be undercut and become outflow dominant as it propagates eastward
with a northerly storm track. Looking farther south, two areas of
convection exist in western CO including 1) scattered showers and
storms aided by diurnal heating over the higher terrain of the
Mt Zirkel Wilderness moving north/northeast, and 2) another line
recently developing closer to the analyzed surface cold front
that will continue to move east/northeast into southwest Carbon
Co over the next hour.

Overall, still looks like storms will continue to develop early this
afternoon with cold pool organization aided by the cold frontal
passage. Storms will look to intensify rapidly from a potentially
broken line of storms crossing the Laramie Range to a possible MCS
by the time precipitation reaches the NE panhandle early this
evening. Strong winds (70+ mph) remain to be the main hazard today
with these storms, especially in western NE where the HREF
probabilities of wind gusts over 55 mph are greatest.

One feature that will continue to be an evolving situation
as storms develop and cold pool organization commences is the
location of the strongest wind with this complex. So far, the slower
eastward progression of the upper level low has led to a slightly
more broad lee trough than previously forecast. This has limited
southerly moisture advection into the NE panhandle into late this
afternoon where surface dew points remain in the mid-40s while
northeast CO sites are reporting pockets of 50+ degree dew points.
If this persists into later this afternoon, instability may be more
limited across the NE panhandle keeping the focus for stronger
storms, cold pool, and associated wind gusts along and south of the
I-80 corridor. Will continue to monitor future observational trends
into this afternoon.

&&

.SHORT TERM /THROUGH WEDNESDAY NIGHT/...
Issued at 335 AM MDT Tue Sep 17 2024

Active weather expected over the next 24 to 30 hours with
multiple impacts and concerns. Current IR Satellite loop shows
the potent Pacific storm system lifting northeast across the
Great Basin region and into Idaho early this morning. This storm
system, and the potent midlevel shortwave extending southward
into four corners region, are expected to continue moving east
across Wyoming and Colorado today, and eventually eject east
into the Great Plains tonight. All models show a sharp
negatively tilted mid to upper level trough lifting northeast
across southeast Wyoming and western Nebraska this afternoon.
Synoptic models show strong forcing across the area today with
intense low level frontogenesis and upper level level diffluence
associated with the jet stream over a large area (along and
east of the Laramie Range, including western Nebraska). Not only
is this storm dynamic, but we`re still dealing with favorable
thermodynamic parameters for a substantial convective threat for
mid-late September. Followed the NAM and the HRRR for
convection potential today since boundary layer parameters are
initializing much better compared to the GFS, especially surface
moisture and MLCAPE. Expect thunderstorms to begin developing
across Carbon county over the next few hours as the area of
strong upper level diffluence begins to move into the forecast
area. This cluster of thunderstorms is forecast to eventually
form a squall line as the Pacific storm and associated
negatively tilted short wave trough axis rapidly moves northeast
into the Front Range. This set up is reminiscent of the June
6th 2020 Derecho event where much of the area received wind
gusts between 60 to 80 MPH across southeast Wyoming and western
Nebraska. SPC added far eastern Wyoming and most of the western
Nebraska panhandle in an Enhanced Risk for severe thunderstorms
and highlighted wind gusts in access of 70+ MPH for the region.
All CAMS and high resolution guidance show a solid line of
severe thunderstorms, with additional bowing segments,
developing across Albany county with rapid eastward propagation
into western Nebraska through the afternoon. Brief very heavy
rainfall, hail, frequent lightning, and damaging winds over 70
MPH are possible with this line between noon and 800 PM this
evening. This line should impact the I-25 corridor between the
hours of 300 to 500 PM, and into western Nebraska between 500 PM
and 700 PM. Once the front moves across the area, temperatures
will quickly drop into the 50s and 60s during the afternoon and
early evening hours.

Otherwise, tricky wind forecast (specifically: non-convective
winds) for today with some models showing a lack of a surface
pressure gradient, while others show a strong west to east
surface pressure gradient that persists a few hours even behind
the main line of thunderstorms. This gradient does relax
tonight, but then strengthens again behind the storm system with
cold air advection near the surface through early Wednesday
morning. Kept the High Wind Watch going for the secondary wind
prone areas of southeast Wyoming since we still have at least 24
to 30 hours of lead time for these zones. Upgraded the Watch to
a High Wind Warning for the Arlington and Elk Mountain zone for
several favorable scenarios for strong gusty winds: (1) Late
morning/early afternoon as the line of strong thunderstorms
starts to organize over the area (2) convective showers
lingering behind the main line of thunderstorms late in the
afternoon through the late evening hours. This set up has a
history of producing brief gusts as high as 70 MPH in this area
over the last several months (3) increasing low level pressure
gradients and low-midlevel subsidence late tonight through
Wednesday morning. There is also a good chance for portions of
I-25 from Casper to Bordeaux/Wheatland to see strong wind gusts
as well with some indication of a mountain wave developing on
the lee- side of the Laramie Range late tonight.

Strong winds are expected to gradually weaken by late Wednesday
morning and Wednesday afternoon. It will be cooler on Wednesday
with highs in the 60s to low 70s west of I-25, and in the 70s to
low 80s east of I-25...warmest temperatures will be found in the
lower elevations of western Nebraska.

&&

.LONG TERM /THURSDAY THROUGH TUESDAY/...
Issued at 230 AM MDT Tue Sep 17 2024

A progressive weather pattern is being advertised for the extended
forecast. Our region will be under the influence of weak upper level
ridging for Thursday and Friday, causing the area to be
approximately 5 degrees above average for the daytime high, but that
will change by this weekend as temperatures transition to
potentially cooler than 10 degrees below normal for the week of the
autumnal equinox. We will have multiple shortwave disturbances
propagating into our region, bringing a much cooler weather regime
to the area. Areas in the high plains, especially the mountain
zones, will see well below normal temperatures making it feel like
fall outdoors. Towards the beginning of next week, we will continue
to see additional opportunities for precipitation and another
potential round of cooler temperatures. Overall, expect a below
average temperature pattern and higher chances for much needed
precipitation. Daytime highs in the higher terrain portions of the
mountain zones from this weekend into early next week will struggle
to reach the 40s, with lower elevations seeing the upper 50s to low
70s. Keep the umbrella handy due to higher chances of rain
showers/thunderstorms, and an extra layer for the cooler
temperatures that are favored in the extended forecast.

&&

.AVIATION /00Z TAFS THROUGH 00Z THURSDAY/...
-- Changed Discussion --
Issued at 521 PM MDT Tue Sep 17 2024

A cold front will sweep west to east across the terminals
tonight, with scattered showers and thunderstorms ending from
west to east this evening.

Wyoming TAFS...Scattered to broken clouds from 5000 to
10000 feet will prevail through this evening, then skies will
become clear. Occasional thunderstorms will occur at Rawlins and
Laramie until 01Z, and at Cheyenne until 02Z, with wind gusts
up to 50 knots and visibilities as low as 4 miles. Outside of
thunderstorms, winds will gust to 45 knots.

Nebraska TAFS...Scattered to broken clouds from 4000 to
10000 feet will prevail through this evening, then skies will
become mostly clear. Occasional thunderstorms will occur until
04Z, producing wind gusts to 50 knots and visibilities to
3 miles. Outside of thunderstorms, winds will gust to 40 knots.
-- End Changed Discussion --


&&

.CYS WATCHES/WARNINGS/ADVISORIES...
-- Changed Discussion --
WY...High Wind Warning from midnight tonight to 3 PM MDT Wednesday
     for WYZ101-103>105.
     High Wind Watch from midnight MDT tonight through Wednesday
     morning for WYZ107.
     High Wind Warning until noon MDT Wednesday for WYZ110.
NE...None.
-- End Changed Discussion --

&&

$$

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