611
FXUS65 KCYS 042330
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
530 PM MDT Sat May 4 2024

.KEY MESSAGES...

- Mainly dry and warmer conditions are anticipated for the
  weekend.

- Strong winds (60+ mph) return to southeast Wyoming and western
  Nebraska Monday with a prolonged period of elevated winds
  continuing for wind prone locations through midweek.

&&

.SHORT TERM /THROUGH SUNDAY NIGHT/...
Issued at 153 PM MDT Sat May 4 2024

Ridging continues to develop over much of Wyoming and western
Nebraska today with sunny skies dominating the forecast.
Temperatures, while chilly, will only end up about 2-4 degrees
below average in most places as heights rise and full sun allows
for maximized solar heating and mixing evidenced in forecast
soundings. To the east of the Laramie Range, a switch to south-
southeasterly surface flow will begin to user in mild air.
Overnight lows will tend to run a bit higher than guidance
averages given the breezy surface flow east of the mountains.
Overall expect mainly clear skies into Sunday morning.

By Sunday afternoon, a potent low-pressure system will barrel
through the central Rockies. Ahead of this system, fast south-
southwesterly flow in the mid and upper levels will spread
eastward through the Great Basin and intermountain region.
Forecast ensemble guidance and blended output depicts a broad
synoptically-driven high wind event affecting all of south-
central Wyoming as well as northwest Colorado. Ensemble
statistical output places surface gusts in the higher end of
climatology, favoring a possible high wind event for portions of
western Carbon County through the afternoon and evening hours.
While recent national blend output in similar events has ended
up higher than reality, have opted to issue a High Wind Watch
for the aforementioned zones on Sunday afternoon given the
strong signal in global forecast guidance.

East of the Laramie Range in the high plains, moisture return
in southeasterly flow out ahead of the advancing low pressure
system will transport upper 40s to near 50 dewpoints into far
eastern Wyoming. As height falls start inching toward the high
plains by 0z, some longer-range CAM guidance suggests the
possibility for convection to fire in the 0-2z timeframe. High-
res guidance remains quite dispersive with potential outcomes
on Sunday evening. Stronger forcing and a sharper dryline could
be enough to overcome rather strong capping evidenced by a large
warm-nose at 700mb in forecast soundings. This particular
outcome is quite evident in the past several runs of the NAM
Nest, which has consistently suggested a cluster of fast- moving
organized convection in the Carbon/Niobrara County area, with
more isolated single-cells as far south as the Platte/Goshen
County region. The HRRR, given its dry boundary-layer bias, has
naturally been at the opposite end of the spectrum indicating
only a few isolated high-based areas of shower activity in a
deeply mixed environment. Other CAM guidance lies somewhere
between these two extremes with varying degrees of meager
dryline convergence in the NSSL, ARW, and RRFS. Ultimately the
convection forecast on Sunday evening will come down to how
quickly we can overlap the height falls from the west with
moisture from the east - a slightly faster trough and quicker
moisture advection would certainly be enough to realize the more
bullish CAM solutions.

&&

.LONG TERM /MONDAY THROUGH SATURDAY/...
Issued at 153 PM MDT Sat May 4 2024

An active, windy pattern continues to take shape starting early this
week and continuing through mid-week. A potent, upper-level,
negatively tilted trough will dig into the region early Monday with
strong cyclogenesis expected across southwestern South Dakota by
Monday afternoon. As this cyclone develops, strong westerly flow is
expected across the region from 250mb down to the surface,
suggesting a widespread high wind event is possible once again for
southeast Wyoming and western Nebraska. 700mb westerly winds are
progged to peak around 75kt throughout the day Monday as the surface
low develops and rapidly strengthens as it moves northeasterly
across the Dakotas. NAEFS Mean Zonal Wind at 700mb suggests that the
700mb winds will be in the 99.5th percentile (or greater) for this
period of time, showing how unusually strong the 700mb jet is
expected to be for this time of year. Along with this strong 700mb
flow, steep lapse rates are expected to help mix the wind down to
the surface further supporting very strong gusts for many locations.
Strong downward omega fields are also evident in the GFS, with maxed
out values expected across the Laramie Range Monday afternoon. These
downward omega fields continue to foster support for a very strong
downslope wind event Monday through early Wednesday. In addition to
this, Craig to Casper 850mb height gradients have soared to the 80-
113m range throughout the day Monday and into the overnight hours.
Similarly, 700mb Craig to Casper gradients have increased to the 85-
90m range. Most of the time, the Craig to Casper height gradients
can be used as a rough estimate of peak winds gusts near Arlington.
With these unusually high values, winds could peak in the 70-80mph
range at Arlington through the Monday/Tuesday period. Finally, in-
house guidance continues to remain confident in a prolonged high
wind event with 80-90% probability for high winds across the winds
prones and into the adjacent plains. This setup will need to
continue to be monitored and a High Wind Warning will likely be
needed for many locations to start the work week. In good news,
winds are expected to decrease by early Wednesday morning, with an
additional brief lull possible Tuesday morning.

The remaining long term forecast features unsettled, cooler weather
as the upper-level low rotates over the Northern Plains before
slowly ejecting east by Thursday afternoon. Behind this low, a
secondary, flat trough is expected to impact the region, once again
favoring daily precipitation chances across much of southeast
Wyoming and western Nebraska. Long range models begin to differ
significantly after Thursday, with the GFS suggesting another cut-
off low rotating just west of the CWA, while the ECMWF moves the
whole system off the east before another system approaches the West
Coast. Both models still favor precipitation chances and cooler,
wetter weather, so kept PoPs between 30 and 60% for the remaining
long term forecast. Expect cloudy, cool, and unsettled for the late
week after the strong winds decrease early Wednesday morning.

&&

.AVIATION /00Z TAFS THROUGH 00Z MONDAY/...
Issued at 524 PM MDT Sat May 4 2024

VFR expected for all terminals through the forecast period. Wind
gusts this evening and overnight for southeast WY terminals will
be 20-30 knots at times. KRWL will dissipate to less than 12
knots through Sunday morning. Wind gusts for southeast WY
terminals will ramp up quick on Sunday morning, with gusts of
30-45 knots. The NE Panhandle terminals will see elevated winds
this evening and overnight between 25-35 knots. By midday
Sunday, wind gusts across the NE Panhandle terminals will be
strong, with gusts of 35-50 knots present through the afternoon
hours.

&&

.CYS WATCHES/WARNINGS/ADVISORIES...
WY...High Wind Watch from Sunday morning through Sunday evening for
     WYZ104-109-111.
NE...None.

&&

$$

SHORT TERM...MAC
LONG TERM...AM
AVIATION...BW