-- Discussion containing changed information from previous version are highlighted. --
209
FXUS65 KCYS 171158
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
558 AM MDT Mon Jun 17 2024

.KEY MESSAGES...

- A strong capping inversion in place throughout the day will
  lead to fairly benign weather. However, if the cap breaks
  later this evening, severe thunderstorms with large hail and
  gusty winds could be possible.

- Strong winds are expected late Monday night and early Tuesday.
  Wind gusts in excess of 50-55mph are favored in the wind prone
  corridors of southeast Wyoming. Please see the latest High
  Wind Watch for further details.

- A cold front will bring cooler temperatures for Tuesday and
  Wednesday. A warming trend will develop for Thursday through
  Saturday, along with daily chances for afternoon and evening
  showers and thunderstorms.

&&

.SHORT TERM /THROUGH TUESDAY NIGHT/...
Issued at 412 AM MDT Mon Jun 17 2024

Today looks almost like a repeat of yesterday, with fairly quiet
weather expected during the day, followed by the potential for storm
development in the evening. Easterly flow early this morning will
increase moisture in the low-levels, likely leading to the
development of low status east of the Laramie Range. Southwesterly
flow out ahead of an incoming trough will advect warmer and drier
air into the CWA throughout much of the day. High temperatures will
be a few degrees warmer than yesterday, especially for areas south
of the North Platte River Valley. This southwest flow will also work
to push drier air further east, quickly eroding low stratus across
the far eastern plains of Wyoming as a dryline is pushed up against
the Wyoming/Nebraska border. Aside from upper 50 to low 60 degree
dewpoints in the panhandle and the presence of a dryline, model
soundings do show a suitable environment for potentially severe
storms to develop. Best environment appears to be in the southern
Nebraska panhandle, but the northern panhandle still has a decent
set up as well. Model soundings from the GFS and RAP both show
MUCAPE values over 2000 J/kg and effective shear values over 45 kts
in the southern panhandle. This lends itself to a large hail threat.
Strong wind gusts over 60 MPH could also be possible with DCAPE
values over 1000 J/kg. Lastly, it is worth mentioning that rotating
updrafts and an isolated tornado could be possible, mainly in the
southern half of the panhandle. Effective shear values support
supercell development, while approximately 1700 J/kg of MLCAPE and
SRH values of 200 m^2/s^2 and greater support tornadoes. However,
all of this comes with a big "if". Hi-Res guidance has not been too
keen on storms developing in the CWA at all. This is most likely due
to a strong capping inversion caused by warm air advection aloft.
GFS and RAP soundings do show a strong cap, but have it mostly
eroded away by Monday evening. Therefore, if storms do develop, it
looks most probable that they would develop during the evening and
continue through the first half of the overnight. So again, another
conditional severe weather forecast dependent on whether the cap
will erode. If it does, there is a higher likelihood that storms
will be severe as the environment is favorable and there is
fairly good forcing from an incoming cold front.

Aside from potential severe weather this evening, a strong cold
front will push across the area late tonight. This will lead to a
gradient driven wind event as the MSLP gradient over the CWA alone
could be over 20 mb! A High Wind Watch is already in place for the
southeast Wyoming wind prones from 12 AM to 9 AM Tuesday. Luckily
this looks like a quick hitting event, mainly effecting the area as
the cold front pushes through. Decided to leave the headline as a
Watch for now to get at least another round of model data in. Given
the time of year and weaker subsidence than we`re accustomed too for
winter wind events, did not have full confidence to upgrade just
yet. However, in-house guidance does show decent 40 percent
probabilities for high winds and GFS 750 mb winds are maxing out
around 50 kts.

Temperatures will be much cooler behind the frontal passage on
Tuesday. Highs will be anywhere from 10 to 20 degrees cooler than
Monday! Highs west of the Laramie Range will likely not make it out
of the 60s, while areas east of the Laramie Range will see mostly
low to mid 70s. Storm chances on Tuesday will be minimal as high
pressure builds in northeastern Wyoming behind the front. Will have
to keep an eye out for the potential of frost development in the
high elevations and high valleys as overnight lows will be fairly
chilly. However, increased cloud cover over the western CWA will
likely limit any frost.

&&

.LONG TERM /WEDNESDAY THROUGH SUNDAY/...
Issued at 412 AM MDT Mon Jun 17 2024

Medium Range models continue to be in good agreement for the
middle part of the week as a cooler airmass settles over the
region behind Monday Night`s cold front. Deterministic models
and ensembles appear to be settling on the cooler solution and
right around the 25th percentile from a day or two ago. 700mb
temperatures Wednesday afternoon should be between 2c and 6c
across the eastern plains with a slight rebound in temperatures
west of I-25. With broken to overcast skies, highs in the mid
60s to low 70s looks good and tweaked the official forecast to
lower the south facing ridges and slopes a few more degrees due
to persistent south to southeast winds and cloudy skies. Kept
POP between 15 to 30 percent for widely scattered showers and
isolated thunder, mainly due to WAA aloft expected later in the
day.

For Thursday and Friday, models indicate the surface cold front
ejecting northeast and weakening across the plains as weak upper
level ridging develops across the central and northern plains.
Expect warmer temperatures, closer to average, later in the week
with highs in the upper 70s to mid 80s...coolest further north
(Lusk, Chadron, etc) which will likely be fighting persistent
cloud cover ahead of of the stationary/warm front. Models
continue to indicate a weak Pacific trough moving onshore early
Thursday, with a pair of notable upper level disturbances
ejecting northeast into Wyoming late Thursday and the second one
later Friday into Friday evening. NAEFS continues to show 95th
to 99th percentile PWATs, this time over most of the forecast
area and not just confined to the high plains. The subtropical
jet will also become more active during this time period.
Continued to increase POP and thunderstorm coverage over the
whole area, including Albany and Carbon counties. Nocturnal
convection also looks like a good possibility east of I-25 each
night, but did not go quite as high on POP as the NBM suggests.

Looking ahead into next weekend, models remain in pretty good
agreement showing the polar jet retreating northward into
southern Canada with low amplitude ridging over the eastern
Great Basin Region and the Front Range. A drier airmass pushes
into the intermountain west with 700mb temperatures climbing to
15c to 20c, resulting in hot temperatures for most of southwest
Wyoming and western Nebraska by Sunday afternoon.

&&

.AVIATION /12Z TAFS THROUGH 12Z TUESDAY/...
-- Changed Discussion --
Issued at 558 AM MDT Mon Jun 17 2024

A strong early summer cold front, currently over Oregon and
Washington state this morning, will gradually move southeast into
Wyoming this afternoon. Very windy conditions and much cooler
temperatures are expected as this front moves through tonight.
Isolated strong to severe thunderstorms are possible, mainly for the
western Nebraska terminals, late this afternoon through tonight.

HAZARDS/WEATHER TRENDS: IFR, or near IFR, CIGS will continue this
morning for KCYS and all western Nebraska terminals. Can`t rule out
some patchy fog as well. Clouds will lift by 18z, and then attention
will shift to potential strong to severe thunderstorms for mainly
the western Nebraska terminals late this afternoon through early
tonight (23z Monday through 08z Tuesday). As of right snow, very low
confidence TAF due to uncertain timing and low coverage, so kept out
of most TAF sites for now and will wait for better high resolution
model agreement. At this time, looks like KSNY and KCDR have the
best chance to see thunderstorms. Otherwise, strong northwest winds
will develop behind the cold front after 04z for KRWL and KLAR, and
after 08z further east...including KCYS and the southern Nebraska
terminals where gusts of 45 knots are expected.
-- End Changed Discussion --

&&

.CYS WATCHES/WARNINGS/ADVISORIES...
WY...High Wind Watch from late tonight through Tuesday morning for
     WYZ106-110-116-117.
NE...None.

&&

$$

SHORT TERM...SF
LONG TERM...TJT
AVIATION...TJT