827
FXUS65 KCYS 150948
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
348 AM MDT Sun Sep 15 2024

.KEY MESSAGES...

- Scattered thunderstorms may develop late this afternoon and
  evening along and east of I-25. Isolated storms will have the
  potential to become strong to severe.

- Chances for showers and thunderstorms continue through
  Tuesday, followed by another chance for precipitation and
  cooler temperatures late in the week.

&&

.SHORT TERM /THROUGH MONDAY NIGHT/...
Issued at 345 AM MDT Sun Sep 15 2024


Mostly clear skies are present over the area this morning. The sharp
dryline has retreated westward overnight, now located roughly from
Cheyenne to Van Tassel. Moist air with dewpoints in the 50s is
located to the east of this boundary, while those to the west remain
quite dry, with dewpoints mostly in the 20s. Another night of light
winds is allowing temperatures in the dry sector to plummet once
again, but we are not as cold as 24 hours ago thanks to the warmer
airmass encroaching into our area. This dryline will be an important
player in today`s weather, as afternoon thunderstorms return to the
forecast.

While a deep longwave trough digs southward along the West Coast
today, a modest shortwave traversing around the leading edge of the
trough will approach the area. This is currently visible on GOES
water-vapor imagery just beginning to move into southwest Wyoming.
Synoptic scale lift ahead of this trough axis will help support the
development of thunderstorms this afternoon. Forecast soundings show
a fairly potent environment for this time of year, with MLCAPE
between 1000 and 1500 J/kg and lapse rates exceeding 8 C/km.
However, soundings also show substantial CIN in place this
afternoon. This, combined with the late arrival of the shortwave
trough axis, should result in fairly slow development of storms. It
may take until about 3-4PM before an updraft is able to overcome the
cap. The latest run of the HRRR also presents a failure mode
scenario today, in which storms are nearly totally unable to
overcome the cap resulting in very few if any storms. This is
plausible based on environmental parameters, but is currently not
the favored scenario since this model may be progressing the dryline
too far to the east and thus resulting in a too-dry boundary layer
that wouldn`t support thunderstorms. Most of the convective activity
(if it occurs) should be mostly between 5PM and 11PM, coming to an
end by around midnight when the shortwave trough axis clears the
area and we enter a synoptic scale substance region. Isolated
strong to severe thunderstorms will be possible through the late
afternoon and evening hours. While vertical wind shear will be
present, it looks too weak to support supercellular storm modes.

Looking ahead to Monday, the strong West Coast trough will dive into
central California, supporting amplification of the downstream ridge
to our east. Another subtle shortwave will push out of the south as
this amplification occurs, bringing with it a fairly significant
surge of lingering moisture out of the monsoon region. Integrated
water vapor transport Monday evening is over the 90th percentile of
climatology. This will be concurrent with notable isentropic lift
associated with the amplification of the standing Rossby wave
pattern. Thus, look for scattered showers and thunderstorms
developing mainly along and west of the Laramie range Monday
afternoon and evening, which may carry into the overnight hours as
well.

&&

.LONG TERM /TUESDAY THROUGH SATURDAY/...
Issued at 345 AM MDT Sun Sep 15 2024

Primary forecast concerns in the long range will primarily
relate to the longwave pattern change expected by mid to late
week, and the potential for some moderate to locally heavy
rainfall amounts (snow accumulations in the higher mountains)
next weekend as the Rex Block over the eastern United States
results in very slow movement of Pacific storm systems moving
east across the Front Range. It looks like the warm late summer
temperatures will come to an end on Thursday, with a more fall-
like pattern setting up across the region after September 20th.

Early this week, the Front Range will still be under the
influence of southwest to southerly flow aloft with 700mb
temperatures near 12c Tuesday. This translates to high
temperatures in the 80s to near 90 for the high plains and low
to mid 70s west of the Laramie Range. Models show the first of a
series of Pacific upper level troughs moving eastward across
the Great Basin Region on Monday and across Wyoming on Tuesday.
Most of southeast Wyoming and western Nebraska will be in the
region of diffluent flow aloft ahead of the trough axis. With
some low level instability, afternoon showers and thunderstorms
are expected...especially once the trough axis lifts northeast
and a surface cold front begins to move into central and eastern
Wyoming. Not expecting severe weather at this time due to
limited moisture, but a few strong storms are possible due to low
level and midlevel forcing, decent 0-6 km shear, and moisture
advection through the afternoon hours. Surface cold front will
push through the area and into western Nebraska by Tuesday
night, with daytime temperatures closer to normal for this time
of the year Wednesday. Windy conditions are expected Tuesday and
Tuesday night with the wind prone areas likely seeing gusts up
to 45-55 MPH Tuesday and early Wednesday, with gusts up to 40
MPH outside of the wind prone areas.

Temperatures will rebound slightly on Thursday as the next
Pacific trough moves across the southern Great Basin region.
This is when models start to diverge considerably as the speed
of this trough is nearly 24 hours faster with the GFS compared
to the less progressive ECMWF and Canadian. Thankfully, models
come into better agreement for next weekend as the strong Rex
Block over the eastern United States develops and remains in
place through the extended. There is high confidence with a
gradual cooling trend over southeast Wyoming and western
Nebraska Friday through next weekend, but confidence is limited
with precipitation amounts. This is pretty typical with
blocking patterns, and forecast confidence may not change much
over the next several days. Lower than average confidence is
mainly influenced by the strength of the Rex Block and exact
position when/where it develops. Gradually increased POP Friday
and Saturday for widespread rainfall, with the possibility of
considerable early season snowfall across the mountains above
9500 feet. With good ensemble support, kept the cooling trend
going towards the end of the week and next weekend with highs in
the upper 50s and 60s by next weekend. Can`t rule out the first
freeze of the season Saturday morning, but Sunday morning looks
more probable for areas west of the Laramie Range as another
strong upper level trough, this time digging south out of
western Canada, pushes across Montana and Wyoming with 700mb
temperatures lowering towards -3c by late next weekend.

&&

.AVIATION /06Z TAFS THROUGH 06Z MONDAY/...
Issued at 1033 PM MDT Sat Sep 14 2024

Southwest flow aloft will continue, while a weather disturbance
brings increasing chances for showers and thunderstorms Sunday
afternoon and evening.

Wyoming TAFS...Clear skies will prevail until late Sunday
morning, then clouds will be scattered near 10000 feet. Winds
will gust to 22 knots from 16Z to 01Z.

Nebraska TAFS...Clear skies will prevail until midday Sunday,
then clouds will be scattered to broken near 9000 feet. Expect
thunderstorms in the vicinity from 00Z to 03Z. Winds will gust
to 25 knots from 18Z to 00Z.

&&

.CYS WATCHES/WARNINGS/ADVISORIES...
WY...None.
NE...None.

&&

$$

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