-- Discussion containing changed information from previous version are highlighted. --
741
FXUS65 KCYS 160346
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
946 PM 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 320 PM MDT Sun Sep 15 2024

A shortwave pushing into the CWA this afternoon will be responsible
for the development of showers and thunderstorms later this
afternoon and into the overnight hours. Hi-Res guidance is in good
agreement with storms developing off the Laramie Range and gradually
pushing east into Nebraska by this evening. Radar still remains
fairly quiet, with just a few showers over the high terrain in
Colorado. Models also show considerable amounts of low-level CIN for
storms to overcome this afternoon, hence the likely later convective
initiation time. However, as previously mentioned, Hi-Res guidance
seems confident in storm development despite the CIN. Model
soundings do show relatively impressive CAPE values for this time of
year, ranging from 1000 to 1500 J/kg. This along with synoptic scale
lift from the incoming trough should be enough to get scattered
storms. Cannot rule out a strong to marginally severe storm today
given the environment. The HRRR does show perhaps one or two more
organized cells early this evening lending itself to a hail threat,
before becoming a more disorganized cluster of storms with more of a
strong wind threat. As mentioned, storms will continue into the
overnight hours, but should clear out by early Monday morning.

Looking at likely a drier day on Monday as strong southerly flow
ahead of the upper-level trough to the west continues to advect warm
air into the region. Still expect mild, above average temperatures
across much of the area. Cannot rule out a few isolated showers west
of the Laramie Range as the southerly flow may also advect some
moisture into this area. However, models have backed off from
previous runs regarding how much moisture to push into our CWA.
Currently, the most favorable area for moisture looks like
southwest Colorado. Whatever moisture does make it into the CWA
could be enough to spark a few showers during the afternoon, but
even Hi-Res guidance is not very excited about the possibility
of much precipitation.

&&

.LONG TERM /TUESDAY THROUGH SUNDAY/...
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 TUESDAY/...
-- Changed Discussion --
Issued at 939 PM MDT Sun Sep 15 2024

Southwest flow aloft will continue. Scattered to broken clouds
from 8000 to 12000 feet will prevail. Winds will gust to
30 knots at Chadron and Sidney until 09Z, then gust to 25 knots
at the Wyoming terminals and Sidney from 15Z to 01Z, and gust
to 25 knots at Chadron and Alliance after 18Z.
-- End Changed Discussion --

&&

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

&&

$$

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