-- Discussion containing changed information from previous version are highlighted. --
318
FXUS65 KCYS 170345
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
945 PM MDT Mon Sep 16 2024

.KEY MESSAGES...

- Isolated to scattered showers and thunderstorms are possible
  this afternoon and evening, mainly along and west of I-25.

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

- Strong winds may continue in the wind prone areas through
  Wednesday morning. A High Wind Watch has been issued.

&&

.SHORT TERM /THROUGH TUESDAY NIGHT/...
Issued at 307 PM MDT Mon Sep 16 2024

Latest GOES WV imagery shows a large scale upper level low over
much of the western CONUS with a leading stream of moisture
extending from CO up into southeast WY. This has led to
scattered showers developing early this afternoon with
occasional lightning. Despite upper level moisture, low-levels
remain relatively dry leading to inverted-V profiles supportive
of gusty winds in the vicinity of storms. Dixon AWOS station
recently reported a gust over 45 mph. Storms will continue into
the early evening before slowly dissipating.

Main forecast concerns for Tuesday will involve strong winds as
thunderstorms move west to east across southern WY into the NE
panhandle early in the evening. The upper level low over the
western CONUS will continue its slow progression eastward as the
250mb jet rounds the base of the trough today. This will provide
favorable upper level divergence supportive of widespread
coverage of storms as early as late morning moving into portions
of Carbon Co, despite marginal instability profiles. As the
previous discussion noted, any enhancement of the lee trough
Tuesday morning will further increase the wind threat with
storms moving east of the Laramie Range with a strong MSLP
gradient. Additionally, richer boundary layer moisture will be
placed east of the lee trough across the NE panhandle where
storms should transition to a possible broken line into a larger
scale MCS with damaging winds.

Based on latest CAMs, timing of the storms with strong winds looks
to be 1-3 PM for Laramie, 2-4 PM for Cheyenne, 4-6 PM for
Torrington/Scottsbluff. This upper level set up for severe
thunderstorms draws comparisons to the 6 June 2020 Rocky
Mountain Derecho where numerous wind gust reports over 70 mph
were observed. However, low-level wind fields appear to be weak
tomorrow than that event. SPC has made additions to their Day 2
Outlook to include a hatched area for significant winds
possible.

As far as headline modification go, decided to add portions of
northern Carbon Co through Converse Co into the High Wind Watch
for Wednesday. Strong 700mb flow in the wake of the frontal
passage will extend across the North Laramie Range with
downsloping enhancement possible into the I-25 corridor north of
Wheatland. Latest NAEFS guidance shows climatological 99th
percentile 700mb winds crossing through central WY with this
system.

&&

.LONG TERM /WEDNESDAY THROUGH MONDAY/...
Issued at 420 AM MDT Mon Sep 16 2024

Long range models are in reasonable agreement through Friday
night, but are now showing significant discrepancies for the
weekend regarding the potential for prolonged rainy and cool
weather for the weekend.

For the middle of this week, models in good agreement with the
timing of a pretty strong cold front pushing across the area by
early Wednesday morning. High temperatures on Wednesday will
likely be 10 to 15 degrees cooler compared to Tuesday, but still
should be around average for this time of the year. It will be
pretty windy behind the cold front with 700mb winds over 55
knots for central and east central Wyoming, with some evidence
of mountain wave activity and stronger winds on the lee-side of
the Snowy Range and Laramie Range. Increased winds a bit more
with in- house wind models showing the wind prone areas flirting
with high wind criteria, including areas near the I-25
corridor.

Temperatures will rebound slightly on Thursday as the next
Pacific trough moves across the southern Great Basin region and
moderate to strong southwesterly flow aloft develops along the
Front Range. Models are in better agreement for Thursday and
Thursday night as the GFS has backed off on its more
progressive solution...and now is more in line with the Canadian
and ECMWF solutions with the Pacific upper level trough slower
and much further west compared to yesterday`s 00z run. Most of
Friday is looking drier and warmer in comparison to what some
models were showing a few days ago, mainly due to the Pacific
trough taking its time moving eastward and still over the far
eastern Great Basin region, so increased high temperatures into
the 70s for most locations with POP generally below 30 percent
through the day.

For Friday night and the weekend, this is when models start to
diverge considerably with the 00z ECMWF and 00z GFS showing
completely different solutions compared to yesterday`s 00z
cycle. This is to be expected with blocking patterns as models
struggle to resolve them in the medium to extended forecast
range. Regardless, it looks like unsettled and cool weather for
next weekend. The potent upper level trough is forecast to
cross into the central Front Range and slow down...potentially
forming a closed low around the Colorado/Wyoming/Nebraska border
on Saturday. The ECMWF is now showing a prolonged moderate to
heavy rain event with embedded thunderstorms Saturday afternoon
through Sunday evening...mainly because of low to midlevel
convergence associated with strong frontogenesis and
cyclogenesis across the region. The GFS on the other hand is now
more progressive at the onset of the event, mainly because it
delays the Rex Block formation across the eastern U.S. until
later in the weekend. Therefore, not as cool and somewhat drier
weather for southeast Wyoming. The Canadian maintains its
aggressive solution, similar to the ECMWF, but is 18 to 24 hours
earlier...showing moderate to heavy rainfall developing as
early as Friday evening. Still expect a gradual cool-down this
weekend with highs in the 60s and nighttime lows in the 30s and
40s. There is still lower than average confidence in POP and
timing of rainfall and mountain snowfall. All models have
trended warmer in the 700mb level, so heavy mountain snow
accumulations look less likely at this time. Kept POP between 30
to 50 percent, but increased the length of time through
Saturday night. Unsettled weather could linger well into Sunday,
as the 00z ECMWF is suggesting.

&&

.AVIATION /06Z TAFS THROUGH 06Z WEDNESDAY/...
-- Changed Discussion --
Issued at 941 PM MDT Mon Sep 16 2024

Southwest flow aloft will strengthen through the period, while a
potent cold front moves west to east across the area Tuesday
afternoon and evening, producing scattered showers and
thunderstorms.

Wyoming TAFS...Scattered to broken clouds from 8000 to
15000 feet will prevail until 01Z, then skies will be clear.
Thunderstorms will be in the vicinity of Rawlins from 16Z to
21Z, at Laramie from 18Z to 00Z, and at Cheyenne from 21Z to
01Z. Winds will gust to 35 knots at all terminals after 16Z
Tuesday.

Nebraska TAFS...Scattered to broken clouds from 8000 to
15000 feet will prevail. Thunderstorms will be in the vicinity
from 00Z to 03Z. Winds will gust to 35 knots after 18Z Tuesday.
-- End Changed Discussion --

&&

.CYS WATCHES/WARNINGS/ADVISORIES...
WY...High Wind Watch from late Tuesday night through Wednesday
     morning for WYZ101-104-105-107.
     High Wind Watch from Tuesday afternoon through Wednesday
     morning for WYZ110.
NE...None.

&&

$$

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