802
FXUS65 KCYS 141740
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
1140 AM MDT Sat Sep 14 2024

.KEY MESSAGES...

- Critical fire weather conditions continue this afternoon,
  mainly in Carbon, Albany, and Converse counties. A Red Flag
  Warning is in effect.

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

&&

.SHORT TERM /THROUGH SUNDAY NIGHT/...
Issued at 330 AM MDT Sat Sep 14 2024

Mostly clear skies overnight and light winds have lead to a chilly
morning for many across the area. 30s are showing up in a few
valleys west of the Laramie range once again with some spots to the
east dropping into the mid 40s. The influence of the strong trough
from the last few days has weakened as the feature lifts up into
Canada, its eastward propagation halted by a Rex Block over eastern
North America. However, a subtle secondary shortwave is on its way
through the area this morning. This can be identified as a slight
bend in the 500-mb height contours and a region of higher mid to
upper level moisture in GOES water vapor channels. Its passage will
briefly enhance height/pressure gradients across the area, and lead
to another afternoon of breezy westerly winds in Carbon, Albany and
Converse counties. With very dry air remaining in the boundary layer
on the Wyoming side, we`ll have critical fire danger on track once
again this afternoon. See the Fire Weather section below for more
details. Further east, the low-level moisture has retreated into
central Nebraska, but is expected to push a bit back to the west
today as SSE winds take hold this morning. This will result in a
sharp dryline forming over the NE panhandle. While a fairly unstable
environment is expected to develop to the east of this, recent model
trends have accelerated the weak shortwave`s passage today. As a
result, the dryline boundary may be on its subsident flank by peak
heating today. HiRes models have consequently backed off on showing
any thunderstorm activity this afternoon, but due to the
environment, low end PoPs will be carried through in this forecast
update since it can`t be totally ruled out yet.

Strengthening surface high pressure to our east tonight will help to
nudge the moisture boundary further west, likely ending up just west
of the WY/NE border by Sunday AM. Some patchy fog or low clouds
could be possible to the east of the boundary, but confidence was
too low to add to the forecast at this time. Meanwhile, the next
potent upper level low will begin to dive south along the West Coast
Sunday, amplifying the downstream ridge and enhancing south to
southwest flow aloft. Another shortwave will start to climb out of
the southwest on Sunday, moving over our area during the afternoon
and evening hours. Synoptic scale lift ahead of this feature should
help to flare up a few isolated to widely scattered thunderstorms
along the moisture boundary Sunday afternoon, which should be
pushing up against the Laramie range by this time. While forecast
soundings show a fair amount of CIN in place, the environment east
of the dryline could be conducive to an isolated strong to severe
thunderstorm, since the synoptic lift should give burgeoning
updrafts a little boost. Any showers/storms that manage to develop
will probably continue well into the evening until the trough axis
clears the area around midnight and shuts off the synoptic
support.

&&

.LONG TERM /MONDAY THROUGH FRIDAY/...
Issued at 330 AM MDT Sat Sep 14 2024

Models and 00z ensembles seem to be coming into better agreement
regarding the long term pattern, showing more fall-like weather
for southeast Wyoming and western Nebraska as we head towards
the official start of fall (September 22nd). Early in the week,
the Front Range will still be under the influence of southwest
to southerly flow aloft with 700mb temperatures near 15c Monday
and part of Tuesday. This translates to high temperatures in the
80s to near 90 for the high plains and mid 70s west of the
Laramie Range. So a few more warm and pleasant days are
expected as we head into the next workweek. Models show a
Pacific upper level trough moving eastward across the Great
Basin Region on Monday. Most of southeast Wyoming will be in the
area of diffluent flow aloft on the eastern side of the trough.
With some low level instability, afternoon showers and
thunderstorms are expected. Conditions are more favorable on
Tuesday as the upper level trough lifts northeast and a surface
cold front begins to move into central Wyoming. Not expecting
severe weather at this time due to limited moisture, but a few
strong storms are possible given moderate instability, low level
forcing, and ample 0-6 km shear. Surface cold front will push through
the area and into western Nebraska by Tuesday night, with
daytime temperatures closer to normal (70s) for this time of the
year Wednesday afternoon. Areas across western Nebraska, and near the
Wyoming/Nebraska border, will likely still have highs in the
upper 70s to low 80s. Windy conditions are expected with the
wind prone areas seeing gusts up to 50 MPH Tuesday and early
Wednesday, with gusts up to 40 MPH outside of the wind prone
areas.

For mid to late next week, medium range models show a longwave
trough settling into the western United States. According to NAEFS,
this trough (500mb geopotential heights) will be nearly 4 standard
deviations below climatology for the area with mountain snowfall likely
for the central and northern Pacific states. This trough is forecast to
gradually slide eastward into Wyoming through by the end of the week.
Although deterministic models slightly differ on how this evolves, all
models show cooler temperatures and a better chance for widespread
rainfall by next Friday and Saturday. In fact, the higher mountains
above 9500 feet may see their first snowfall of the season as early as
Thursday night. With good ensemble support, trended temperatures lower
towards the end of the week and next weekend with highs in the 60s to
low 70s by Friday and next Saturday. 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. Kept POP between 30 to 50 percent as
precipitation becomes more stratiform by the end of the forecast
period. With the ongoing Rex Block over the eastern United States, will
have to watch for Pacific systems stalling over or near the forecast
area next weekend and into the last full week of September. &&

.AVIATION /12Z TAFS THROUGH 12Z SUNDAY/...
Issued at 540 AM MDT Sat Sep 14 2024

A weak disturbance aloft will move northeast across the region
this morning. Other than a few thunderstorms and breezy conditions,
mostly sunny skies are expected with minimal concerns to Aviation
over the next 24 hours.

HAZARDS/WEATHER TRENDS: VFR conditions will prevail today and tonight.
The best chance for thunderstorms will be across the western Nebraska
Panhandle this afternoon and this evening, but coverage should be
isolated at best. Kept VCTS out of the western Nebraska TAFs for now
due to high uncertainty.

&&

.AVIATION /18Z TAFS THROUGH 18Z SUNDAY/...
Issued at 1140 AM MDT Sat Sep 14 2024

Expect VFR conditions through the day and into the evening hours.
Occasional gusts of 20 to 25 kts will be possible throughout the
afternoon. Weak southeast flow early Sunday morning could lead to
the development of patchy fog in the southern Nebraska panhandle and
North Platte River Valley. Low status may also be possible in this
area which could lead to MVFR conditions Sunday morning.

&&

.FIRE WEATHER...
Issued at 330 AM MDT Sat Sep 14 2024

Critical fire weather conditions are expected once again this
afternoon, mainly in portions of Carbon, Albany, and Converse
counties. Red Flag Warnings remain in effect. Increasing winds
associated with a weak passing disturbance today along with very low
humidity will lead to critical fire weather conditions. Winds will
be predominantly westerly, gusting up to 30 to 35 MPH. The dryline
will move back to the west tonight, but it is not expected to reach
the Laramie Range. Thus, the areas of concern today will see poor to
fair RH recoveries tonight. Sunday looks warm and very dry again,
but the winds will be lighter than today. Another push of moisture
will arrive Sunday night, leading to better RH recoveries Monday AM
and reduced fire weather concerns for Monday and Tuesday.

&&

.CYS WATCHES/WARNINGS/ADVISORIES...
WY...Red Flag Warning until 8 PM MDT this evening for WYZ418>422-
     427.
NE...None.

&&

$$

SHORT TERM...MN
LONG TERM...TJT
AVIATION...SF
FIRE WEATHER...MN