-- Discussion containing changed information from previous version are highlighted. --
957
FXUS65 KCYS 182304
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
504 PM MDT Wed Sep 18 2024

.KEY MESSAGES...

- Near-critical fire weather conditions today for most with
  gusty winds and dry conditions. Red Flag Warning is in effect
  for the northern Nebraska panhandle through early this
  evening.

- Dry conditions continue through the remainder of the week
  before precipitation chances return over the weekend.

&&

.SHORT TERM /THROUGH FRIDAY NIGHT/...
Issued at 325 PM MDT Wed Sep 18 2024

Latest GOES WV imagery shows a mature large scale occluded cyclone
over eastern MT with notable dry air and strong winds just to the
south over our area. 700mb winds continue to decrease over southeast
WY this afternoon limiting the high wind potential, but a few waves
are still present on GOES imagery. A few sensors gusted 60-70 mph
earlier this morning near Elk Mountain as well as in the immediate
lee of the North Laramie Range along I-25 south of Glendo. However,
winds have dropped to 35-50 mph for most sites and will continue to
do so headed into this evening with hi-res and in-house guidance
support. However, gusty southwest winds over the northern Nebraska
panhandle have resulted in critical fire weather conditions
developing this afternoon, aided by downsloping winds off the
Laramie Range. Therefore, a Red Flag Warning was issued through
early this evening.

The remainder of the week will be dry with gradual warming
temperatures as the upper level low continues to climb into the
north-central CONUS while a secondary upper level low digs off the
CA coast. Thursday afternoon temperatures will be around normal for
mid-September while Friday will be 5-10F degrees warmer in the mid-
70s to 80s as 700mb temps reach 10-13C.

&&

.LONG TERM /SATURDAY THROUGH WEDNESDAY/...
Issued at 325 PM MDT Wed Sep 18 2024

The first portion of the long-term period will focus on a storm
system moving through the central and southern Rockies and out into
the high plains from Saturday through Sunday. At the same time, a
broad scale trough in the northern Rockies will swing through,
dragging a surface front south along the high plains up against the
front range. The combination of these two events will bring a rather
notable change to the weather in southeast Wyoming and western
Nebraska, including a cool-down in temperatures and the possibility
for some precipitation. Ensemble forecast guidance remains in
excellent agreement showing a strong signal for a cool-down in
temperatures on both Saturday and especially on Sunday behind the
frontal system. Temperatures will likely run around 10 degrees below
average on Saturday, with temperatures upwards of 15 degrees below
average on Sunday as cool air from the north funnels southward along
the high plains and seeps into the mountains and basins farther
west. Despite strong confidence in the temperature forecast, the
precipitation forecast remains much less certain. This is due to a
wide dispersal in the exact location and track of a surface low
pressure system expected to develop in the lee of the southern Front
Range of Colorado on Saturday into Sunday. Some ensemble guidance
groups favor a more southern track and develop the low farther east
into the great plains. These scenarios would result in a much lower
chance for precipitation north of the Colorado border into western
Nebraska. Other ensemble groups still favor a track close enough to
the Colorado Front Range to toss some better moisture north of the
border. In these scenarios, afternoon showers and thunderstorms
would be possible in the higher terrain of southeast Wyoming, with
general light rain spreading eastward through the evening hours
behind any stronger convection. With cooler mid-level temperatures
in place on the north side of this system/behind the cold front, the
highest elevations of the Snowy Range including Medicine Bow Peak
could be in line for a light snow accumulation. However, this would
only take place if this system takes the more northern track. The
latest guidance trends certainly support the drier, more southern
track resulting in much lower precipitation chances for southeast
Wyoming and western Nebraska. We will continue to monitor this
system for any changes in guidance.

Dramatic differences in ensemble guidance emerge even by Sunday and
last through the middle of next week, casting a large amount of
uncertainty on the long-term period. ECMWF ensemble guidance favors
a slower departure of our weekend storm system, with clouds and some
shower chances lasting through the day on Sunday, compared with the
drier GEFS. We will favor the drier trend for now given the pattern
shown in recent model cycles but can not fully eliminate the chance
for precipitation and especially clouds on Sunday. A very weak
trough will likely pass through sometime on Monday, with a
reinforcing cold front that will keep temperatures on Monday and
Tuesday below average. Most guidance supports a dry passage of this
system without a Pacific moisture connection. A ridge of high
pressure then looks to develop by midweek over the southwestern
CONUS however some ensemble guidance keeps this feature suppressed
allowing for repeated trough passages over the central and northern
Rockies, whereas other guidance favors a more sprawling ridge.
Temperatures do look to warm a bit by Wednesday onward, possibly
returning to average or even slightly above average for the last
full week of September.

&&

.AVIATION /00Z TAFS THROUGH 00Z THURSDAY/...
-- Changed Discussion --
Issued at 501 PM MDT Wed Sep 18 2024

Gusty conditions across southeast Wyoming and western Nebraska
with clear to mostly clear skies. VFR conditions are expected
to continue through the 00Z TAF period. Winds begin to decrease
over the next 1-3 hours, with overnight winds fairly calm. Winds
will begin to increase across western Nebraska and KRWL late
tomorrow morning and remain elevated through the afternoon. No
precipitation expected at this time.
-- End Changed Discussion --

&&

.CYS WATCHES/WARNINGS/ADVISORIES...
WY...None.
NE...Red Flag Warning until 7 PM MDT this evening for NEZ434.

&&

$$

SHORT TERM...MB
LONG TERM...MAC
AVIATION...AM