236
FXUS65 KCYS 240140
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
640 PM MST Sat Nov 23 2024

.KEY MESSAGES...

- Elevated to critical fire weather conditions and strong gusty
  winds will continue for portions of Carbon, Albany, and
  Converse counties through the early evening hours.

- A storm system will bring colder temperatures and a chance for
  snow showers Sunday and Sunday night. Light accumulations are
  expected for the I-80 corridor west of Cheyenne. Winter
  Weather Advisories have been issued for the mountains.

- Another round of snow showers and colder temperatures may
  impact holiday travel on Tuesday into Wednesday.

&&

.UPDATE...
Issued at 631 PM MST Sat Nov 23 2024

Strong winds with gusts over 40 MPH continue for the Arlington/Elk
Mountain area this evening. Downsloping winds in this zone are
also keeping relative humidity at critical thresholds, despite
the sun going down. Observations from this zone show wind gusts
between 35 and 45 MPH with relative humidity values below 15
percent. Due to the persistent critical fire weather conditions,
did decide to extend the Red Flag Warning for the Arlington zone
until 9 PM. Hopefully by then, humidity will recover above
critical thresholds with lighter winds. However, will re-
evaluate before the Red Flag Warning expires as the HRRR
continues to show relative humidity below 15 percent through
about midnight.

&&

.SHORT TERM /THROUGH MONDAY NIGHT/...
Issued at 200 PM MST Sat Nov 23 2024

A Pacific jet streaming across the Western CONUS this afternoon is
producing plenty of mid to high level cloud cover over the area
visible on the latest satellite imagery. While a low amplitude but
fairly strong ridge is still holding on across the area, this will
break down over the next 24 hours as the messy trough over the
Pacific Northwest moves eastward. Currently, two distinct air
masses are positioned over the area: a warm and remarkably dry
airmass covering most of the area, while a surface high to the
northeast is keeping cooler and more moist air in place mainly
along/north of the North Platte River Valley. The boundary layer
is extremely dry for this time of year southwest of the
boundary, producing fairly widespread critical fire weather
conditions when factoring in the gusty southwest winds.
Additionally, gradients are strong enough to produce a few gusts
in the 55-60 MPH in Carbon County. As we head into the evening
hours, the surface high over western Colorado is expected to
weaken, reducing the cross-barrier MSLP gradient and allowing
high winds to die down. High Wind Warnings and Red Flag Warnings
should be able to expire on time at 6PM, but RH will probably
remain near critical to critical for a few hours after.

Heading into tonight, an approaching shortwave riding along the jet
stream will nudge into the area. Mid-level moisture will rapidly
recover this evening, allowing orographic snow showers to return to
the mountains after midnight. The boundary layer will still be
extremely dry in the valleys, so it may take a few hours to moisten
up the lower levels and allow snow showers to spread to the lower
elevations. This should happen quickly after the shortwave trough
axis and associated frontal boundary arrives, which is expected
to be around 12z near Rawlins and Douglas. Expect rapid low-
level moistening after this, which will then quickly increase
snow shower coverage, driven primarily by frontogenesis. The
cool-down and wind shift will happen fairly quickly over the
High Plains, but the mid- level boundary and associated lift
will be slower to clear out, not finishing up until a secondary
trough axis/vorticity maximum clears the area Sunday evening. As
a result, snow showers are expected to increase in coverage
over the High Plains Sunday late morning into the afternoon, but
downslope flow along/south of the North Platte River valley
will undercut the lift. Accumulating snow is likely to be
focused further north, in the Pine Ridge and vicinity where up
to 1-2" will be possible. Further south, expectations are for
just a few snowflakes to a half inch or so on the high end.
Modest impacts are also likely for the I-80 corridor from the
summit westward, where a general 1/2-3" can be expected. There
is about a 20% chance of a higher end scenario which would
warrant a Winter Weather Advisory for the Rawlins to Arlington
corridor, but confidence is too low to issue any products at
this time. As usual, the highest amounts will be in the
mountains where low end advisory amounts of about 4-9" are
expected. Therefore, a Winter Weather Advisory has been issued
for the higher mountains in the Sierra Madre and Snowy range.

Once the secondary trough axis clears the area Sunday evening,
expect a quick end to precipitation as much drier air moves in along
a subtle ridge axis. Expect cooler temperatures Monday, but the
quick rebound of the ridge should lead to highs ending up just a few
degrees cooler than average.

&&

.LONG TERM /TUESDAY THROUGH SATURDAY/...
Issued at 200 PM MST Sat Nov 23 2024

Fairly zonal flow aloft expected for Tuesday ahead of an incoming
shortwave trough from the northwest. Significant jet energy is
expected Tuesday as the upper-level, 160kt jet moves overhead.
Westerly flow remains throughout the atmosphere, leading to a 40kt
jet across the Laramie Range Tuesday morning. Elevated winds will be
likely along and just east of the Laramie Range along the I-25
corridor. High Winds are currently not expected, as the 700mb trough
is not quite strong enough and in-house random forest guidance does
not favor high winds for Tuesday. Rising motion associated with the
upper-level jet combined with westerly to southwesterly 700mb and
surface winds favor the development of upslope flow and moderate
snow across the Sierra Madre Range, with the Snowies being mostly
shadowed due to the wind direction. Near-average precipitable water
values will favor continued development of snow showers across the
higher terrain. The Sierra Madre Range could see over a foot of snow
Tuesday morning through Wednesday evening. However, there is some
disagreement between long range models of how much QPF will be
available across the mountains. The ECMWF favors much higher QPF
throughout the duration, while the GFS keeps slightly lower values.
These differences result from the displacement of the surface low
between models. the GFS keeps the low further south, while the ECMWF
keeps it further north. The further north track is more favorable
for heavier precipitation, due to wrap around moisture, while the
southern path keeps much of the better moisture out of the region.

An attendant cold front associated with the incoming upper-level
trough will push through late Tuesday night into early Wednesday
morning. Cold temperatures will pour into the region behind this
front, with the upper-level trough quickly moving overhead. Long
range models have come into better across on the progression of the
cold front, with both the ECMWF and GFS pushing the front through by
early Wednesday morning. Northerly flow will develop behind the
passing front, though winds are expected to remain relatively light.
The warmest temperatures on Wednesday are expected around midnight,
with temperatures dropping steadily into the 20s and 30s area-wide.
Overnight lows will drops into the teens and low-20s Wednesday night
into Thursday morning. With the upper-level trough pushing through,
rising motion will be present across the region, leading to the
threat of isolated to scattered snow showers. The higher terrain is
expected to continue to see snow throughout the day Wednesday, with
lower-lying areas seeing the potential for snow throughout the day.
Accumulations are not expected to be much across the region, outside
of the mountains, due to the relatively warm conditions in the days
leading up to this event. Any snow that falls will likely melt, due
to the warmer ground conditions.

The remaining forecast period remains unsettled, as primarily
northwesterly flow remains aloft. Snow chances for the lower-terrain
taper off by Thursday morning, but snow chances in the Sierra Madre
and Snowy ranges continue through the period. With northwesterly
flow aloft, winds are expected to remain elevated, especially across
the wind prone regions of southeast Wyoming. Temperatures remain in
the 30s and 40s as northwesterly flow acts to keep temperatures
cooler by bringing in cooler Canadian air into the region.

&&

.AVIATION /00Z TAFS THROUGH 00Z MONDAY/...
Issued at 444 PM MST Sat Nov 23 2024

An incoming system overnight will increase clouds and precipitation
chances over the area. A cold front will move from west to east over
the forecast area tonight and through the day Sunday. KRWL will be
the first terminal affected with low stratus and scattered snow
showers forecast late tonight and early Sunday morning. Expect IFR
conditions from both poor visibility in falling snow and low CIGs.
Next precipitation and low CIGs will spread into KLAR by Sunday
morning. Terminals east of the Laramie Range will see precipitation
and low CIGs later in the day on Sunday.

&&

.CYS WATCHES/WARNINGS/ADVISORIES...
WY...Red Flag Warning until 9 PM MST this evening for WYZ427.
     Winter Weather Advisory from 3 AM Sunday to midnight MST
     Sunday night for WYZ112-114.
NE...None.

&&

$$

UPDATE...SF
SHORT TERM...MN
LONG TERM...AM
AVIATION...SF