641 
FXUS65 KCYS 262349
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
449 PM MST Sat Nov 26 2022

.SHORT TERM...(This evening through Monday night) 
Issued at 315 PM MST Sat Nov 26 2022

Active weather expected starting tonight, and continuing through
Monday, as several potent Pacific troughs push across the 
Northwest United States. Cooler afternoon temperatures today in
contrast to highs of around 60 degrees yesterday with northerly 
winds behind a weak cold front. Visible Satellite loop shows 
increasing cloudiness across central and eastern Wyoming this 
afternoon ahead of a fast moving but potent upper level trough 
currently moving into MOntana and western Wyoming. Snow is 
currently falling in Riverton, Sheridan, and just started at 
Rawlins. This trend is expected to continue this evening with 
light snow developing along the I-25 corridor between 5 and 9 PM, 
and slightly later for the southern Nebraska Panhandle. Generally 
expect light snow intensity with small periods of moderate snow 
possible through early Sunday morning. Even though accumulations 
won't be all that impressive, there is enough of a travel impact 
to justify Winter Weather Advisories from Elk Mountain to just 
west of Cheyenne. Expect 1 to 3 inches of snow in these areas with
slightly higher amounts across west to northwest facing ridges 
and slopes. 

Upper level trough will quickly move south of the area and weaken
early on Sunday with partly cloudy skies and near average
temperatures for this time of the year through the afternoon.
Models in agreement with the next strong wind event with mid to
upper level flow shifting into the west ahead of the next stronger
Pacific upper level trough/storm system. Low level pressure
gradients will increase rapidly as most of southeast Wyoming will
be south of the initial 120kt jet max translating across Wyoming
late Sunday and Sunday night. Model guidance and in-house wind
models continue to be in very good agreement with strong winds
starting as early as Sunday morning at Arlington/Elk Mountain, and
into Sunday evening for the rest of southeast Wyoming. Used the
NBM 4.1 max gust probs to to tweak wind gusts across the wind
prone areas and decided to upgrade the High Wind Watch to a
Warning for most of the zones previously in the Watch. High
probability (50%+) of gusts around 75 MPH for the wind prone areas
and gusts around 65 MPH for the "sub" wind prone areas (north of
Wheatland, Douglas, northern Carbon County). Kept central Carbon
County and the I-80 Summit/foothills in the High Wind Watch due to
pinning down the timing/location and intensity of mountain wave 
activity. Will brief the next shifts accordingly to try to pin 
down the timing, but do expect these zones to hit criteria at some
point during the event. Guidance shows lower probabilities for 
strong winds at Saratoga, Laramie, and Cheyenne...so will continue
to keep these locations out of the High Wind headlines for now, 
but will continue to monitor as we head through Sunday. Note: if
the location of the forecast mountain waves change and trend
further east, may need to extend the High Wind Watches and 
Warnings across the eastern plains.  

Forecast concerns then quickly change back to snow and travel
impacts starting Monday afternoon west of I-25 (Monday night east
of I-25). Strong gusty winds will slowly diminish for the wind 
prone areas, so kept the Arlington/Elk Mountain area going until 
early Monday morning. Snow is forecast to quickly develop west of
I-25 as winds diminish across the area. Models and ensembles
remain consistent with this system as strong forcing pushes across
Wyoming and western Nebraska. Models continue to indicate low
level frontogenesis, upper level jet dynamics, and ample
WAA/overrunning aloft within the dendritic growth zone. Increased
POP between 75 percent for the lower elevations to near 100
percent for the mountains. This event looks like a good snow
producer due to expected forcing due to additional frontogenesis
as the front stalls along the mountains. Expect most of the
forecast area to have some sort of travel impacts early Tuesday
morning.  

.LONG TERM...(Tuesday through Saturday) 
Issued at 129 PM MST Sat Nov 26 2022

Little changes to the long term forecast, with an active pattern 
setting up from midweek through the early part of the weekend. The 
longwave trough digging from the Pacific Northwest earlier in the 
week through to the Central Plains will usher in increased snow 
chances on Tuesday as a strong cold front pushes through the CWA. 
Much colder temperatures are expected under the north to 
northwesterly flow with 700mb temperatures dipping as low as -19C 
across southeastern Wyoming and around -17C in the Nebraska 
Panhandle. Very similar conditions will be observed, as seen from 
the the week prior, with daytime highs in the teens and overnight 
lows in the single digits. Main challenges with this forecast 
package exist with the progressive nature of the cold front, and as 
a result, the overall snowfall totals expected under this system. 
Models are overall consistent in placing measurable precipitation 
from west to easts across the CWA, before tapering off in the 
overnight hours Tuesday night into Wednesday.  GFS has continued to 
remain a bit more aggressive with this system, keeping much higher 
24 hour snow totals with the frontal boundary stalling along and 
just north of the Rockies. On the opposite side, the Euro has a much 
more progressive system, having the cold front push further eastward 
and ushering in westerly flow behind the system and creating more a 
downsloping component, effectively eliminating precipitation chances 
much sooner. Majority of the ensemble members have consistently 
echoed these two solutions, with increased confidence for snow 
through early Tuesday afternoon and the totals remaining uncertain. 
Currently favoring the GFS solution, with the positioning and 
movement of the frontal boundary and the higher likelihood of it 
stalling out along the Rockies as seen in past events. Will need to 
continue monitoring this event with winter weather highlights 
possible in future updates. 

Warming trends are expected throughout the day on Wednesday, with 
700mb temperatures increasing from -19C to -9C by 18z, as upper 
level ridging continues to move eastward from the western seaboard 
and large scale subsidence tracks into the CWA. Daytime highs will 
rebound into the high-20s, and even low-30s, for most areas east of 
I-25 on Wednesday and climbing towards seasonal averages by 
Thursday. Despite the warming trends, winds are also likely to 
increase from mid to late week, with a series of minor disturbances 
propagating within the flow. 

&&

.AVIATION...(For the 00Z TAFS through 00Z Sunday evening)
Issued at 435 PM MST Sat Nov 26 2022

Snow showers are moving into the area from northwest to southeast
at this time, and will lead to dropping CIGs and VIS at most
terminals. The main change from the prior TAF package is a slight
acceleration in timing of dropping CIGs/VIS this evening. KRWL 
has already been seeing MVFR to IFR conditions, which will 
continue for several more hours. Expect CIGs/VIS to drop at KLAR, 
KCYS, and KBFF in the next 1-3 hours, with IFR likely in Wyoming 
and possible at KBFF. This storm system is moving quickly, and 
snow is expected to wind down around 06-09z. However, a few hires
models continue to show IFR CIGs moving back in to Wyoming 
terminals after a brief break, and lasting through morning.
Confidence in this second period is low, but enough to include in
the 00z TAF. 

Further east, snow will be more limited, but a period of light
snow with MVFR conditions is likely at KCDR and KAIA, but
conditions should improve quickly after around 05z. KSNY can
expect a brief period of snow and IFR CIGs, but expect improving
conditions after 08z. Cloud cover will decrease during the 
morning Sunday, and be replaced by gusty winds, particularly in 
Wyoming where 35kt gusts are possible. 

&&

.FIRE WEATHER...
Issued at 156 PM MST Sat Nov 26 2022

No fire weather concerns expected this weekend through next week
due to several chances for accumulating snow and very cold 
temperatures.

&&

.CYS WATCHES/WARNINGS/ADVISORIES...
WY...High Wind Warning from 11 AM Sunday to 5 AM MST Monday for 
     WYZ101-104>107.

     High Wind Watch from Sunday morning through late Sunday night 
     for WYZ109-116-117.

     Winter Weather Advisory until 5 AM MST Sunday for WYZ110-115-116.

     High Wind Warning from 5 AM Sunday to 2 PM MST Monday for WYZ110.

NE...None.
&&

$$

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