172 
FXUS65 KCYS 092355
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
455 PM MST Fri Dec 9 2022

.SHORT TERM...(This evening through Sunday night)
Issued at 250 PM MST Fri Dec 9 2022

Forecast challenges in the short term deal with strong winds today
through Saturday morning. Small chance for strong winds again
Saturday night into Sunday. Followed by the potential for a
significant winter storm to start the work week next week.

Currently...Surface trough lays along the east slopes of the Laramie 
Range this afternoon. 1026mb surface high over northwestern 
Colorado creating a strong surface pressure gradient between the 
two features. Getting strong wind gusts at Arlington and more 
recently...over the south Laramie Range. Vedauwoo gusting to 
63mph...so went ahead and started the High Wind Warning now for 
the Summit and eastern foothills. 

Craig to Casper 850/700mb height gradients continue to ramp up
this evening...up to 61/56mtrs at 03Z. Random Forest high wind
guidance also showing this happening as well. GFS 700/750mb winds
up to 60kts across the Laramie Range from 03 to 09Z tonight...so
this is when we can expect our strongest winds. Night crew may
need to extend the warning through Saturday morning as
Craig/Casper height gradients remain elevated through 18Z.

Could see a minor strong wind event Saturday night into Sunday as
Craig/Casper gradients begin to increase once again. Random Forest
guidance not particularly pegging on this event at this time. GFS
700/750mb winds though are showing 50-55kts over the wind prone
areas...so chance is there for a short term high wind event.

Strong low pressure system moves into Nevada Sunday afternoon. For
us...Sunday looks warm and breezy. All guidance showing
precipitation remaining west of our CWA.

.LONG TERM...(Monday through Friday)
Issued at 250 PM MST Fri Dec 9 2022

KEY MESSAGES:

1) Strong winter storm remains possible in the long term forecast 
with weather impacts beginning Monday morning and lasting through 
Wednesday. Primary hazards will be heavy snowfall and strong winds.

2) Uncertainty remains high at this time as even a slight shift 
in any direction could drastically change impacts.

3) The period of greatest impacts appears to be early Tuesday 
morning until Wednesday morning across east central Wyoming and 
northern Nebraska panhandle where there will likely be moderate to 
heavy snowfall and strong winds 40 to 50 mph.

DISCUSSION: The primary forecast concern for the long term forecast 
period remains a potential strong winter storm starting Monday over 
western portions of our cwa, lasting through Wednesday. Confidence 
is HIGH that the main hazards with this storm will be heavy snowfall 
and strong winds, however confidence is LOW on where exactly these 
impacts will occur. The location will be heavily dependent on how 
the system sets up synoptically. Model runs have remained relatively 
consistent over the last 24 hours, but one of the more significant 
changes is that the model is pushing the low east faster and sooner.

Tuesday is the likely day of highest impacts due to models showing 
the low positioned right near the NE/CO/KS border injecting deep 
Gulf moisture around the north side of the trough with favorable 
isentropic lift in the Nebraska panhandle, just crossing the eastern 
Wyoming border. The NBM v4.1 has stayed consistent pinging the 
mountains (Snowy, Sierra Madre, and North Laramie Ranges), the Pine 
Ridge area from Chadron to Lusk, and the south side of the North 
Platte River valley as the areas of highest snowfall totals. There 
is a sharp gradient between potential 3 to 5 inches along the WY/CO 
border and 8 to 12 inches in that area just south of the North 
Platte River valley. Outside of the previously named areas, forecast 
snowfall totals remain near or just above ensemble means.

Winter Weather Headlines will likely be coming either tomorrow or 
Sunday depending on how the ensembles and future model runs develop. 
Keeping in mind that this storm is still five days out, stay tuned 
in the coming days for forecast updates!

The arctic airmass stays over southeast Wyoming and the Nebraska 
panhandle through at least the rest of the long term forecast 
period, so temperatures will stay very cold. Especially in areas 
that receive a fresh snowpack, temperatures will be in the teens to 
possibly low 20s in the afternoon and single digits overnight.

&&

.AVIATION...(For the 00Z TAFS through Saturday afternoon)
Issued at 455 PM MST Fri Dec 9 2022

A progressive upper level trough will push across Wyoming and 
western Nebraska this evening. A line of snow showers will continue 
moving east across Carbon and Albany Counties, mainly impacting KRWL 
and possibly KLAR until 03z. Otherwise, a period of gusty westerly 
to northwest winds are expected as this upper level trough moves 
across the region until 10z early Saturday morning.  

HAZARDS/WEATHER TRENDS: A brief snow shower at KRWL this evening may 
reduce VIS below 2 miles at KLAR between 01z and 03z. Conditions are 
expected to improve back to VFR conditions overnight tonight with 
VFR conditions at all other terminals. Very low chance (below 10%) 
for fog around KRWL tonight, so will keep out of the TAF for now. 

Gusty winds up to 35-40 knots are expected with the passage of the 
upper level trough for most locations, earlier (now until 06z) for 
KRWL and KLAR, and a little later this evening (03z to 10z) for KCYS 
and western Nebraska. Some mountain wave activity is possible with 
LLWS mainly at KLAR and KCYS when surface winds become light at 
times.  

&&

.FIRE WEATHER...
Issued at 250 PM MST Fri Dec 9 2022

Strong winds ongoing this afternoon in the wind prone areas of 
southeast Wyoming. Looks like afternoon humidity could become near
critical just east of the Laramie Range in downsloping west winds.
But fuels are not ready and the three hour criteria for low
afternoon humidity not expected. Should see humidity beginning to
rise as temperatures drop. Pleasant weather Saturday and Sunday
with afternoon humidity expected to remain above critical levels.
Major winter storm looking likely Monday through Wednesday next
week with significant snow accumulations for most areas.

&&

.CYS WATCHES/WARNINGS/ADVISORIES...

WY...High Wind Warning until 8 AM MST Saturday for WYZ106-110.

     High Wind Warning until 8 AM MST Saturday for WYZ116-117.

NE...None.
&&

$$

SHORT TERM...GCC
LONG TERM...LK
AVIATION...TJT 
FIRE WEATHER...GCC