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