081 FXUS65 KRIW 191750 AFDRIW Area Forecast Discussion National Weather Service Riverton WY 1150 AM MDT Sat Oct 19 2019 .SHORT TERM...(Today through Monday) Issued at 437 AM MDT Sat Oct 19 2019 A fall storm system will bring significant snowfall across much of western Wyoming today through Sunday morning. The heaviest snowfall will occur this afternoon through around midnight tonight. Very windy possibly high wind is possible tonight and/or Sunday across N. Johnson and Sweetwater County. A fast moving trough early this morning is pushing into Pacific Northwest. This will pull east southeast into the Intermountain West today, and then across the northern Rockies tonight. Models continue to show as this trough jumps the Divide late tonight, it will slow down with a mid-upper level low developing. By 00Z Monday, this upper low is progged to be across central South Dakota/ central Nebraska area. Although there are isolated snow showers over the west now, the onslaught of the main precipiation shield looks to hold off until around 18Z today. The 700 MB flow this morning backs to the southwest limiting isentropic upglide/orographic and overall chances of snowfall. Considered backing off the timing of winter highlights to 18Z, but kept them unchanged. By midday into the afternoon the overall synoptic lift with this system will be overspreading the forecast area. Models show strong QG forcing and upper-level divergence. This combined with the dendritic growth zone around 50 MB in depth should result a 6 to 12 hour period where snowfall should be heavy. Snowfall rates could approach 1 inch. There is also the slight possibility of thunder snow as there are steep lapse rates and weak instability. Snowfall rates impacted by the convective element could see a burst of snow with rates of 2+ inches/hour. The strong dynamics can result in the differences in snow amounts between valley and mountains not being as stark. However temperatures in the 30s today and a warm ground in the western valleys will limit snow accumulations through late afternoon. The exception will be in bursts of heavy snow which will likely result in roads in the valleys becoming briefly slick before melting occurs. East of the Divide, and across Sweetwater County will be mainly dry with windy southwest winds as a lee, surface trough develops. The strong cold front with this system will push into western Wyoming this afternoon and sweep east across the forecast area tonight. Very windy west to northwest will occur in wake of the cold front. Have elected to issue a high wind watch for N. Johnson County late tonight into Sunday. In wake of the cold front in that area there is a combination of strong pressure rises, 700 MB northwest winds near 50 KT and the overall favorable northwest winds for N. Johnson County. There is the potential of some snow, but the overall hazard looks to be winds as temps look to be in the lower to mid 30s. Thought about hoisting a winter weather advisory for Sweetwater County with the combination of light snow and very windy westerly wind, but snow amounts look to be under an inch. Will let day shift look at again to see if an advisory is warranted. The snow east of the Divide tonight into Sunday look to be relatively disorganized with the best chances of snow over Johnson and Natrona. Sunday afternoon snow showers are expected which could result in brief heavy snow with models showing Casper being hit by this activity. Snow should gradually end Sunday evening over the east, but a favorable northwest flow will keep chances of snow over the west and northwest Sunday night and Monday. .LONG TERM...(Monday through Saturday) Issued at 437 AM MDT Sat Oct 19 2019 Northwest flow across the region will keep showers mainly over the northwest and extreme north Monday night into Tuesday. There will be gusty winds in the usual wind prone areas on Tuesday, with temperatures a bit warmer. On Wednesday a new weather system will move across the area. With this trough and cold front, the showers in the northwest will spread further south into portions of central Wyoming and stick around through Thursday. Gusty winds will also accompany this next system across much of the area. Thursday looks to be the coldest morning with the models showing H7 temperatures at -8 to -14 C for surface temps in the teens and low to mid 20s. Friday should be cold as well, however a dry day. && .AVIATION...(For the 18Z TAFS through 18Z Sunday afternoon) Issued at 1137 AM MDT Sat Oct 19 2019 West of the Continental Divide: Precipitation is starting to move into far western WY currently, and will overspread through the rest of the day. MVFR conditions will develop as the precipitation moves in, with -SN reaching JAC soon, and remaining sites late this afternoon. Have opted to include tempo groupings for snow, as precipitation will not be consistent, but will have a good breeze blowing it around. All precip should be done by sunrise Sunday, except for lingering SHSN at KJAC. Very gusty wind expected at KRKS during the day Sunday, with some gusts near 40kts. East of the Continental Divide: For the most part, VFR conditions will continue through the period. Still expecting gusty winds to develop this afternoon and will likely continue through the night as the next storm system sweeps through. Some scattered shower activity may briefly bring down vis after midnight, due to light snow blowing around. Breezes continue into the day Sunday for all areas. && .FIRE WEATHER... Issued AT 437 AM MDT Sat Oct 19 2019 Significant snowfall is expected over much of western Wyoming through noon Sunday with the heaviest snowfall this afternoon through midnight tonight. Mainly dry conditions over the south and areas east of the Divide with windy southwest flow mainly between Rock Springs and Casper. Critical fire weather thresholds will not be met as min RH will be in the mid 20s to near 30 percent. A strong cold front will push across the area tonight with precipitation spreading across the south, as well as some areas east of the divide. Precipitation could begin as rain, but should transition to snow quickly after frontal passage. Very windy west to northwest wins are expected in wake of the cold front lasting through the day Sunday. && .RIW WATCHES/WARNINGS/ADVISORIES... Winter Weather Advisory until noon MDT Sunday for WYZ001-002- 013>015-023-024. Winter Storm Warning until noon MDT Sunday for WYZ012. High Wind Watch from late tonight through Sunday afternoon for WYZ010. Winter Weather Advisory from 3 PM this afternoon to noon MDT Sunday for WYZ027. Winter Weather Advisory from midnight tonight to 6 PM MDT Sunday for WYZ008-009. && $$ SHORT TERM...Murrell LONG TERM...Swanson AVIATION...Straub FIRE WEATHER...Murrell