486 FXUS65 KRIW 062007 AFDRIW Area Forecast Discussion National Weather Service Riverton WY 207 PM MDT Wed Oct 6 2021 .SHORT TERM...(This evening through Thursday night) Issued at 121 PM MDT Wed Oct 6 2021 South-southwesterly flow aloft has caused widespread cloud cover today as increased upper-level moisture streams northward. A shortwave trough moving northward in the larger scale pattern will provide modest lift, leading to scattered afternoon and evening showers. These showers will be primarily confined to the Upper Green River Basin and locations west of the Divide, where dynamics and moisture is most abundant. A tenth to a quarter inch of rain is possible in this region. A few weak thunderstorms are also possible this afternoon, as high-resolution model consensus predicts about 100 to 250 J/kg of CAPE for this region. East of the Divide, the low levels remain fairly dry, making rainfall less likely. A few of the scattered virga showers could make it to the ground, leading to isolated light rainfall. Surface winds today will be breezy, especially in the wind corridor from Rock Springs to Casper to Kaycee WY. Gusts 30 to 40 mph will be common through 8 PM MDT. Temperatures will be near normal west of the Divide, and about 5 degrees above average east of the Divide where less low cloud cover is present. Cloud cover continues overnight, but a cooler air mass and less wind leads to early AM temperatures about 10 to 15 degrees cooler than the mild, summer-like Wednesday morning temperatures. The western Wyoming mountain ranges are the main focus area for another round of showers Thursday morning into the early afternoon. Later in the afternoon and evening these showers move eastward, eventually leading to scattered showers for a majority of western and central Wyoming. Temperatures Thursday are likely to be a few degrees cooler than on Wednesday, as the longwave trough over the Pacific Northwest slowly edges toward the Rockies. Snow levels remain 10 to 11 thousand ft AMSL. Showers continue into early Friday morning. .LONG TERM...(Friday through Wednesday) Issued at 205 PM MDT Wed Oct 6 2021 Strong moist southwest flow continues on Friday as a weak disturbance moves from NW WY into SE MT. Models are showing a brief respite of precipitation across the northwest part of the state as the core of the trough digs in central California. Incoming flow will become more southerly as a result and precipitation increasing across all of western Wyoming during the afternoon and evening. Precipitation is expected to continue into the evening and cross over the divide during the evening hours for widespread rain in the lower elevations, and snow across the northwest mountains. Both the NAM and GFS indicate a ripple in the 500mb heights on Saturday morning across northern WY with the moisture axis stretching from SE MT to west-central Nevada. The mean trough stays over the forecast area as the surface low moves into the Dakotas, keeping southwest flow going overnight through the wind corridor. The Bighorns and Johnson County should continue with precipitation with wrap around on the wave with scattered showers elsewhere. Weak ridging pushes into western WY late Saturday and into Sunday, though temperatures will be on the cool side with 700mb temps between +2C and -2C. By Sunday night though, moisture returns to northwest WY due to a deep trough moving into the Pacific Northwest and again pushing moist southwest flow ahead of it. Models are still showing a significant storm shaping up from late Monday through Wednesday. The MSLP low over SE WY is expected to deepen below 1000mb creating strong wind late Monday. Meanwhile, a cold front will push into western WY late Monday, with northerly flow also pushing into the Bighorn Basin. Western WY will see rain changing to snow Monday night as snow levels drop. With the surface low deepening over NE Colorado, moisture and cold on the NW side of the system will push over the forecast area to bring widespread precipitation from early Tuesday into Wednesday. The GFS is currently being more progressive and eastward with the system than the ECMWF, but either solution looks to bring a very strong fall storm to the area. Temperatures will aid in snow falling in the lower elevations though melting will occur in the days to follow. && .AVIATION...(For the 18Z TAFs through 18Z Thursday) Issued AT 1140 AM MDT Wed Oct 6 2021 Low pressure system moving from northeast Utah into south central Wyoming today, increasing moisture and clouds with some light rain showers already this morning. VFR conditions will generally prevail, though likelihood of MVFR increases under and around storms as isolated thunderstorms develop across SW WY after 06/20Z. Gusty south-southwest wind will continue for most locations this afternoon through about 07/02Z. Most shower activity will remain past sunset but diminish by 07/06Z. Higher terrain to remain mostly obscured throughout the next 24 hours. Another disturbance moving from Idaho to Montana will bring showers back to western WY before sunrise Thursday. KJAC, KBPI, and KPNA should see occasional MVFR before 07/18Z, and KJAC could see MVFR during the afternoon. South- southwest wind returns by 07/18Z as well at all sites. Scattered showers continue over NW WY Thursday afternoon, with isolated showers and thunderstorms possible from SW WY northeast through central WY. Please see the Aviation Weather Center and/or CWSU ZDV and ZLC for the latest information on smoke, icing, and turbulence forecasts. && .FIRE WEATHER... Issued AT 121 PM MDT Wed Oct 6 2021 Fire weather concerns are limited over the next several days, as moisture moves in from the southwest, leading to cloudier, cooler, and wetter conditions across the region. Minimum RHs today should remain in the 30 to 50 percent range for the low elevations, with the higher values west of the Divide. Winds will be breezy, especially from the Rock Springs to Casper WY wind corridor, where gusts 30 to 40 mph are likely this afternoon. Scattered showers develop this afternoon, with the focus for wetting rainfall over southwest to south central Wyoming. Another round of showers are possible Thursday, this time with western mountains being the focus for wetting rains. Temperatures cool by a few degrees and winds will be lighter compared to Wednesday. Widespread wetting rains are expected across western and central Wyoming Friday night into Saturday. Temperatures are forecast to have a general cooling trend over the weekend, reaching well below seasonal normals by the first part of next week. && .RIW WATCHES/WARNINGS/ADVISORIES...NONE. && $$ SHORT TERM...VandenBoogart LONG TERM...McDonald AVIATION...McDonald FIRE WEATHER...VandenBoogart