243 FXUS65 KRIW 310504 AFDRIW Area Forecast Discussion National Weather Service Riverton WY 1004 PM MST Tue Dec 30 2025 .KEY MESSAGES... - Another cool night for western Wyoming valleys, with patchy fog again possible due to efficient cooling of the near surface air layer. - Above normal temperatures are forecast for New Years Eve and New Years Day (10 to 15 degrees above normal). - Dry weather tonight through tomorrow (Wednesday), with a storm system bringing light to moderate rain and snow (above approximately 7000 ft elevation) to western and southern Wyoming Thursday into Friday. Several inches of snow are expected for the western WY mountains. && .UPDATE... Issued at 1250 PM MST Tue Dec 30 2025 Only minor updates with the forecast as relatively quiet weather continues over western and central WY today into Wednesday. Some pesky shallow fog did grace itself over the northern portions of Jackson Hole this morning and briefly impact visibilities at Jackson airport. The fog is quickly dissipating this afternoon over Jackson Lake with daytime heating and full sunshine with the area of high clouds moving southeast. Mostly clear skies will be over the forecast area late this afternoon and evening as the area of clouds exits the state and upper-level high pressure builds over the Great Basin. With the ridge axis stretching from the Four Corners area toward west-central Idaho, decent surface high pressure will also build over northern Utah, keeping a pressure gradient directed SW to NE across the state. As a result breezy and gusty southwest winds across central WY and the eastern foothills of the mountains ranges will continue overnight and into Wednesday. Relatively light winds will occur elsewhere, which may or may not give rise to more shallow fog in the western WY valleys under steep inversions (can you hear my uncertain tone?). With the mostly clear skies on Wednesday, max temperatures will increase to the low 40s to low 50s, which is 10 to 15 degrees above normal, continuing the warming trend for December. Clouds will increase across southwest WY Wednesday late afternoon and overnight, providing cover and expectantly warming low temperatures in the cold western valleys. With the latest model runs, precipitation should start across western and southwest WY by sunrise Thursday morning, with snow in the mountains but a rain/snow mix in the lower elevations since 700mb temps will be 0 to -2 C. The Thu-Fri system should bring 4-6 inches of snow to the Tetons and Salt Range, with less than half an inch in the valleys. && .DISCUSSION... Issued at 151 AM MST Tue Dec 30 2025 Quiet weather is expected through the end of the year. A ridge situated over the southwestern US brings mild temperatures and relatively light winds to the area the next couple days. Highs today are in the mid-30s to low 40s for much of the area, which are 5 to 10 degrees above normal for the end of December. Have dropped the western valley temperatures slightly, as has been the trend over the last couple days. The next weather system of any note arrives as early as Thursday morning, with increasing clouds and precipitation from the west- southwest. This is in response to an upper-level low entering into southern California. However, this system gets absorbed into the flow and does not have any indications of anything potent. Light snow will be possible over the western and southwestern mountains as early as Thursday morning, but more likely later Thursday into Friday morning. Snow levels of 6500 to 7000 feet means light rain is possible for lower elevations. High pressure returns after this quick system, so temperatures won't fluctuate much with this disturbance. Looking into next weekend, a trough to the west could bring some gusty winds as the pressure gradient tightens, but there is weak, if any, evidence of strong winds (55 mph or greater) at this time. With the ridge still in place, and some winds to mix the air, temperatures continue to warm, with currently forecasted highs (mid-40s to mid-50s) hovering near daily records to start January. && .AVIATION /06Z TAFS THROUGH 06Z THURSDAY/... Issued at 1000 PM MST Tue Dec 30 2025 VFR conditions are forecast for most locations over the next 24 hours, with high clouds moving into the region by Wednesday evening ahead of the next more impactful round of aviation weather. KJAC is the one TAF site that could see intermittent MVFR or even IFR conditions if patchy shallow fog develops again on Wednesday morning similar to Tuesday morning. Not much has changed weather pattern-wise (with strong ridging aloft and nearly calm surface winds), and in such cases, persistence (the same weather as the previous day) is often times a good forecast. Satellite imagery will be monitored overnight, with a strong hinting of fog in the current 06z TAF. Elsewhere, minimal impactful aviation weather is expected with only minor wind shifts throughout the next 24 hours. && .RIW WATCHES/WARNINGS/ADVISORIES... None. && $$ UPDATE...McDonald DISCUSSION...Wittmann AVIATION...VandenBoogart