Area Forecast Discussion
Issued by NWS Riverton, WY
Issued by NWS Riverton, WY
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525 FXUS65 KRIW 170419 AFDRIW Area Forecast Discussion National Weather Service Riverton WY 1019 PM MDT Sun Jun 16 2024 .KEY MESSAGES... - A cold front pushes across the forecast area Monday increasing the chance for precipitation, especially across the north. Gusty west to northwest wind follows the front along with much cooler temperatures. - The colder air leads to light snow over the northwest corner of the state Monday night, including lower elevations of Yellowstone. Sub-freezing temperatures look likely for the western valleys/basins. - After an unseasonably cool Tuesday, temperatures begin to rebound Wednesday and climb to above normal Thursday and beyond. && .DISCUSSION... Issued at 100 PM MDT Sun Jun 16 2024 Mid and high clouds have persisted over the region through early afternoon Sunday aided by jet-level energy. Dry conditions have prevailed as stable conditions and limited moisture have been unfavorable for any convective development. Instability finally increases late Sunday afternoon in Natrona and Johnson Counties were surface dewpoints around 40 are at least a tad more favorable. Any convection that develops will be short-lived and headed eastward by midnight. Elsewhere, cloud cover will be on the increase overnight as a shortwave in the Pacific Northwest begins an eastward trek toward Wyoming. Seasonal overnight temperatures are in store. The approaching shortwave and associated cold front reach far west Wyoming by midday Monday. Pre-frontal gusty southwest wind ramps up in advance of the front during the late morning. Surface winds veer to the west and northwest following frontal passage. Jet streak dynamics and increasing mid-level moisture favor a steady increase in rain shower coverage across the northwest beginning mid-morning Monday, spreading across mainly the northern-third of the region through the afternoon. The best instability and strongest convection looks to be across Johnson County late Monday afternoon, although all areas east of the Continental Divide are susceptible to isolated thunderstorms. While western locales trend much cooler Monday, areas of central and southwest Wyoming will still see seasonal temperatures ahead of the cold front. Scattered showers continue across the north and east through Monday evening as the front progresses east. Attention then shifts to wrap- around moisture and cold air in northwest flow moving into the western mountains and Yellowstone Monday evening through at least sunrise Tuesday. Mid-level temperatures around -6C/-7C favor snow levels falling to the valley floors in Yellowstone. Despite recent warm weather, there is a 50-60 percent chance of light snow accumulations up to around one inch above 7000 ft MSL. The Tetons and northwest mountains above 9000 ft have a 60-80 percent chance of 2 to 4 inch accumulations, but a less than 30 percent chance of 6 inches or more by sunrise Tuesday. Cloud cover plays a role in how cold the western valleys and basins get Monday night. Cloud cover should be less extensive in Star Valley, south Lincoln County, and the Upper Green River Basin, allowing for better radiational cooling. HREF plots shows 29-32F by sunrise Tuesday for these areas and Jackson Hole. This is a tad warmer than what was seen yesterday, so confidence in a hard freeze <28F is not as high. The better chance for lows <28F will be at Old Faithful and Canyon in Yellowstone. Light orographic-driven snow showers continue much of Tuesday over the northwest mountains with scattered rain showers across lower elevations of the north. Daytime highs Tuesday will be 8 to 15 degrees below normal. Temperatures across the western valleys and basins will again be cold Tuesday night given less cloud cover. Locations east of the Divide mainly see lows in the lower 40s. Weak ridging begins to build back into the region Wednesday as the early week trough lifts to the northeast. Temperatures rebound closer to seasonal readings and many areas will be dry. A warming trend continues into next weekend with above normal temperatures beginning Thursday. A general westerly flow aloft will favor late-day convection along and east of the Divide each day. && .AVIATION /06Z TAFS THROUGH 06Z TUESDAY/... Issued at 1014 PM MDT Sun Jun 16 2024 VFR conditions are expected through tonight with lingering midlevel clouds for many terminals. A cold front will begin to push into western Wyoming around 18Z Monday. Showers will impact the KJAC area first, with lower chances of showers and thunderstorms then spreading east through the rest of the afternoon and evening. The best chances for direct terminal impacts and possibly MVFR conditions will be at KJAC and KCOD, with lesser chances at KCPR and KWRL. Otherwise, wind will also be an aviation concern ahead of and behind the front, with gusts of 25 to 35 knots fairly widespread through Monday evening. Please see the Aviation Weather Center and/or CWSU ZDV and ZLC for the latest information on smoke, icing, and turbulence forecasts. && .RIW WATCHES/WARNINGS/ADVISORIES... None. && $$ DISCUSSION...Jones AVIATION...Myers