Area Forecast Discussion
Issued by NWS Riverton, WY
Issued by NWS Riverton, WY
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103 FXUS65 KRIW 162200 AFDRIW Area Forecast Discussion National Weather Service Riverton WY 400 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 /00Z TAFS THROUGH 00Z TUESDAY/... Issued at 400 PM MDT Sun Jun 16 2024 Mostly VFR conditions will prevail through the forecast period. Breezy winds will diminish by 02 to 04Z this evening. Cloud cover will increase Monday morning the period as a cold front approaches the region. Ahead of this cold front, southwest winds will also begin to increase. The cold front will begin to move into the west by around 18Z Monday, with rain showers and MVFR conditions expected to occur at KJAC around 18Z. KJAC may see IFR conditions (30% chance) by later in the afternoon. KCOD may also see rain showers late in the period, but hi-res models are still seeing inconsistencies in timing of the heavier precipitation with the cold front. Have included MVFR with -RA at 20Z for now, but only 40% confidence in MFR conditions at this time. There is a less than 25% chance of showers at all other terminals. With the frontal passage, winds will also continue to increase. Gusts 30 to 40kts will be possible at KCPR and KRKS, as well as 20% chance of gusts exceeding 30kts at KBPI, KPNA, KLND, and KRIW through the afternoon. 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...CNJ AVIATION...Hensley