Area Forecast Discussion
Issued by NWS Riverton, WY
Issued by NWS Riverton, WY
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209 FXUS65 KRIW 022155 AFDRIW Area Forecast Discussion National Weather Service Riverton WY 355 PM MDT Sun Jun 2 2024 .KEY MESSAGES... - A plume of Pacific moisture impacts western Wyoming Monday. Precipitation totals of one-third to two-thirds of an inch will be common in the far west mountains by sunset Monday. - A ridge of high pressure will dominate the western United States for the latter half of the coming week. Temperatures will be about 10F degrees above normal, with widespread lower elevation temperatures in the mid 70s to upper 80s. - The above normal warmth allows for continued melting of the mountain snowpack. The most notable rises are expected in smaller tributaries of the Snake River and smaller creeks and streams fed by snowpack in the northern Bighorn Range. && .DISCUSSION... Issued at 100 PM MDT Sun Jun 2 2024 Weak troughing is across the forecast area early Sunday afternoon. The coverage of showers and isolated thunderstorms becomes scattered as instability increases a tad this afternoon in the heat of the day. The far north and far south are favored areas given available moisture and ongoing activity. Peak convective coverage is expected between 3pm and 7pm Sunday with a rapid decrease by sunset. Outflow wind of 30 to 40 mph would be the primary hazard with any convection. The upper-level flow then backs more to the west and southwest overnight in response to a longwave trough along the Pacific Northwest coast. This trough ejects a more vigorous shortwave toward Wyoming and the Northern Rockies Monday and Monday night. A healthy dose of precipitable water feeds eastward into the western portion of the state 12-15Z/Monday and persists for about 9- 12 hours. QPF of one-third to two-thirds of an inch will be widespread in the western mountains. The orographically-favored Teton Range and southwest Yellowstone National Park have about a 60 percent chance of QPF exceeding one inch over the 24-hour period ending 6pm Monday. Scattered showers spread east of the Continental Divide Monday afternoon and evening, with the best chance coming over the far north and the I-80 Corridor through southwest Wyoming. Precipitation chances gradually diminish Monday evening as the shortwave tracks eastward through Montana and drier air arrives from the west. Showers do linger over the far north through much of Monday night. As for temperatures, readings will be cooler Monday, particularly across the far west given precipitation and cloud cover. The shortwave will be near the Montana/Saskatchewan border by 12Z/Tuesday. Cyclonic flow lingers over the northern-third of Wyoming on the backside of this system through Tuesday morning allowing for a few showers to persist over the northern mountains. Moisture and dynamics wane Tuesday afternoon with west-northwest flow aloft leading to dry and and breezy conditions regionwide as the shortwave tracks northeast. West to northwest surface wind 15 to 30 mph will be common by mid afternoon. Tuesday night sees a ridge building in from the southwest with a clear sky and slightly above normal temperatures on tap. Wednesday finds a dominant ridge setting up over the Great Basin with the axis extending north to the Idaho Panhandle. Temperatures rise to around 10F above normal Wednesday, a trend that continues through at least Friday as the ridge remains in place. Models have been consistent in advertising sub-tropical moisture emanating from a weak low off the Baja coast rotating north around the backside of the mid-level high by Friday. Blended model solutions have responded by gradually increasing the chance of precipitation for Friday into the weekend. This probably leads to another round of high-based convection, potentially beginning as early as late Thursday across southwest Wyoming. A look at SNOTEL data indicates many mountain locations have lost 4 to 6 inches of snow water equivalent (SWE) over the past five days. Given these trends and the ripening of the snowpack, the expectation is that much of the mountain snowpack below 9,500 feet will melt by the end of this period. The exception will be the far west where even at 8K feet there was still 8 to 12 inches of SWE in the northern Salt/Wyoming Ranges and the mountains of Teton County. The relatively warm airmass and slug of Pacific moisture Monday could also lead to rain-on-snow in the 8-10K range of the western mountains. Smaller tributaries of the Snake are the most susceptible to minor flooding over the coming several days. There is a good bit of capacity remaining in waterways east of the Continental Divide, but a loss of 2 to 3 inches of SWE a day can quickly change the situation. Smaller tributaries of the west slope of the northern Bighorn Range will be monitored given that approximately 12 inches of SWE remains. && .AVIATION /00Z TAFS THROUGH 00Z TUESDAY/... Issued at 355 PM MDT Sun Jun 2 2024 The few remaining showers around the area should dissipate the first couple hours of the period. Winds also decrease this evening and into the overnight hours. A weather system approaches western Wyoming overnight, with increasing precipitation chances for KJAC starting around 12Z/Mon. KBPI and KPNA see rain chances increase after 12Z/Mon, with the best chances by around 18Z/Mon. Chances spread eastwards during the afternoon. Chances remain 20% or less for locations east of the Divide based on the latest models. Winds also increase ahead of this system, with gusts 20 to 25 knots before 18Z at many sites. Windiest locations will be KCPR and KRKS during Monday afternoon, with gusts over 30 knots. Please see the Aviation Weather Center and/or CWSU ZDV and ZLC for the latest information on icing and turbulence forecasts. && .RIW WATCHES/WARNINGS/ADVISORIES... None. && $$ DISCUSSION...CNJ AVIATION...Wittmann