Area Forecast Discussion
Issued by NWS Riverton, WY
Issued by NWS Riverton, WY
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863 FXUS65 KRIW 041943 AFDRIW Area Forecast Discussion National Weather Service Riverton WY 143 PM MDT Sat May 4 2024 .KEY MESSAGES... - Breezy afternoon for some, but otherwise warm and sunny today. - Impactful storm starts Sunday. Gusty winds 35 to 50 mph, with 60+ possible for some locations. A High Wind Watch/Warning is in effect. - Precipitation moves into western Wyoming Sunday. A Winter Storm Watch is in effect for the Tetons and Yellowstone. Precipitation becomes widespread across the area by Monday. - Unsettled and windy workweek. Uncertainty in snow and rain amounts Tuesday and beyond. && .DISCUSSION... Issued at 142 PM MDT Sat May 4 2024 Not much to note about today (compared to what`s coming). Temperatures are notably warmer today, with many places peaking in the 60s; a few low 70s as well today. Besides some high clouds passing through, the sky remains mostly clear as well. A breezy southerly wind develops this afternoon for some locations, with gusts 15 to 30 mph, nothing too unusual for a Wyoming spring day. Current water vapor imagery shows the next approaching weather system, a Pacific low centered just west of Oregon. This will be the weather-maker over the next week. The low makes landfall tonight, its center moving towards Wyoming. As it does so, it tightens the pressure gradient, and brings a 100 knot jet. Southerly winds begin to increase after sunrise Sunday, first impacting the higher elevations and southern Sweetwater County. By mid-morning, gusts in excess of 50 mph are likely (70%) for portions of Sweetwater County into southern Fremont County as the southerly flow increases. By afternoon, gusts 30 to 50 mph will be widespread for much of the area, including the Green River Basin, the Wind River Basin, portions of Natrona and Johnson Counties, and possibly into the Bighorn Basin to name a few. Gusts for southern Sweetwater County are likely (80%) to be in excess of 58 mph. During peak afternoon heating and mixing, high wind is possible across portions of Sweetwater, Fremont, and Natorna Counties; a High Wind Watch has been issued for the applicable locations. Sweetwater County has been upgraded to a Warning given better confidence there. Shift of Tails continues to highlight southern Sweetwater County in the 90th percentile and a tongue of 80th+ percentiles for the locations just mentioned. Of note is the Lander foothills, where high winds are possible in the afternoon, as well as the evening, as gusts begin to turn southwesterly. Another point of interest is Thermopolis, where downsloping winds could reach up to 50 mph given the favored direction. Expect travel impacts to I-80 and South Pass/Red Canyon as strong crosswinds occur. Winds decrease some Sunday night as mixing lessens. Winds also shift southwesterly as the low progresses eastward. Monday morning, winds begin to increase again. By late morning, high winds (gusts over 58 mph) are again possible (70%) for areas of Sweetwater County, southern Fremont, and into Natrona County (Wind Corridor). These winds should continue to around sunset Monday. The High Wind Watch extends into Monday for this reason. As mentioned, it`s important to note that high winds are not likely (greater than 90%) to occur overnight Sunday into Monday. Winds pick up again for Tuesday. Currently not seeing indication of high winds, as 700mb winds are a bit lower (30 to 40 knots, rather than 50 to 60) and wind direction is a bit more westerly (not as favored). Moderate confidence on this at this time. Taking a step back in time now, the other element to this system is precipitation. Starting on Sunday, precipitation chances move into far western Wyoming by 12Z (6am). Chances spread eastward through the day, but should mainly impact locations west of the Continental Divide Sunday. Not much mountain snow is expected through the morning and afternoon as the best dynamics are slower to approach. By later afternoon and evening, a 100 knot jet passes through, bringing the left exit region dynamics to the area. This should enhance snowfall for the mountains of western Wyoming Sunday night. Through the day Monday, as the low tracks eastward, favored westerly flow and ample moisture enhances snow further. Heaviest snow occurs Monday morning and afternoon for the Tetons and southern Yellowstone, and to a lesser extent, the Salt/Wyoming Ranges. Snow continues into Tuesday. Wrap around moisture brings another better wave of moisture to these same locations Tuesday morning and afternoon. Confidence is lower in this given uncertainty in the low`s track. A Winter Storm Watch is in effect for the Tetons and Yellowstone Sunday evening through Tuesday night to cover this prolonged snow and two waves of better snowfall. Through Tuesday night, up to 2 feet of snow is possible for the higher elevations of the Tetons. A foot to 20 inches for other Teton locations and southwest Yellowstone. Up to 12 inches for other western mountain locations. Again, it should be noted there is less confidence in snow amounts for the latter half of this system, so totals are subject to change. Moving onto precipitation east of the Divide. Instability east of the Divide could bring an isolated (15% or less) shower, or even thunderstorm, Sunday afternoon. Better precipitation chances move in Sunday night. Snow levels drop to around 6000 feet, so mainly rain is expected for the lower elevations. Just before sunrise, however, the cooling temperatures overnight may bring snow levels a bit lower, so a few flakes could be seen early Monday morning. Widespread rain chances through Monday, best over the foothills and mountains where elevation will enhance the showers. Rain chances continue Tuesday as the low is slow to move, stuck by a ridge over the eastern United States. Moisture continues to get pumped in through the start of the week. Favored locations will be over the northern half of the state. The best place for moisture will be the Bighorn mountains and into Johnson County due to the upper-level low location. Models are still inconsistent in the placement of this low, which will have great impacts on the total moisture into the area, specifically the Bighorn Mountains and Johnson County. Dry slot placement and proximity to the low will effect precipitation totals. There is confidence in the Bighorns getting some good moisture, at least initially, so have issued a Winter Storm Watch for the snow in the initial round. Low confidence in the storm`s track into Tuesday, so difficult to forecast snow amounts then and beyond. The low becomes almost stationary Tuesday and Wednesday. This is why the storm`s track is so important, because it will determine where the best moisture, and lack of, occurs these couple days. In any case, the entire area sees cooler temperatures and unsettled conditions through next week. There is some model indication for the low finally moving eastward Thursday, but lots of variance in models keeps confidence very low. Too much variance for Friday and beyond, with the potential for either a continued unsettled pattern or high pressure building in. Cluster analysis agrees with this assessment, showing uncertainty in 500mb heights on Day 7 over the western United States. && .AVIATION /18Z TAFS THROUGH 18Z SUNDAY/... Issued at 1235 PM MDT Sat May 4 2024 VFR conditions will prevail through this evening, with clear ceilings and wind generally remaining on the lighter side. An incoming system will begin to bring showers to western Wyoming after 06Z Sunday. Have introduced precipitation impacts to KJAC, KBPI, and KPNA late in the period as this activity moves east. There is some indication from hi-res models on showers also moving into the Bighorn Basin during this time, but low confidence on this precludes mention at KCOD or KWRL for now. Otherwise, wind will begin to increase for most terminals late Sunday morning as the system approaches. 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... Winter Storm Watch from Sunday evening through late Tuesday night for WYZ001-012. Winter Storm Watch from Monday morning through Tuesday morning for WYZ008-009. High Wind Watch from Sunday morning through Sunday evening for WYZ018. High Wind Watch from Sunday morning through Monday evening for WYZ019-020. High Wind Warning from 9 AM Sunday to 9 PM MDT Monday for WYZ028>030. && $$ DISCUSSION...Wittmann AVIATION...Myers