Area Forecast Discussion
Issued by NWS Riverton, WY
Issued by NWS Riverton, WY
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929 FXUS65 KRIW 210401 AFDRIW Area Forecast Discussion National Weather Service Riverton WY 1001 PM MDT Mon May 20 2024 .KEY MESSAGES... - Wet and cool today and tomorrow. Highest rain totals east of the Divide. - Mountain snow, heaviest Monday night through Tuesday morning. Expect winter travel conditions along the mountain passes. - An active pattern continues through the rest of the week. A couple more weather systems impact the area through the end of the week and this weekend.&& .DISCUSSION... Issued at 1259 PM MDT Mon May 20 2024 A trough centered over Idaho is currently moving eastward. This is the main weather driver today and tomorrow, as it crosses through Wyoming. Today, noticeably cooler temperatures have already made their way into the state. Highs peak in the 50s for most, or about 10 to 15 degree below normal for mid-May. Besides the temperatures, precipitation has also moved in the area. Current radar shows a swath of rain extending from southwest Sweetwater County into Natrona County. This is where the best rain occurs today, but rain chances spread to much of the area through the afternoon. The last place to get rain chances is western Wyoming. In summary, everywhere has a shot to see rain today, with some thunder possible. For the mountains, snow levels are 7000 to 8000 feet today, with light snow accumulation for most of the higher elevations. Up to a few inches is possible for the highest terrain for the Absarokas, Bighorns, and Winds through this afternoon. The trough moves eastward tonight, centered over Wyoming. This brings a more enhanced period of precipitation Monday night through early Tuesday afternoon, primarily focused over locations east of the Continental Divide. Latest models have the low slightly northward of previous runs, influencing locations of highest rain/snow totals slightly. Starting with the lower elevations, rain totals Monday through Tuesday are highest across Fremont, Natrona, and Johnson Counties. Totals here are roughly 0.50 to almost an inch. HREF indicates about a 20% chance for locations to exceed an inch. A few locations may get repeated heavier showers resulting in over an inch, but for the most part, amounts are less than an inch. There is a bit uncertainty with amounts towards Natrona and Johnson Counties, with some models showing a heavier band of rain in that area. This would result in higher totals than currently forecast. Elsewhere, generally under half an inch is forecast through Tuesday, with a few spots of higher totals possible for Sweetwater County. For the mountains, the highest snowfall will occur where the best, favored upslope occurs. For the projected track of the trough, this is the eastern sides of the Absaroka, Bighorn, and Wind River Mountains. Due to the track looking to be a touch more north, amounts in the Absarokas and Winds have decreased a little since yesterday. The eastern Wind River Range could see up to 10 inches of snow. For the Absarokas, mainly the southern range will get the better snow amounts, with a little over a foot for the higher elevations. The Bighorns are a bit higher due to favored flow, with near a foot and half. Looking at impacts, expect winter travel conditions on mountain passes late Monday through Tuesday, the heaviest snow falling Tuesday. Winter weather Advisories are in effect for the Wind River and Absaroka mountains. Heaviest snow falling during the day will somewhat limit road impacts. A Winter Storm Warning is in effect for the Bighorns for the higher forecasted amounts. Precipitation comes to an end Tuesday evening as the trough exits to the east. Tuesday night/Wednesday morning will be cold, with forecast lows in the lower 30s. For locations that have started their growing season, like the Bighorn Basin, freezing temperatures are possible. Though the current forecast is for many locations there to be around 34 or 35 degrees, cold prone/low areas could be a few degrees cooler. Right near the surface also tends to be colder than the forecasted temperature due to radiational cooling, so plants in the ground may develop frost. A Freeze Watch is in effect for the Bighorn Basin Tuesday night/Wednesday morning to cover this potential. Slight, and brief, ridging builds for Wednesday. This should help bring temperatures back towards normal. The next weather system, however, is quick to approach. Rain/mountain snow chances increase across western Wyoming as early as the afternoon. This upper low`s main impact comes Thursday/Friday and looks to move through quickly. Temperatures drop a little during its passage. Favored location for precipitation is northern Wyoming, but given we are still a few days out, have at least a small chance everywhere in the forecast. Winds also increase across the area these days, strongest across the Wind Corridor. The weekend stays active, with yet another low impacting the area Saturday, potentially into Sunday. Past that, it looks like ridging may be in the cards, but its difficult to give confidence 9 days out. && .AVIATION /06Z TAFS THROUGH 06Z WEDNESDAY/... Issued at 959 PM MDT Mon May 20 2024 West of the Divide...KBPI/KJAC/KPNA/KRKS Terminals A slow moving weather system will pivot through the region over the next 24 hours, which will bring rain chances to KRKS/KPNA Tuesday afternoon. At KJAC/KBPI mostly dry conditions are expected, but an isolated rain shower is possible (20-30%) Tuesday afternoon. Westerly/northwesterly wind will increase Tuesday afternoon and gusts will be 20-25 knots through the afternoon hours, quickly fading Tuesday evening. Overall conditions will remain VFR, but a drop to MVFR is likely (80%) if a heavier shower moves directly over a terminal. Mountaintop obscuration will be prevalent through tuesday afternoon. East of the Divide...KCOD/KCPR/KLND/KRIW/KWRL Terminals A slow moving weather system will pivot through the region over the next 24 hours, which will bring rain to all terminals tonight into Tuesday morning. Steady rain tonight will give way to rain showers late Tuesday morning through Tuesday afternoon at KRIW/KLND. A dry period is likely (70%) at KCOD from 15Z-20Z/Tues before more rain showers develop in the afternoon. A steady rain will hang on at KCPR the longest, until 23Z, and then rain showers hang on for a few hours before rain ends Tuesday evening. Wind increases at KCPR late in the afternoon and will remain breezy for a few hours. At KWRL rain ends at 20Z/Tues. As rain occurs MVFR/IFR conditions will prevail at terminals. Mountain obscuration will develop and persist through Tuesday afternoon. 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 Weather Advisory until 6 PM MDT Tuesday for WYZ002-015. Freeze Watch from late Tuesday night through Wednesday morning for WYZ003>006. Winter Storm Warning until 6 PM MDT Tuesday for WYZ008-009. && $$ DISCUSSION...Wittmann AVIATION...Rowe