Area Forecast Discussion
Issued by NWS Riverton, WY
Issued by NWS Riverton, WY
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262 FXUS65 KRIW 221048 AFDRIW Area Forecast Discussion National Weather Service Riverton WY 448 AM MDT Wed May 22 2024 .KEY MESSAGES... - Showers return to western Wyoming today; breezy to windy, warmer and mainly dry East of the Divide. - Snow returns to the western mountains tonight. - Much cooler Thursday with snow in the northern mountains, the heaviest in the northern Bighorns. - Unsettled weather expected for the holiday weekend, although details for timing and placement of precipitation remain uncertain. && .DISCUSSION... Issued at 329 AM MDT Wed May 22 2024 Things are quiet as I write this discussion around 2 am this morning. However, the quiet will not last. The next round of precipitation will arrive in the west, courtesy of an upper level low now circulating over the Pacific Northwest. Moisture will advect into northwestern Wyoming starting this morning, but precipitation amounts should remain on the light side through the daylight hours. Dry and warmer conditions will be found east of the Divide, but it will become windy as the pressure gradient tightens ahead of the advancing system. High wind is not expected today, but the favored areas in this pattern, like Casper and Rock Springs, will get quite gusty. Deeper moisture begins to move into the west as well as better upper level dynamics in the form of an 100 knot jet streak to enhance upper level divergence. This will bring steadier precipitation to the western mountains. Snow levels will remain around 7500 feet or higher through the evening though before falling to the valley floors later at night. Most guidance is fairly consistent in keeping the heaviest precipitation at night largely between 6 pm and midnight. There could be some decent rates though, around 1 in 2 chance of 1 inch an hour at times. Probabilistic guidance gives around a 2 in 5 chance of over 6 inches of snow in the higher elevations of the Tetons. However, this is over a longer period. So, we will not issue any Winter Weather highlights but will issue a Special Weather Statement for tonight. If the cold air can get in fast enough, there is a chance (about 1 out of 3) of some light accumulations of snow in the western valleys as well. The other area that could see decent snow tonight is the Absarokas, but this would fall over areas with almost no people or roads, so no highlights here. The concern tonight east of the Divide is wind, mainly in the southwesterly flow areas. There are a few 50+ knot wind barbs in the favored areas of southeastern Fremont and Natrona Counties later tonight. But this is for a short period of time around 12Z before it lessens. There could be a few wind gusts of over 58 mph, but this would be mainly in the higher elevations where impacts would be few. The best chance in the lower elevations would be in places like Wyoming Boulevard and Hat Six Road near Casper, but these look isolated. A Special Weather Statement would probably cover this as well. As for precipitation, some showers will spread into the area, mainly across the northern half of the area. But any decent precipitation would hold off until Thursday. And now for Thursday. This was the most challenging period, with the most difficult decision and the potential for highlights. There were several factors that went into this decision. First was QPF. This system does have a decent amount of moisture to work with. However, the deepest moisture looks to remain over Montana. Some guidance does show a bullseye over the northern Bighorns with over an inch of QPF. This is a very small part of the our area though. The other concern is that there could be a very tight gradient in regards to the amounts. Southern portions of the range may see little to no precipitation. The second concern is wind. There will be a lot more wind with this system on Thursday, especially in the afternoon as the low moves to the east and the favorable northwest flow / cold advection pattern sets up. Wind could get quite strong, with around a 2 in 5 chance of wind gusts to 50 mph around Buffalo. There is not quite enough mid level wind for High Wind Warnings though. The third consideration was timing. Guidance is in very good agreement in the bulk of the precipitation occurring during the day. And with the strong late May sun, snow may have trouble sticking to the roads at this time. So, with a greater than 2 in 3 chance of 6 inches of snow or more north of Powder River Pass, we went with a Winter Weather Advisory. Chances of 12 inches or more are less than 1 out of 2 except for the far north and highest peaks, so we have not gone with watches or warnings. As for the rest of the area, this looks like a blustery and cool day with showers scattered through the area. The vast majority would fall across the north with less of a chance in the south. For the most part, wind should fall short of high wind criteria. Temperatures will average 10 to 20 degrees below normal across the area. More settled weather returns Friday with shortwave ridging over the area, bringing a warmer and more settled day. As for the holiday weekend, things are more uncertain. The third in the series of upper level lows will cross the area and bring a chance of showers for Saturday and Sunday and possibly into Monday as well. There is a larger spread in guidance here though. So, although we have a high confidence in unsettled weather for Saturday and Sunday, confidence in the details on timing and placement of precipitation remains rather low. Memorial Day is also up in the air depending on the speed of the low. Ridging should then build into the area and bring drier and warmer weather for the middle of next week. && .AVIATION /12Z TAFS THROUGH 12Z THURSDAY/... Issued at 440 AM MDT Wed May 22 2024 West of the Divide...KBPI/KJAC/KPNA/KRKS Terminals Prevailing VFR conditions will continue this morning, as high clouds stream over the area from the west. Rain showers will move into western Wyoming by midday, but will be mostly confined over the mountains. This will also impact KJAC, but conditions will predominantly remain VFR even as showers pass through. Steadier rain is expected at KJAC Wednesday evening after 01Z, so reductions in vis to MVFR are possible (40%). Isolated thunderstorms will develop Wednesday afternoon into Wednesday evening, mainly in the mountains around KJAC. Not enough confidence in thunderstorm coverage for VCTS at KJAC at this time. Other terminals will remain dry, although there is a 20-30% chance of rain at KBPI/KPNA Wednesday evening. Wind will increase at all terminals late Wednesday morning from the southwest, with gusts of 20-30 knots at all terminals through the afternoon hours. A wind shift from the northwest will occur late Wednesday evening. Mountaintop obscuration will begin late this morning as clouds continue to increase and continue through the TAF period. East of the Divide...KCOD/KCPR/KLND/KRIW/KWRL Terminals Prevailing VFR conditions will persist over the next 24 hours, but wind will increase for all terminals late this morning as another storm system approaches the state. Wind will be southwesterly sustained in the teens, gusting 20-30 knots from 14/16Z through the afternoon, gradually decreasing between 00Z/02Z. A shift to northwesterly wind is likely (70%) this evening for KWRL/KRIW/KLND. Rain will spread over northern portions of the Bighorn Basin after 06Z. Snow is not expected at this time, but the rain could change over to snow if the cold air arrives faster than expected. 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 from 3 AM to 6 PM MDT Thursday for WYZ008-009. && $$ DISCUSSION...Hattings AVIATION...LaVoie