795 FXUS65 KRIW 231802 AFDRIW Area Forecast Discussion National Weather Service Riverton WY 1102 AM MST Sun Feb 23 2020 .SHORT TERM...Today through Tuesday night All in all, today looks to be a quiet day with a mix of clouds and sunshine. A warming trend will continue in the basins as inversions continue to grow shallower and the air mass continues to modify. Still some question if the inversions can break though. Nevertheless, it should be the warmest day of the past several since most areas will be starting off warmer than previous days. Any snow in the west from the approaching Pacific system should hold off until after sunset. ANd here is where we will split the discussion into West and East of the Divide. West of the Divide...That aforementioned system will move into the west and bring a period of snow tonight. At this point, the steadiest snow should occur between around 9 pm and 3 am. Since it is a fairly quick hitter and does not have a lot of moisture to work with, snowfall amounts should be below advisory levels. However, it could get nasty travel wise for a few hours either side of midnight as the trough axis moves through. It will become windy with that passage as well leading to areas of blowing and drifting snow. For now, we will go with a Special Weather Statement. The only places that look to get advisory level slow would be the top of the mountains where impacts would be negligible to even wanted in the case of the ski resorts. Strong winds will also be possible in Sweetwater County with the frontal passage. It is borderline for high wind though. Again, we will go with a special weather statement to cover for the wind. Steady snow should end by sunrise. However, with northwest flow continuing there will be some snow showers with the orographic flow. There could even be a couple of those dastardly, pain the neck snowbands that develop. This would be localized though.We maintained chance to likely POPs but with lower QPF to cover for this. High pressure then builds in from the west Monday night and should shut off the snow by late Monday night. Tuesday at this point looks to be a dry day. Temperatures look to remain below normal but nothing unusual for late February. East of the Divide...This system will press across the area later tonight and into Monday. There is still a question how much snow can make it across the Divide, especially given some downsloping flow. Most of the models give the best chance of snow to northern Wyoming but even here amounts do not look that high. In addition, the snow will be more banded and hard to pinpoint where showers will POP up as mesoscale models generally have no clue more than 12 hours out. For now, we have gone with slight chance to chance POPs but kept QPF low. There could be a bit more moisture in far eastern areas, including around Casper as the system may slow down a bit. Timing here may be a problem since it could be around the morning commute so this has to be watched. As the system passes, strong winds will likely develop in the favored northwest flow locations as well late tonight. This is especially so around northern Johnson County. Some guidance indicates high wind but others don't. This would be fairly brief though so no highlights here for now. A second system, this one coming southward out of Canada will move into the area Monday night into Tuesday. Given the origin of the system from Canada, there is not a ton of moisture to work with. As a result, most areas would see only light snow. The exception may be the northern Bighorn Mountains where the favored northwest flow may allow for local advisory amounts of snow. The main threat will be strong wind. The NAM continues to show 700 millibar winds as high as 60 knots Monday night in Johnson County. This looks to be the best chance of high winds of the period. Still some time to evaluate though. The snow should taper off later Tuesday as the system moves away to the south and east with winds gradually decreasing through the day. Tuesday night at this point looks to be dry and cold. .LONG TERM...Wednesday through Sunday night Northwest flow will be in place over the Cowboy State Wednesday, in the wake of the storm. Embedded shortwaves could bring another round of light snow to far western portions in the afternoon, spreading across northern and eastern portions Wednesday night. Gusty winds will once again be in place across the Wind Corridor Wednesday, as the surface gradient remains tight. Snow chances will continue over northwestern portions Thursday, with chances for snow over the Bighorns, Johnson and Natrona counties late Thursday afternoon into Thursday night. Gusty winds will likely continue Thursday, with the surface and 700mb gradients remaining tight. However, the jet is not particularly strong (70+kt) and 700mb winds will range 30-40kt. The ridge axis will build and shift over the Northern Rockies Friday, pushing the jet eastward and warming 700mb temperatures. The ridge will remain in place Saturday, before shifting eastward over the Plains. A major longwave trough could develop over the western CONUS by Sunday. This could bring a moist, southwest flow to the far west through the day as the main low center digs over southern CA. Temperatures will slowly moderate through the rest of the week, with the warmest days likely occurring Friday and Saturday. However, overnight lows will remain cold with readings in the single digits && .AVIATION...(For the 18Z TAFS through 18Z Monday afternoon) Issued at 1053 AM MST Sun Feb 23 2020 VFR conditions will prevail through 00Z west of the divide and through 06Z east of the divide. A weather system will move into western WY after 00Z spreading some snow across the region with MVFR to IFR conditions occurring through 12Z. Expect some mountain obscuration in the west after 00Z. East of the divide, scattered snow will develop after 06Z with areas of MVFR to IFR conditions and some mountain obscuration. A gusty wind will develop over many areas tonight through 12Z Monday as the cold front moves through the region during this time. Breezy conditions will continue through 18Z behind the front, but not as strong. One exception will be KBYG where winds will likely continue gusting up to 45 knots. && .FIRE WEATHER... High pressure will bring another dry and cold day to much of the area today. Wind should remain light to moderate today with humidity remaining above critical levels. Mixing and smoke dispersal will be generally fair to poor. A system approaching from the west will bring some snow to the west tonight and areas further east from late && .RIW WATCHES/WARNINGS/ADVISORIES... NONE. && $$ SHORT TERM...Hattings LONG TERM...LaVoie AVIATION...Lipson FIRE WEATHER...Hattings |