173 FXUS65 KRIW 061008 AFDRIW Area Forecast Discussion National Weather Service Riverton WY 308 AM MST Sat Feb 6 2021 .SHORT TERM...Today through Monday night We will divide the discussion into West and East of the Divide. West of the Divide...The area is currently in a lull between systems right now. There is still some wind and some blowing snow. This lull will not last long. The next in the series of short waves will approach the area this morning and bring in another round of snow. There will still be impressive upper level support in the form of an 140 knot jet streak moving over the area. This system does not appear to have quite as much moisture to work with. It will have more wind though and this will have some travel impacts. As a result, we will issue new Winter Weather Advisories in similar areas as the last highlights and add the western slopes of the Wind River Mountains. This looks to be a longer lived event as well, with snow showers continuing into Sunday. However, the heaviest snow will fall from around mid afternoon through around midnight tonight before the passage of the trough. Precipitation then becomes more showery and banded and harder to nail down exactly where they will set up. There could be some warning level amounts, but these would be in the higher elevations where either impacts would be minimal or where the snow is desired. As for placement of the heaviest snow, most models favor northern areas, which will have the most favorable dynamics. The other concern is wind. Southern Wyoming will be in the right front quadrant of the jet, which is favorable for downward motion. With 700 millibar winds expected to increase to 50 to 60 knots, some high winds are possible. The one concern is that the peak boundary level winds occur at night, when mixing is not as pronounced. The strong winds do continue into Sunday. We will issue a High Wind Watch starting at 5 pm, but this is far from a slam dunk. We will let the day shift make the final decision on whether or not to upgrade or cancel. Snow showers will continue into Sunday night as well, although favorable northwest flow will keep some around. Coverage should really decrease for Monday for the most part. However, with a cold front laying near the northern border, there could be some more organized snow in portions of Yellowstone Monday afternoon. There are differences in placement though, so we kept PoPs low for now. East of the Divide...The cold front has slid southward toward the Divide early Saturday morning. This has resulted in snowbands to form. Most are of the variety that move, not stationary ones. So snowfall looks mainly light for most areas, with a few exceptions. Most models show a decreasing tread with the showers today, as the front tries to lift northward again. Temperatures will be tricky today, depending on how far northward the front lifts as well, especially in central portions of the state. Showers will begin to increase again tonight, as the boundary begins to sag southward again. Here, differences begin to show up as to how heavy the showers get. The ECMWF is the wettest with a decent amount of QPF across the northern Big Horn Basin and northern Johnson County. The NAM/GFS are drier though. The ECMWF did a good job with the snow on Friday night though, the only model to have the snow as far south as it got tonight. We split the middle for now, extending snow as far south as the Big Horn Basin and northern Johnson County. As for highlights, maybe the western Bighorn Range, but it is borderline. The more significant snow will hold off until tonight, so there is still time. The big question for Sunday and Monday will be temperatures and how quickly the cold air moves into the area. The models have different solutions and are showing poor run to run consistency. Northern Johnson County will almost certainly be in the cold air, as well as the northern border locations. However, a place like Worland and even as far south as Riverton has as much as a 20 degree difference in high temperatures on different guidance for Monday. For the most part, we didn't change continuity too much given the uncertainty. As for snow, the deepest moisture will be moving away to the east. And given the continental origin of the air mass behind the front, any snowfall amounts would be light for the most part. However, with the jet over the area and a disturbance moving through, this is a favorable pattern for decent snow across northern Wyoming. We added some PoPs to account for this, but it is far from certain. .LONG TERM...Tuesday through Friday The persistent northwest zonal flow pattern will retreat slightly northward by Tuesday. Winds will decrease across the area as a result. Otherwise, bitter cold temperatures will continue over northern portions. The push with the arctic front looks to occur Wednesday, as a trough rotating around the polar low located over Saskatchewan/Manitoba pushes southward over MT. Light snow will begin over the far north by midday Wednesday behind the front. This snow will spread slowly southward through the rest of the day, reaching central portions through the overnight hours and eastern portions of Sweetwater County by 12Z Thursday. Snow will quickly end from north to south Thursday morning, as the drier, arctic air infiltrates the state. At this time, snow will end across the state by sunset Thursday evening. Model divergence begins Thursday, with respect to the trough. The GFS is the more progressive model, however, the ECMWF places less emphasis on it. This is due to a building ridge over the EPAC. This building ridge will cause a channel jet to form between the ridge and the polar low. The jet will be located over the PACNW and ID/WY. The GFS has this channel jet as well, but keeps it further south over the Great Basin, due to the rotating trough. If the ECMWF solution verifies, it would be another windy, raw day over the Cowboy State. && .AVIATION...For the 12Z TAFS through 12Z Sunday West of the Divide...KBPI/KJAC/KPNA/KRKS Through about 15Z...most terminals should have VFR conditions with some very light snow/flurries mainly over the mountains. Between 15Z and 19Z, widespread snow is expected to develop over the far west. KJAC will likely have a prolonged period of IFR visibilities in snow with MVFR ceilings through much of the remainder of the TAF period. KBPI and KPNA will be impacted with MVFR/IFR conditions at times after 18Z, but overall the time period of those conditions will be much less than what occurs at KJAC. The main concern for KRKS will be the very strong westerly wind through the TAF period. East of the Divide...KCOD/KCPR/KLND/KRIW/KWRL Terminals Any lingering snow showers over the east will quickly dissipate. Areas with MVFR ceilings remain possible, mainly at KCOD and KCPR, and should improve to VFR by 15/16Z. VFR conditions are then expected for the rest of the day. Another cold front will push into northern Wyoming between 03Z and 06Z, but could stall out before reaching the Continental Divide. In wake of this cold front, there could be areas with MVFR ceilings and some snow shower activity. KCOD terminal would be most impacted by these conditions. && .FIRE WEATHER... Another weather system will approach western Wyoming this morning and bring another round of moderate snow that will last into Sunday. East of the Continental Divide, snow showers will become less numerous through the day and likely be limited to northern areas by noon. A gusty to strong wind will continue as well, especially across southern Wyoming. Relative humidity will remain above critical levels. Mixing and smoke dispersal will range from poor in the north and east to good to very good in the west and south. && .RIW WATCHES/WARNINGS/ADVISORIES... Winter Weather Advisory from 9 AM this morning to 5 PM MST Sunday for WYZ001-002-012>014-023-024. High Wind Watch from this afternoon through Sunday afternoon for WYZ027>030. && $$ SHORT TERM...Hattings LONG TERM...LaVoie AVIATION...Murrell FIRE WEATHER...Hattings