101 FXUS65 KRIW 012031 AFDRIW Area Forecast Discussion National Weather Service Riverton WY 131 PM MST Fri Feb 1 2019 .SHORT TERM...Tonight through Monday Quite an active period is shaping up across the area. Given the contrast in the weather, we will split the discussion into west and east of the divide for the most part. As for tonight, expect dry weather everywhere although clouds will be on the increase. Winds will also gradually increase across the wind corridor. West of the Divide...The main concern here is snow. All the models have continued to slow the progress of the Pacific system. The day should start dry for most locations and even most of the morning should end up precipitation free. Some snow will begin to spread in later in the morning. However, at this point, Saturday does not look too bad. Most of the models have mainly light precipitation until later in the afternoon. In addition, a couple of models show warmer air moving in ahead of the cold front with the warm advection and raising 700 millibar temperatures to around -3 to -4. This would be enough to mix or even change over some of the valleys to rain for a time in the afternoon and evening before colder air moves in behind the front and drops snow levels back down to the valley floors later Saturday night. At this point, all locations look to see the heaviest snow from later Saturday night into Sunday morning with the passage of the cold front, trough axis and the best jet dynamics through the area. As a result, we have hoisted winter weather advisories across the area starting at 11 am, although the worst conditions would to be until later on. Snow will lighten after around noon although light snow should continue through the afternoon and into the evening. Another wave will the move into the area either later Sunday night or Monday, depending on what model you believe. Again, the NAM is the fastest. However, like with most Pacific systems, it is probably too fast. As a result, we went with the higher POPs on Monday. However, there will be colder air in place for this system so this system looks to be all snow. We may need more highlights for this event in the future. East of the Divide...Here the attention turns mainly to wind. We continue to have the gusty winds across the wind corridor and around the crossroad of the universe, Clark. 700 millibar winds are borderline through. As a result, we will stick with a special weather statement rather than a highlight. Things also look borderline around the high wind watch, although we do have 50 knots at 700 millibars and the favored southwest direction. We still have time for the upgrade there though. Otherwise, dry weather should continue through Saturday. Inversions have pretty much mixed out at this point and that should be the case tomorrow as well. As a result, expect the mildest day for most locations than we have seen in a while with some areas rising into the 50s at times. A cold front will approach from the north on later Saturday night into Sunday into northern Wyoming. We do get into some uncertainty now though as to how far south it will get and in addition how much if any precipitation will fall. All models indicate some snow breaking out across the northern Big Horn Basin either later Saturday night or into Sunday. There are large differences in amounts however. In addition, there are some differences as to how far south it will extend. For now, we have gone with the high POP, low QPF approach for now. Another difficulty is that the snow here is likely to be banded and as a result hard to pinpoint this far out. A loot depends on the exact track of the jet streak moving across the area. We have included at least a slight chance of precipitation across the entire area Sunday morning though to account for the uncertainty. The other question is for high temperatures. Cooler temperatures will likely press into the north following the frontal passage. Further south, they will be VERy tricky. For now, we made few changes to continuity until we can resolve more details. It will be the same problem for Monday. The difference will be all areas should be dry. The position of the front is critical. For example, the extended GFS MOS guidance has a high of 18 in Buffalo and 42 in Casper for Monday afternoon. Again, we kept temperatures largely close to continuity and played the middle ground until the models can come into better agreement. .LONG TERM...Monday night through Friday Timing of the next system has increased 24 hours and is now progged to impact the forecast area Tuesday and Wednesday. This Pacific storm system will dig southward just off the PACNW coast, moving onshore over N CA by 12Z Tuesday. The system will then move eastward over the Great Basin through Tuesday night. Periods of light snow will remain over the west, with chances of light snow along and north of the stationary front (roughly from Cody to Buffalo and north). The heaviest snow is expected to push into the west and southwest on Tuesday and Tuesday night, as the Pacific system moves into the Great Basin. A second, more arctic system, will quickly slide southward from northern Canada and become in phase with the CA/Great Basin system. The associated arctic front will combine with the stationary front along the WY/MT state line and finally push southward along and east of the Divide Tuesday night and Wednesday. Light snow will become widespread Tuesday night, as the front and trough begin to pass through the area. A band of moderate to heavy snow could be possible from Rock Springs to Casper early Wednesday morning, as the left front quad of the jet is over this area before exiting eastward. Light snow, gusty north winds and plunging temperatures (single digits above and below zero Tuesday night, followed by highs only in the teens on Wednesday), will continue through the day Wednesday. Wind chill values Tuesday night and Wednesday morning are expected in the -10F to -20F range across north and central Wyoming. Snow will finally end across the area by Wednesday evening. The area will remain in the grip of this modified Arctic airmass on Thursday, and possibly on Friday. A transition to a milder westerly Pacific flow looks more likely by the weekend, with the arctic airmass retreating. Chances of light orographic snow will remain over the NW mountains in this westerly flow.&& .AVIATION...00Z Issuance VFR conditions with mainly SKC through tonight. Mid- high level clouds will begin to pass over the area after 00Z Saturday. Windy southwest flow will continue from South Pass to Casper through the TAF period, increasing slightly by 09Z. Gusty winds will also continue along the lee side of mountain ranges east of the Divide, and into some adjacent foothills. LLWS will also increase over several locations by 09Z Saturday, despite the gusty sfc winds. These conditions are expected to last through 00Z Sunday as well. An approaching storm system will increase low cloudiness late tonight over the west, with snow showers increasing as well. Precipitation is expected to start as -RASN at KJAC, as temperatures are expected to be slightly above freezing. VCSH will be possible at KBPI and KPNA between 18Z Saturday and 00Z Sunday. && .FIRE WEATHER... Mainly dry weather will continue through Friday night with flat ridging in place over the area. Inversions have weakened today and should continue to weaken tomorrow although they may still be in place at times. A Pacific weather system will move in from the west and bring some snow starting Saturday, although the heaviest snow is likely to hold off until Saturday night. Enough warm air may move in Saturday afternoon for the snow to mix with rain in some of the valleys for a time. && .RIW WATCHES/WARNINGS/ADVISORIES... Winter Weather Advisory from 11 AM Saturday to noon MST Sunday for WYZ001-002-012>015-023>025. High Wind Watch from late tonight through Saturday evening for WYZ019-020. && $$ SHORT TERM...Hattings LONG TERM...LaVoie AVIATION...LaVoie FIRE WEATHER...Hattings