Area Forecast Discussion
Issued by NWS Boise, ID
Issued by NWS Boise, ID
411 FXUS65 KBOI 300223 AFDBOI Area Forecast Discussion National Weather Service Boise ID 823 PM MDT Mon Apr 29 2024 .DISCUSSION...A cold front moved across southwestern Idaho late this afternoon and south-central Idaho this evening. The front brought wind gusts to near 60 mph in the Lower Treasure Valley, and blowing dust that briefly reduced visibility to a quarter mile, but the front gradually weakened as it passed through the Upper Treasure Valley and into the Magic Valley. A Wind Advisory expired at 7 PM MDT. Gusts to 40 mph and blowing dust were still being reported in the Magic Valley this evening. Conditions will improve the rest of tonight, but temperatures will be colder in all areas overnight through Wednesday as an upper low passes directly overhead. Scattered instability rain and snow showers will develop Tuesday, and Wednesday morning will be cold enough for snow showers down to valley floors although moisture will be limited. Current forecast describes things well. No updates. && .AVIATION...VFR. Scattered snow showers in the north and isolated rain showers elsewhere. Mountains obscured. Snow levels lowering to 3000-4000 ft MSL by Tue/12Z. Surface winds: W 15-25 kt with gusts to 30 kt this evening, decreasing to 5-15 kt by Tue/12Z. Winds aloft at 10kft MSL: W 15-25 kt. KBOI...VFR with mid clouds. 20% chance of -SHRA from 02-04Z, but most likely virga. Surface winds: NW 10-15 kt, decreasing after 06Z, then becoming light and variable after 09Z. && .PREV DISCUSSION... SHORT TERM...Tonight through Wednesday night...An unseasonably cool upper level trough is moving across the Pacific NW today. This system is bringing a cold front through the area, and has reached Ontario, OR as of 2pm MDT. This front will continue to track southeast through the afternoon, and temperatures will slowly fall behind the front, with rain, snow and graupel showers mainly north of a line from Burns OR to Boise ID. Isolated thunderstorms, too. The front is forecast to generate windy conditions in the upper Treasure and Magic Valleys and Camas Prairie through this evening where a wind advisory remains in effect. The upper level trough will remain over the area on Tuesday, keeping temperatures around 10 to 15 degrees below normal. The coldest air aloft will reach -30C at 500mb and extend from a line north of Burns to Boise on Tuesday. Slightly warmer 500mb temperatures near the NV border will lessen the chance of showers. Scattered showers will redevelop on Tuesday afternoon with a slight chance of thunderstorms, where the coldest air aloft is. Another shortwave moves south into the upper level trough on Wednesday morning, enhancing the threat of snow showers over the area. 500mb temperatures reach -33C on Wednesday morning, which is near record cold for this time of year. Light snow possible down to the valley floors, but precipitation will be showery in nature, so some places may see snow, while others will not. Snow showers will linger over the mountains on Wednesday afternoon, but warmer air aloft and ridging will bring drier and more stable conditions by Wednesday evening. LONG TERM...Thursday through Monday...Long-term models are showing poor agreement that gets worse with time. As a result, most aspects of the forecast have a high degree of uncertainty. A compact low will move in from the northwest on Thursday, bringing precipitation through Thursday night with breezy afternoon winds. Chance of precip is fairly low, 25-35% in higher terrain and ~10% in lower valleys. Most precipitation will be rain, but above 5000 ft MSL snow becomes possible since some colder air will linger from the low earlier in the week. Instability in central Oregon will support a slight chance of thunderstorms. The track of this low varies a little model to model, but most of the disagreement is in the coverage of the precipitation, hence the mediocre precipitation chances. A brief calm period will persist Friday into Saturday morning thanks to a ridge of high pressure. At the same time, a deep closed low from the Gulf of Alaska will track southeast along the Canadian west coast. While models are beginning to show more agreement with each run, they are still not quite there. The GFS continues to show the low making landfall in California, and then putting us under a deformation zone as it moves east, while the 12z EC now shows the core of the low moving directly overhead, putting us under the coldest part of the airmass. This is a fairly big change from the 06z EC that showed the low becoming a longwave trough leading to still unsettled but a notably weaker weather pattern. The National Blend consensus has area-wide 30-50% precip chances beginning Saturday afternoon, dropping to 20-40% Sunday afternoon through the rest of the long-term. Saturday onward each afternoon will have a chance of thunderstorms and breeziness. The spread in maximum temperature forecast is 10-20 degrees each day, with the consensus being normal temps. && .BOI WATCHES/WARNINGS/ADVISORIES... ID...None. OR...None. && $$ www.weather.gov/Boise Interact with us via social media: www.facebook.com/NWSBoise www.twitter.com/NWSBoise DISCUSSION...LC AVIATION.....SP SHORT TERM...KA LONG TERM....JM