Marine Weather and Tide Forecast for Boise City, ID

AVIATION
VFR and mostly clear with few high clouds continuing through Saturday. Patchy fog possible in sheltered valleys with the highest chance (60%) at KBKE. Surface winds: Mostly variable under 10 kt, except W-SW 10-15 kt in Magic Valley Saturday afternoon. Winds aloft at 10kft MSL: W-NW 20-35 kt.

KBOI...VFR with clear skies. Surface winds: variable below 5 kts overnight.

Sunday Outlook...Mostly VFR with increasing high clouds by late evening. Low stratus and patchy fog possible in sheltered valleys overnight/early morning on Sun. Light and variable winds.

AIR STAGNATION
An upper level ridge will remain over the area through the weekend, resulting in stagnant conditions. Mixing heights will peak each afternoon at around 1,000 to 2,000 feet AGL through the weekend, and winds will generally remain less than 10 mph resulting in poor ventilation. An Air Stagnation Advisory will remain in effect through Sunday, and has been expanded to also include the West Central Mountains, Boise Mountains, and Camas Prairie. There is potential for stagnant conditions to persist into Monday before an active pattern brings increased mixing and winds, as well as precipitation, on Tuesday and Wednesday.

PREV DISCUSSION
SHORT TERM
Tonight through Sunday night
An large upper level ridge will remain over the area. Expect mostly clear skies with light winds. Afternoon temperatures will be 15 to 20 degrees above normal for this time of year. The light winds, low mixing heights, and a strong night time temperature inversion will also trap air pollution in valley locations. An air stagnation advisory has been issued for these conditions which will continue through the weekend. There isn't enough moisture to produce widespread foggy conditions that are typical during an inversion, but patchy morning fog is still possible in lower valley locations.

LONG TERM
Monday through Friday
A longwave Pacific trough is expected to steer into the region Monday, ramping up precipitation chances across the forecast area late Monday as the first wave of Pacific moisture arrives with this trough. Temperatures will initially stay mild Monday and Tuesday as the colder airmass remains to the north, supporting relatively high snow levels at approximately 7000 feet or higher. A prominent and windy cold front is forecast to move through east Oregon and southwest Idaho Tuesday evening into Wednesday. This front will be accompanied by a potent atmospheric river plume, spreading notable precipitation across the region. Max temperatures will drop by approximately 10 degrees for Wednesday, with snow levels also lowering to 3500-4500 feet MSL.
While precipitation will become likely area-wide, this flow will most favor the higher terrain of west-central/central Idaho.

There remains considerable guidance uncertainty on the positioning of the polar jet stream beginning late Wednesday and beyond, which decreases forecast confidence in temperatures/snow levels for the rest of the week. However, ensembles suggest that a fairly persistent plume of Pacific moisture will continue to pummel the Pacific Northwest throughout the rest of the week. As a result, significant precipitation totals are possible across the mountains throughout next week, reaching over an 1" of new liquid equivalent over the west-central/central Idaho terrain. Depending on the oscillation of snow level during these series of moisture pushes, snowfall totals could reach 1 to 2 feet for higher elevations during this extended period.

BOI WATCHES/WARNINGS/ADVISORIES
ID...Air Stagnation Advisory until 5 AM MST Monday IDZ011>014-016-028-033.
OR...Air Stagnation Advisory until 5 AM MST /4 AM PST/ Monday ORZ061>064.