textproduct: Boise

DISCUSSION

A high amplitude upper level ridge will remain the dominant weather feature over the Intermountain West through the short term period. This pattern will maintain dry and stable conditions across southeast Oregon and southwest Idaho. As the ridge strengthens, valley inversions will deepen, trapping cold air near the surface while temperatures aloft warm significantly. This will result in a continuation of below normal temperatures in the lower valleys, particularly the Snake Plain, Magic Valley, and Baker Valley, while mountain locations and ridge tops experience unseasonably warm conditions.

Scattered high clouds are present today over much of the area. Low stratus and fog have formed in the favored sheltered valleys this morning, including Long Valley and portions of the Western Magic Valley. This stratus is expected to lift partially this afternoon but likely reform and expand in coverage tonight and Sunday morning as the stagnant air mass reinforces the inversion. Winds will remain light and terrain driven, generally less than 10 mph. No precipitation is expected through Monday.

AVIATION

Mostly VFR. Patchy fog in mountain valleys resulting in some IFR/LIFR this morning. Fog/low stratus clearing through the morning. mtns obscured in fog/low stratus. Surface winds: Variable up to 10 kt. Winds aloft at 10kft MSL: Variable 5-20 kt.

KBOI...VFR under high clouds. Surface winds: SE 5-10 kt

Sunday Outlook...Mostly VFR with partly cloudy skies. Localized LIFR- IFR conditions in patchy morning fog/low stratus in sheltered valleys. Generally variable surface winds less than 10 kt.

AIR STAGNATION

The strong upper level ridge centered over the region will suppress atmospheric mixing through at least the middle of next week. Transport winds are forecast to remain very weak, generally under 10 knots, with mixing heights struggling to exceed 1000 to 1500 feet AGL in valley locations. These conditions will limit the dispersion of pollutants, leading to degrading air quality in the lower valleys. An Air Stagnation Advisory remains in effect for all valley zones in southeast Oregon and southwest Idaho until Thursday morning.

PREV DISCUSSION

SHORT TERM...Today through Monday night...High pressure will continue to build into the region through at least the next week, bringing dry conditions with light winds. Moisture aloft from a system to our north will bring occasional cloud cover this weekend. Temperatures in area valleys will remain cool thanks to limited daytime mixing and light winds from a building persistent surface inversion. Mid slope and mountain top temperatures will remain warm. Patchy valley fog will bring low visibility at times, primarily during the morning in the Long Valley and sheltered mountain valleys.

LONG TERM...Tuesday through Saturday...The high pressure will close off over Northern California on Tuesday, bringing the warmest temperatures above the surface inversion by Wednesday. Temperatures in area valleys will remain cool, with air stagnation concerns continuing through at least Thursday. A closed low over central Canada will bring cold air and elevated winds into Southern Idaho by Friday. Some ensemble/deterministic solutions (primarily ECMWF and EC AI ensembles) keep the trough further east, which wouldn't help mixing out stagnant air in area valleys. Solutions then favor a swift return to blocking high pressure with dry conditions by Saturday.

BOI WATCHES/WARNINGS/ADVISORIES

ID...Air Stagnation Advisory until 5 AM MST Thursday IDZ011>014-016-028-033. OR...Air Stagnation Advisory until 5 AM MST /4 AM PST/ Thursday ORZ061>064.