textproduct: Boise

DISCUSSION

Stratus clouds cleared in our northern and western CWA valleys this afternoon, but continue this evening in the Snake Basin from Boise east and in south-central Idaho, as well as in southern Malheur County/OR. Further clearing will stall the rest of tonight but resume Saturday as the air mass continues to dry out. The associated temperature inversion will be harder to completely remove. Colder air aloft and mixing winds near the surface will certainly weaken the inversion through Saturday, and we expect to end the Air Stagnation Advisory at noon MST Saturday. But complete scouring and mixing look unlikely as the main upper ridge persists along the coast and will even rebuild Sunday and Monday. It will take a strong Pacific storm to fully scour the inversion and latest models don't have it.

AVIATION

MVFR-IFR stratus continues in the valleys below 5200 feet MSL, with areas of IFR near KTWF and KJER. Surface winds W-NW 5-15 kt, with gusts 20-25 kt in the higher terrain of eastern OR and west-central ID.

KBOI...MVFR ceiling. Conditions expected to improve to VFR after 18Z Saturday. Surface winds NW 7-12 kt.

Sunday Outlook...VFR with light winds, except patchy low clouds near KJER and KTWF.

AIR STAGNATION

Afternoon satellite imagery showing arrival of cold and drier air aloft as low clouds are eroding from northwest to southeast. Expect this trend to continue into this evening across the lower Snake Plain and SE Oregon zones. Combination of vertical mixing ~2kft AGL and near-surface winds around 10 mph are likely enough to clear out the stagnant conditions across the region on Saturday, though it is borderline at the lowest elevations. The return of an upper ridge and accompanying inversion will start another period of stagnant conditions Sunday night and beyond.

PREV DISCUSSION

SHORT TERM...Tonight through Sunday night...Seeing signs of drier and cold air working in on northerly flow aloft as low clouds have begun to erode this afternoon. Drying aloft will continue through tonight, expanding the erosion of clouds across portions of SE Oregon and the lower Snake Plain. In the Snake Plain, Boise will be on the edge of clearing overnight with areas to the south and east likely remaining under clouds through midday Saturday. Have growing confidence that all areas break out of the low clouds by tomorrow afternoon. A secondary trough will drop along the Continental Divide Sunday, keeping the region under cold northerly flow. While temperatures will remain below normal this will keep widespread fog or low clouds from developing through Sunday night. Any precipitation threat from these upper waves remains well east of the area.

LONG TERM...Monday through Friday...Warming aloft is expected through Wednesday as a ridge aloft again builds over the region, leading to strengthening of the inversion in the valleys. With inversions, the valleys would continue to experience below normal temperatures and mountains above normal temperatures. A closed upper low off north/central California may be close enough to break the inversion in our area Thursday or Friday. For now, conditions are expected to remain generally dry for the entire week ahead.

BOI WATCHES/WARNINGS/ADVISORIES

ID...Air Stagnation Advisory until noon MST Saturday IDZ011>014-016-028-033. OR...Air Stagnation Advisory until noon MST /11 AM PST/ Saturday ORZ061>064.