textproduct: Boise

DISCUSSION

Upper trough over Northern Montana will continue steering out East tonight, sustaining northwest flow that will drop nighttime temperatures into the upper 20s for lower elevations and into the single digits for mountain areas. Colder daytime temperatures will be expected Sunday, with peak valley temperatures in the upper 30s and peak mountain temperatures in the upper 20s. High pressure will build into the area late Sunday, continuing dry conditions. Stagnant conditions and weak winds under 10 mph are still expected late Sunday through Monday.

AVIATION

Mainly VFR. Brief mountain obscuration and MVFR/IFR conditions in snow showers near KBKE Sunday morning. Surface winds: SW- NW 5-15 kt, strongest in the Magic Valley. Winds aloft at 10kft MSL: NW 20-35 kt.

KBOI...VFR with high clouds tomorrow. Surface winds: W-NW 4-7 kt.

AIR STAGNATION

Stagnant conditions develop Sunday as a ridge of high pressure begins to build over the area. Poor mixing heights, light winds, and a temperature inversion expected Monday through Thursday. Temperature inversion will likely trap pollutants, creating haze and patchy fog especially over sheltered valley areas.

PREV DISCUSSION

SHORT TERM...Tonight through Monday night...A trough continues to move east over Montana this afternoon. West to northwest surface winds 10-20 mph across the area this afternoon except 30-40 mph in the Magic Valley. Winds ease after sunset with cool temperatures tonight. Sunday and Monday will be partly sunny and dry with stagnant conditions developing as upper level winds weaken and a ridge builds into the area. Temperatures will be near normal Sunday with slight warming on Monday.

LONG TERM...Tuesday through Saturday...An upper level ridge will bring dry and stable conditions to the area Tuesday through early Thursday. Warming aloft combined with cold air trapped near the surface will create a temperature inversion. As a result, most mid to high elevation sites will see temperatures 5-10 degrees above normal, while the lowest valleys will be near to slightly below normal.

A return to more active conditions is expected late Thursday through Saturday as an upper level trough along the coast helps to direct subtropical moisture from the Southwest northward. The trough is expected to move inland Friday or Saturday. Ahead of the trough, precipitation chances will increase with mild temperatures and snow levels around 4000-6000 feet, possibly rising to as high as 7000 feet if the trough is delayed and the flow aloft becomes more southerly. Cooler air accompanying the trough should lower snow levels, possibly to near valley floors by Saturday. If the trough can direct deeper moisture into our area, as is depicted in a majority of global/AI ensemble members, another round of widespread precipitation with moderate totals and mild temperatures are expected. However, if the trough is faster as is favored by the GEFS, then less precipitation and cooler temperatures are more likely. Overall, the uncertainty results in precipitation chances rising to around 40-70% by Friday morning, then gradually decreasing on Saturday, with temperatures averaging 5-10 degrees above normal.

BOI WATCHES/WARNINGS/ADVISORIES

ID...Air Stagnation Advisory from 6 PM Sunday to 10 AM MST Thursday IDZ012-014-016-028-033. OR...Air Stagnation Advisory from 6 PM MST /5 PM PST/ Sunday to 10 AM MST /9 AM PST/ Thursday ORZ061>064.