textproduct: Boise

LONG TERM...Thursday through Sunday

A much more active pattern will redevelop during the extended. A deep trough off the coast will help to steer a weaker trough near the California coast northward. This weaker trough will be accompanied by subtropical moisture, and rain/snow chances will increase from the south on New Year's Day, reaching 40-70% Thursday night and Friday. This moist airmass, due to its origin in the subtropics, will be mild and snow levels will rise to 5000-6500 feet (possibly as high as 7500 feet). The deep trough off the coast will send shortwave impulses inland over the weekend, bringing a 30-50% chance of rain and high elevation snow. These impulses should gradually bring cooler air aloft into our area with snow levels lowering to 3500-5500 feet. Moderate precipitation totals are expected Thursday through Sunday with significant snow accumulations limited to higher peaks. Winds will mostly be light, but could tick up over the weekend as the shortwaves move through. Temperatures through the period will average 5-10 degrees above normal.

AVIATION

Mainly VFR. Patchy fog developing overnight in mountain valleys. Surface winds: Variable less than 10 kt. Winds aloft at 10kft MSL: N-NE 5-15 kt, becoming NW 10-20 kt after Mon/06Z.

KBOI...VFR. Surface winds NW less than 10 kt this afternoon, becoming SE by mid evening.

AIR STAGNATION

An upper level ridge will build over the region Monday and remain overhead through Wednesday. This pattern will create stagnant conditions in the valleys with poor mixing and light winds as a temperature inversion strengthens. Daytime mixing heights will be below 2000 feet AGL through at least Thursday. An Air Stagnation Advisory is in effect for most valley locations through early Thursday. Conditions may improve late Thursday or Friday as an approaching Pacific system brings precipitation and increases mixing.

BOI WATCHES/WARNINGS/ADVISORIES

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