textproduct: Boise

DISCUSSION

High pressure remains over the area tonight through Wednesday, continuing drier and colder conditions tonight through midweek. Valley temperatures will remain in the upper 20s tonight and mountain temperatures in the teens. Light winds expected from the E-SE, as valley inversions build in tonight. Peak valley temperatures are expected in the upper 30s tomorrow, and peak mountain temperatures in the upper 20s. Low mixing heights are expected tonight through Wednesday, favoring inversions over SE Oregon and the valleys of SW Idaho.

AVIATION

Mostly VFR. Low chance of brief patchy fog and low clouds near KBKE and KMYL early tomorrow morning.. Surface winds: variable less than 5 kt overnight, becoming NE-SE 5-10 kt late tomorrow morning. Winds aloft at 10kft MSL: W 10-15 kt.

KBOI...VFR with mainly clear skies. Surface winds: calm overnight becoming SE 5-8 kt tomorrow morning.

AIR STAGNATION

An upper level ridge will build over the region Monday and remain overhead through Wednesday, creating 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. Haze and patchy fog possible in valleys due to inversions. Conditions may improve late Thursday or Friday as an approaching Pacific frontal system brings precipitation and increases mixing.

PREV DISCUSSION

SHORT TERM...Tonight through Wednesday night...An upper level ridge building along the coast will move overhead Monday through Wednesday. This will bring dry and stable conditions with a temperature inversion that strengthens through midweek. Temperatures will gradually warm through Wednesday, especially at higher elevations that are above the inversion, but will see little change in the lowest valleys across southeast Oregon and far western Idaho where the inversion will be strongest. Highs by Wednesday will be 5-10 degrees above normal in most areas, except near normal in the Lower Treasure Valley due to the inversion. Overnight lows will be cold (10s and 20s) with mostly clear skies and a cool airmass in place. This will also allow patchy valley fog to develop, although dry air will limit potential. Winds will remain light under this pattern.

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.

BOI WATCHES/WARNINGS/ADVISORIES

ID...Air Stagnation Advisory until 10 AM MST Thursday IDZ012-014-016-028-033. OR...Air Stagnation Advisory until 10 AM MST /9 AM PST/ Thursday ORZ061>064.