textproduct: Boise

LONG TERM...Monday through Friday

A broad Alaskan low moves across Canada and the Northern Rockies Monday through Tuesday. While it will bring a 20-40% chance of precipitation across higher terrain, it will also bring strong northwesterly flow to the area. This will push a colder air mass into the area and create gusty conditions near the surface. Temps are expected to drop to near normal on Monday/Tuesday. As the cold front passes on Monday, snow levels drop from 5-6 kft MSL that morning to 3-4 kft MSL that evening, with precipitation possible all day. This would allow precip later in the day to fall as snow in lower elevations. Gusts with this system will be highest Monday at 15-25 mph, higher on ridges. The gusts and cold front should help scour out any lingering inversion from the weekend.

As high pressure builds, later in the day Tuesday will still be cool but drier. The ridge axis will be very broad, with relatively low amplitude, which will work to keep the flow aloft strong. That strong flow will pull in more Pacific moisture, and the weak ridging will warm us up to 5 degrees above normal Wednesday through Friday. Precipitation chances are 20-30% in lower elevations and 40-50% in higher elevations each day. Snow levels rise from 3-4 kft MSL to 5-6 kft MSL on Wednesday and stay that way through the long term. This will prevent much of the possible snow accumulation in mountain towns, but flurries or mixed precipitation is likely. Late Friday, models show a trough quickly moving into the area, increasing precipitation potential. Supplemented by an amplifying ridge in the Pacific, it looks like this system could really cool us down past the long term.

AVIATION

Low clouds remain, though only MVFR-VFR. A band of IFR stratus is generally lifting near the Magic Valley this afternoon. Conditions won't deteriorate again until midnight, where fog returns across the area, most likely in mtn valleys and the Snake Plain. LIFR-IFR fog/stratus will last through the morning. Surface winds: light and variable. Winds aloft at 10kft MSL: variable less than 10 kt.

KBOI...VFR with low ceilings this afternoon generally clearing to partly cloudy. LIFR-IFR conditions expected to return tonight in fog/low stratus. The chance of LIFR is around 40%, with the best estimate of formation time being Sat/06Z. Surface winds remain light and variable.

Weekend Outlook...Areas of valley fog/stratus Saturday/Sunday mornings with improvement each afternoon. Snow levels 5500-6500 ft MSL. Surface winds: SE and variable under 10 kt Saturday and early Sunday, and SW 5-15 kt Sunday night ahead of a cold front Monday.

AIR STAGNATION

High pressure aloft will continue through Sunday night, with daytime mixing heights peaking between 1500 and 2000 feet AGL in the valleys. Light winds and a low-level inversion will continue to limit ventilation in the valleys. A cold front late Sunday into Monday morning will bring increased west to northwest winds, improved mixing, and much better ventilation.

BOI WATCHES/WARNINGS/ADVISORIES

ID...Dense Fog Advisory from 11 PM this evening to 10 AM MST Saturday IDZ012-014-016. OR...None.