textproduct: Boise

DISCUSSION

High clouds are currently passing over the forecast area through early morning, which may act as a limiting factor for fog and low stratus growth tonight until they exit to the southeast. In addition, the clearing of low clouds this afternoon, along with the lack of antecedent precipitation, has led to slightly drier conditions this evening than this time yesterday. Latest hi-res models have picked up on this, reducing dense fog coverage in the Snake Plain and delaying the onset of fog/stratus compared to yesterday. However, the best chance for impact will be south-central Idaho, including the Magic Valley, where stratus current resides and dewpoint depressions remain minimal. With further cooling, the Magic Valley will become susceptible to areas of dense fog throughout the night. Some dense fog is still a concern in portions of the Treasure Valley, with latest HREF run indicating the best chance of occurrence to be mid to late morning. Currently, dense fog chances across the western Snake Plain are 30-50%, but the outliers will be the Boise metro at 10% chance and the area between Boise and Mountain Home at 60% chance. Elsewhere, valleys and basins in eastern Oregon and west-central Idaho are still set to become impacted by areas of fog or low stratus beginning in the next few hours. Drying south to east winds are expected to increase Saturday afternoon, which will help lift and dissipate fog/stratus and yield mostly sunny conditions for most.

AVIATION

Patches of MVFR stratus in the mountains and near the Nevada border this evening. Areas of LIFR-IFR fog/stratus will redevelop in the valleys overnight, improving late Saturday morning. Coverage of fog/low stratus may be lower than last night. Surface winds: light and variable, becoming E-SE 5-10 kt Saturday morning. Winds aloft at 10kft MSL: W-SW 5-10 kt.

KBOI...VFR conditions are expected overnight. A less than 30% chance of LIFR-IFR fog or low stratus will develop at the terminal Saturday morning 12-17Z, with higher chances southwest of the terminal. Surface winds SE around 5 kt.

Sunday Outlook...Areas of valley fog/stratus Sunday morning with improvement in the afternoon. A cold front late Sunday will bring light precipitation mainly to northern areas and mountain obscuration. Snow levels 5500-6500 ft MSL Sunday evening, falling to 3000-4500 ft MSL by Monday morning. Surface winds: SE-SW less than 10 kt early Sunday, then SW-NW 5-15 kt Sunday night.

AIR STAGNATION

High pressure aloft will continue through Sunday night, with daytime mixing heights peaking between 1500 and 2500 feet AGL in the valleys. Relatively light winds and a low-level inversion will continue to limit ventilation in most valleys. A cold front late Sunday into early Monday morning will bring increased west to northwest winds, improving mixing and ventilation.

PREV DISCUSSION

SHORT TERM...Tonight through Sunday night...Latest forecast models all indicate persistence through Sunday, and that means another Dense Fog Advisory for the Magic Valley and Treasure Valley in Idaho late tonight through Saturday morning. Another advisory may be needed Saturday night through Sunday morning, but will address that possibility on Saturday. Little day to day change is expected through Sunday in temperature, wind, humidity, or sky cover. Changes will begin Sunday night as a cold front approaches from the northwest, beginning with increasing clouds then improved low-level mixing via light southeast wind. A 30-40 percent chance of light rain and snow will spread across Baker County and the west-central Idaho mountains by midnight MST Sunday night, then a 40-70 percent chance overnight as far south as the Boise Mountains and highest southern peaks. Snow level will be 4000 to 4500 feet MSL north and 4500-5200 feet south, with new snow up to two two inches.

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.

BOI WATCHES/WARNINGS/ADVISORIES

ID...Dense Fog Advisory until 10 AM MST Saturday IDZ012-014-016. OR...None.