textproduct: Boise

DISCUSSION

Snow is accumulating in the West Central and Boise Mountains, with just over four inches of snow shown on the McCall town snow stake as of 8pm MST. A strong cold front with very cold air aloft will move in tomorrow, bringing bands of moderate to heavy snow across the mountains and south central Idaho. A low chance of snow (20% chance) exists over the Snake River Valley in the early morning as well. This will likely be brief with minimal accumulation if it occurs at all. Hi-res models continue to shift the coldest air and moisture to the northeast, which would eliminate any chance for snow in the Boise metro area.

Breezy winds will accompany the frontal passage, with much colder temperatures over the region anticipated. Colder temperatures combined with the wind will bring low wind chills, especially across higher elevations. Updated the forecast to account for the lower potential for snow tomorrow in the valleys.

AVIATION

Numerous showers, mainly over higher terrain tonight, becoming scattered Sat morning. IFR/LIFR in snow. Mountains obscured. Snow levels: 3k-4k feet MSL tonight, down to valley floors Sat AM. Surface winds: SE-SW 5-15 kt, veering to NW-W 5-15 kt behind cold front early Sat AM. Then, increasing to NW-W 10-20 kt with 20- 35 kt gusts late Sat AM. Winds aloft at 10kft MSL: W 35-50 kt, then NW 20-35 kt by Sat/15Z.

KBOI...VFR. Showers in vicinity over Boise foothills, creating obscuration. Potential for brief snow shower and degraded visibility at KBOI Saturday mid morning through early afternoon. However, forecast confidence is very low as shower activity will be favoring higher terrain. Surface winds: SE-SW 5-10 kt. Shifting to W-NW 8-18 kt with gusts around 25 kt after Sat/08Z, decreasing Sat eve.

Sunday Outlook...Showers ending Saturday evening. VFR, cold, dry Sunday. Surface winds light and variable, except W-NW 10-15 kt with gusts around 25 kt Sun AM in south-central Idaho (KTWF-KJER).

AIR STAGNATION

Stagnant conditions develop Sunday as a ridge of high pressure begins to build over the area. Poor mixing heights with light winds expected Monday through Thursday. A temperature inversion will strengthen Monday through Wednesday of next week as the ridge moves overhead with warmer temperatures aloft.

PREV DISCUSSION

SHORT TERM...Tonight through Sunday night...A brief lull continues in precipitation this afternoon from Boise, ID to Rome OR with mostly clear skies through late this afternoon. Clouds are increasing in central and eastern OR ahead of the next cold front with snow developing at Burns OR as of 220PM MST. The next cold front moves through late this evening into Saturday morning with scattered snow showers and snow levels falling to the valley floors. Precipitation amounts will be light with this system, with minimal snow accumulations below 5000 feet. Above 5000 feet, snow accumulations of 4 to 7 inches possible with locally higher amounts on the peaks where a winter weather advisory remains in effect. Snow amounts will be highly variable due to the showery nature of the precipitation. Heavier bands of snow possible over the west central ID mountains early Saturday morning into the afternoon, but the best dynamics for heavy snow showers will generally be over the central ID panhandle mountains and northeast into western MT where the coldest air aloft is. High temperatures Saturday will be around 10 degrees cooler than today. Breezy west-northwest winds follow the front with gusts of 25-40 mph during the afternoon Saturday. Sunday will be mostly sunny and dry with stagnant conditions developing as upper level winds weaken.

LONG TERM...Monday through Friday...Dry conditions throughout the coming week. Ridging will build in above a low over the Pacific, this blocking pattern (rex block) will allow prolonged high pressure over the region. This set-up will result in strong valley inversions with upper-level subsidence and the low sun angle of this time of year. With plenty of surface moisture to work with, and a favorable pattern, fog and low stratus will be a concern each day as well as decreasing air quality below the inversion. Day time highs may be a little over done, with the potential of a cold pool and reduced surface heating under potential fog/low stratus. Conversely, morning low temps may be a bit under done under the same conditions, with cloud cover reducing diurnal cooling.

The first signs of a pattern change come late next week. Cluster analysis is showing increasing agreement among grand ensemble members; with the favored solution being a trough digging down from the Gulf of Alaska late Thursday into early Friday. Increased mixing with the trough will work to lift the inversion in the late week as well. Precipitation chances will increase Thursday afternoon through Friday area-wide as the trough moves ashore.

BOI WATCHES/WARNINGS/ADVISORIES

ID...Winter Weather Advisory until 5 AM MST Saturday IDZ011-013. OR...None.