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FXUS65 KBOI 282135 AFDBOI

Area Forecast Discussion National Weather Service Boise ID 235 PM MST Tue Nov 28 2023

SHORT TERM
Tonight through Thursday night...Dry and very stable conditions will continue through Wednesday, with the inversion likely breaking Thu afternoon, ending the extended period of stagnant air. The upper level ridge that has basically controlled our weather the past few days will move off to the east Wednesday and a weak trough will move through Wed night and Thu. This trough is not strong enough to produce precipitation, but could bring an increase in cloud cover. A stronger trough is forecast to follow Thu night, and this one will bring light snow to much of the area. Snow level will be low enough for snow at all elevations, but again, only light precipitation is expected. Through Friday morning at 5 AM MST, only about an inch is forecast for the mountains, with less than half an inch in lower elevations. Temps will be cold Wed, with highs nearly 10 degrees below normal in the valleys where the inversion is most strongly felt. We will see just a couple of degrees of warming from Wed into Thu. Overnight lows will continue to be 5 to 10 degrees below normal through Thu morning, but will rise to near normal Fri morning in most areas.

LONG TERM
Friday through Tuesday...Signaling the end of inverted conditions, a series of winter systems will begin early Friday morning. The first system will exit eastward Friday afternoon. While this system is relatively dry at around 0.3 inches of precipitable water, it should be cold enough for snow to reach the ground throughout our area. We are forecasting less than 0.5 inches of snow in lower elevations, and 2-3 inches in higher elevations. As the upstream flow becomes slightly more zonal, snow levels will begin to lift, and by the end of this first system mixed precipitation is possible in the lower Snake Valley. The second system, arriving early Saturday morning, is initially slightly wetter and warmer with 0.35 inches of precipitable water and 3000-4000 ft MSL snow levels. As the upstream flow becomes more zonal Saturday night, our area will see an influx of tropical moisture. This will raise p-wats to peak around 0.6 inches, the 94th percentile for this time of year. While we may lack favorable upper air dynamics as Saturday's trough exits, orographic effect will continue to support precipitation through Tuesday. This setup supports higher terrain seeing significant amounts of snow, summits could see 20-30" and mountain valleys 8-18". Snow levels will continue to rise through the event, 4000-5000 ft MSL on Sunday becoming 6000-7000 ft MSL by Tuesday. While snow is falling in the high terrain, the low elevations will see around 0.5 inches of rain/mixed precip. Models are in good agreement about the synoptic pattern, but model to model differences, especially concerning vertical extent of freezing temperatures, are giving rise to notable differences in snowfall vs rainfall amounts. Max temperatures will rise from about normal on Friday to 5-10 degrees above normal by Tuesday.

AVIATION
Mainly VFR and partly cloudy. Localized fog/stratus may return Wednesday morning, impacts are most likely in the West Central Mtns, possible elsewhere although confidence is low. Surface winds: E-SE 5-15 kt in the Snake River Valley otherwise variable less than 10 kt. Winds aloft at 10k feet MSL: SW 10-15 kt.

AIR STAGNATION
Stagnant conditions and light winds will persist across much of southeast Oregon and lower/sheltered valleys of southwest Idaho through Thursday morning due to an inversion brought on by an upper level ridge. An Air Stagnation Advisory will continue through Thursday morning in eastern Oregon, and the Lower Treasure Valley and Upper Weiser River Valley in Idaho.



BOI WATCHES/WARNINGS/ADVISORIES
ID...Air Stagnation Advisory until noon MST Thursday IDZ012-033.
OR...Air Stagnation Advisory until noon MST /11 AM PST/ Thursday ORZ061>064.

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