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FXUS65 KBOI 281015 AFDBOI

Area Forecast Discussion National Weather Service Boise ID 315 AM MST Tue Nov 28 2023

SHORT TERM
Today through Thursday night...High pressure aloft will continue the current inversion and air stagnation through Thursday morning. Later Thursday the high pressure aloft will break down as a trough approaches from the Gulf of Alaska. The trough will spread light snow across the area from west to east late Thursday through Thursday night and into Friday. The trough will also weaken the inversion, with higher elevations becoming colder while the valleys become milder. Until then, the inversion will remain strong with temperatures changing little from day to day, coldest in the snow-covered valleys, cold in other valleys, and relatively milder at higher elevations. Light winds through Thursday night.

LONG TERM
Friday through Tuesday...Thursday will be the last day of the inverted and stagnant conditions, with temperatures expected to be 5-10 degrees below normal as a weak trough develops over our region on Thursday. A long awaited ending to inverted conditions will likely occur early Friday as a series of upper level troughs begins to enter the region from the northwest. The first trough passage will bring limited precipitation amounts due to low moisture, however, it will be cold enough to support valley snow.
The next troughs will bring much greater moisture aloft, with precipitable water values in the 75th-85th percentiles. However, the upper level flow will shift to the northeast from an advancing Pacific High. This will increase temperatures, with snow levels likely around 4000-5000 feet MSL by Sunday afternoon. Therefore, the best chance for accumulating valley snow is late Friday into early Saturday. Higher elevations above ~5000-6000 feet are likely to do well with 15-30 inches possible from Friday afternoon to Monday afternoon. The current NBM probabilities show about a 20% chance of McCall, ID exceeding 12 inches of total accumulation from Friday-Monday. Models show continued good consensus on precipitation for the weekend, but more variation is shown in amounts, especially amongst Saturday- Sunday ensemble members in the GFS/ECMWF. The clusters show good agreement about the synoptic pattern, with only slight variation in the depth of the successive troughs during the weekend. Temperatures during the long term period will be around 5-10 degrees below normal on Friday, but will increase to about 5 degrees above normal on Monday with the Treasure Valley seeing highs in the upper 40s.

AVIATION
Mainly VFR. Localized valley fog/stratus in the Lower Treasure Valley, Upper Weiser Basin, and Malheur County will briefly form this morning and mix out by early afternoon.
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-W 5-15 kt.

AIR STAGNATION
High pressure aloft will keep the subsidence inversion in place through Wednesday, capping mixing heights below 2000 feet AGL. Stagnant conditions and light winds will persist across much of southeast Oregon and lower/sheltered valleys of southwest Idaho through Thursday. Southeast winds 5 to 15 mph will allow slightly better mixing in the Upper Treasure Valley and Magic Valley today, but lighter winds Wednesday will cause poorer mixing again. 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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