Area Forecast Discussion
Issued by NWS Boise, ID

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FXUS65 KBOI 270337

Area Forecast Discussion
National Weather Service Boise ID
837 PM MST Fri Nov 26 2021

.DISCUSSION...Lingering isolated showers should diminish
overnight. There`s a slight chance for showers to re-develop over
Baker County and west-central ID Saturday. Otherwise, high
pressure will build into the region and yield mostly dry and calm
conditions starting this weekend and into next week. Valley
inversions will develop as high pressure strengthens over the
region. Temperatures will trend above normal. An Air Stagnation
Advisory is in effect through Wednesday.


.AVIATION...Isolated light showers bringing MVFR conditions with
brief mtns obscurement. Elsewhere, pockets of fog developing over
lower mtn valleys in Idahos West Central Mtns including KMYL by
sunrise. Otherwise VFR overnight. Surface winds variable 10 kt or
less. Winds aloft at 10k ft MSL West to Northwest 15 to 30 kts.

Sunday Outlook...VFR. Isolated snow showers over the mountains of
Baker County Oregon and west central Idaho through Sunday
morning. Patchy night and morning fog. Decreasing clouds on Sunday
as a high pressure ridge strengthens over the area.


.AIR STAGNATION...Upper level high pressure is expected to
continue well into next week. An Air Stagnation Advisory has
been issued for eastern Oregon and the valleys of western Idaho
through next Wednesday morning for a prolonged period of low
mixing-height (at or less than 2000 feet AGL) together with
light winds near the surface, and no precipitation. With these
atmospheric conditions in place for the extended period of time,
air quality will eventually worsen. The Air Stagnation Advisory
only addresses atmospheric conditions, not pollution. Air quality
is addressed by Idaho and Oregon Departments of Environmental
Quality (DEQ).


SHORT TERM...Tonight through Sunday night...Extensive mid-level
clouds, and a few faint radar echoes on the Idaho side this
afternoon. South-central Idaho has been sunny, though.  Upper
level ridge will rebuild tonight and Saturday morning after being
flattened today by a weak passing through. A Pacific warm front
will bring a slight chance of rain and snow to northern areas
Saturday night. Sunday looks dry and warmer, and mostly sunny in
the south but continued mostly cloudy in the north. Snow level
will rise to nearly 10000 feet MSL Saturday night and Sunday
behind the warm front, then lower slightly in eastern Oregon
Sunday night. The upper ridge will create an inversion that will
last into next week.  Air stagnation will result at low levels
with mixing heights staying at or below 2000 feet AGL and surface
winds remaining light. These conditions do not by themselves
indicate poor air quality, but they do mean that air quality
will eventually worsen if they continue, which looks likely now.
See the Air Stagnation section below.

LONG TERM...Monday through Friday...Zonal flow aloft Monday will
allow for a chance of showers in the higher terrain in the northern
part of the CWA. Ridging will build back in Tuesday, keeping the
area dry. A weak system will slide by to the north Wednesday and
Thursday, bringing back a chance of showers in the north. At this
time Friday looks dry, but there is the typical model disagreement
at that time frame. While all of the above is occurring, there
will be a surface-based inversion for much of the area, and an
Air Stagnation Advisory has been issued that goes all the way out
through Wednesday morning.


ID...Air Stagnation Advisory until 11 AM MST Wednesday
OR...Air Stagnation Advisory until 11 AM MST /10 AM PST/ Wednesday



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