Nampa, Idaho 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

582
FXUS65 KBOI 131638
AFDBOI

Area Forecast Discussion
National Weather Service Boise ID
938 AM MST Tue Jan 13 2026

.DISCUSSION...The strong upper level ridge remains the dominant
feature over the western U.S., keeping the storm track
displaced well to the north. This synoptic setup ensures dry and
stable conditions will persist across southeast Oregon and
southwest Idaho through the short term period. Subsidence
associated with the ridge is capping the boundary layer,
trapping cold air in the lower valleys while temperatures aloft
and over the higher terrain remain unseasonably warm. As an
example, Treasure Valley temperatures this morning were between
25 and 29 degrees where temperatures at Bogus Basin were 38 to
42 degrees. Have adjusted max temperatures today and tomorrow in
the valleys where cold air has been trapped. This stagnant
pattern will support the development of patchy fog and low
stratus, particularly in the Treasure Valley, Magic Valley, and
lower basins of southeast Oregon during the late night and
morning hours. Updated forecast will be out shortly.

&&

.AVIATION...VFR. MVFR-IFR conditions from patchy fog/low stratus,
mainly in sheltered mountain-valleys and lower basins, mixing out by
late morning. Mountain obscuration in fog/low stratus. Chances for
patchy fog/low stratus return overnight Tuesday into Wednesday
morning. Surface winds: variable under 10 kt. Winds aloft at 10kft
MSL: NW 10-25 kt.

KBOI...VFR. Surface winds: light and variable.

&&

.AIR STAGNATION...High pressure will remain centered over the
region through the remainder of the week. This pattern will
maintain strong valley temperature inversions and very weak
transport winds. Forecast mixing heights are expected to remain
below 1500 feet AGL for an extended period. These conditions
will severely limit ventilation, leading to the potential
accumulation of pollutants in valley locations. The Air
Stagnation Advisory remains in effect until 5 AM MST Monday for
all valley zones in southeast Oregon and southwest Idaho. Little
relief is expected until a more significant pattern change
occurs, which looks unlikely until early next week.

&&

.PREV DISCUSSION...
SHORT TERM...Today through Thursday night...High pressure will
continue to bring stagnant conditions with cool valley
temperatures and patchy fog through the short term period. The
biggest uncertainty in the forecast is where and when valley fog
will develop, contingent primarily on the extent of high clouds
and how quickly temperatures cool overnight. As of 2 am MST,
mist and lower visibility is observed over the Lower Treasure
Valley, with moderate forecast confidence (30% chance) that
fog/mist will continue to develop in the Snake River Valley and
in sheltered mountain valleys bringing visibility less than 5
miles this morning. Coverage will remain patchy tonight into the
morning, with likely redevelopment each night through the
forecast period. Temperatures will remain cool in lower
elevation valleys with mid slopes and ridgetops in the 5000-7000
foot range reaching up to 35-40F each day.

LONG TERM...Friday through Tuesday...The high pressure center
will continue to track northward towards British Columbia
through the weekend, keeping the Western US in a vise grip of
stable conditions. Temperatures will continue to stay similar to
the last week, with slight cooling each day dependent on low
cloud cover and fog through the weekend. Mountain valleys will
continue to be very cold overnight, with lows expected to be in
the teens to single digits each night.

Relief from the stagnant conditions looks to come late next Tuesday
into Wednesday with an upper level trough bringing chances of
precipitation and increased mixing. Ensemble members of the
ECMWF/GFS show varying degrees of precipitation and timing of
the next system, but there is generally good agreement in the
breakdown of the ridge by the end of next week.

&&

.BOI WATCHES/WARNINGS/ADVISORIES...
ID...Air Stagnation Advisory until 5 AM MST Monday
     IDZ011>014-016-028-033.
OR...Air Stagnation Advisory until 5 AM MST /4 AM PST/ Monday
     ORZ061>064.

&&

$$

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DISCUSSION...JDS
AVIATION.....CH
AIR STAGNATION...JDS
SHORT TERM...SA
LONG TERM....SA