Garden City, Idaho 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

217
FXUS65 KBOI 031639
AFDBOI

Area Forecast Discussion
National Weather Service Boise ID
939 AM MST Tue Dec 3 2024

.DISCUSSION...Latest obs and webcams show fog decreasing this
morning in the Upper Treasure Valley but continuing in the Lower
Treasure Valley.  We will end the Dense Fog Advisory at 11 AM MST
in the Upper T.V., but keep it going until 2 PM MST in the Lower
T.V., then start another Dense Fog Advisory this evening through
Wednesday, and likely repeat the process Thursday and Friday.
The Air Stagnation Advisory will also be extended through 5 PM MST
Saturday.

Latest models continue the low-level inversion with air stagnation
and areas of fog (dense in the Lower Treasure Valley) through
Saturday, but the new deterministic GFS, ECM, and CMC bring in a
strong upper trough from the Gulf of Alaska Sunday. The trough
should break the inversion as models bring a surface cold front
and briefly gusty west winds through the area late Saturday night.
The front will be followed by much colder air at higher elevations
while mixing out the inversion at low elevations. The mixing will
warm the cold valleys and disperse the fog. Models then bring the
upper trough in with a chance of snow showers late Sunday/Sunday
night, better chance in the GFS than in ECM. After that the models
show a new, cleaner, and colder air mass in our area Monday under
strong northerly flow aloft, followed by the beginning of another
inversion Tuesday.

&&

.AVIATION...Areas of LIFR fog and low stratus at KONO, KEUL, KMAN,
KMYL, and KMUO this morning as an inversion remains in place. Some
improvement in visibility is expected this afternoon, but low clouds
will persist through the day. Tonight, fog and stratus may expand in
coverage through the Snake Basin with LIFR conditions, with mountain
valley fog also expected. Elsewhere today, it will be clear and VFR.
Surface winds: variable less than 5 kt. Winds aloft at 10kft MSL:
variable around 5 kt.

KBOI...MVFR from mist this morning, with VFR conditions this
afternoon. Winds will be light and variable today. 20% chance of
IFR/LIFR low stratus or fog moving in this afternoon, increasing to
60% chance after 02z this evening.

&&

.AIR STAGNATION...Mixing heights will stay well below 2000 ft
AGL in the valleys through Saturday. Winds should continue 5
mph or less and stagnant conditions will allow air pollutants to
accumulate. Increasing confidence (>50% chance) in a trough
arriving Sunday being strong enough to scour the inversion and
clear out the stagnant air.

&&

.PREV DISCUSSION...
SHORT TERM...Today through Thursday night...The surface-based
inversion remains, and will be with us through the short term
period. A weak upper level trough is expected to come through
Thursday night, but it will bring nothing more than some high
clouds. Winds will remain light for most folks, and low
clouds/fog will likely remain in and near where it was Monday.
There will be little change in temperatures, but the diurnal
range in areas with low clouds and/or fog will become reduced -
lowering daytime highs and potentially slightly increasing
overnight lows. The ridge aloft that is bringing the inversion
is forecast to reach maximum intensity Wed into Thu, resulting
in the warmest temperatures in the mountains. With all that
said, changes in the short term part of the forecast were minor.
Both the Dense Fog Advisory and the Air Stagnation Advisory will
continue as issued.

LONG TERM...Friday through Tuesday...The upper ridge is expected
to weaken Friday and especially Saturday in advance of an upper
level trough forecast to move in Sunday or Sunday night. This
feature has better model agreement this morning compared to
yesterday, although the exact strength and timing are still
uncertain. This system, if it comes through as presently
depicted in the latest operational runs, would push a strong-
enough cold front through the region Sun/Mon. And this front
appears to be strong enough to (at least temporarily) end the
inversion. It is also strong enough to warrant a good chance of
precipitation, especially in the north (40-70%, with 15-40%
across the south), Saturday and Sunday. These precipitation
chances decrease Monday to 15-30% (across the north) and then
become too low to mention Tuesday, with even lower chances in
the south. Temps will remain inverted Friday, perhaps a little
less so Saturday, and should return to a more normal pattern
Sunday through Tuesday.

&&

.BOI WATCHES/WARNINGS/ADVISORIES...
ID...Air Stagnation Advisory until 9 PM MST Friday
     IDZ011>014-016-028-033.
     Dense Fog Advisory until 11 AM MST this morning IDZ012-014.
OR...Air Stagnation Advisory until 9 PM MST /8 PM PST/ Friday
     ORZ061>064.
     Dense Fog Advisory until 11 AM MST this morning ORZ064.

&&

$$

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DISCUSSION...LC
AVIATION.....ST
AIR STAGNATION...LC/SP
SHORT TERM...SP
LONG TERM....SP