Wildfire in Idaho, September 14 to 30, 2020

Area Forecast Discussion
National Weather Service Pocatello ID
246 AM MDT Mon Sep 14 2020

.SHORT TERM...Through Wednesday night. A ridge of high pressure is
currently centered over southern Wyoming, and this ridge will
temporarily shift eastward through Tuesday. As a result, broad,
southwesterly flow will continue to develop across eastern Idaho,
first across mid and upper slopes above nocturnal inversions. The
breeziest conditions will be across the central Idaho mountains
and Arco Desert today, where gusts will top out around 25-30 mph.
This will create critical or near critical fire weather
conditions, and as such a Red Flag Warning is in effect this
afternoon and early evening across the central Idaho mountains.
With the broad southwest flow in place, smoke from the Western
U.S. smoke reservoir will be most prominent across the central
Idaho mountains. Across northern Nevada and northwest Utah, smoke
concentrations are lower, and this air will filter over far SE
Idaho today and Tuesday, so the biggest smoke issues through
Tuesday should be centered over the central Idaho mountains and
perhaps eastern Magic Valley. The ridge of high pressure will
begin to build back across our area by late Tuesday, strengthening
inversions and lowering wind speeds. The ridge is forecast to
remain in place through late Wednesday. This is a similar setup to
what we saw last weekend, and smoke conditions may worsen across
the region as a result. Temperatures will remain warm through the
period, 10-15 degrees above climatological averages, even after
accounting for wildfire smoke.

AD

.LONG TERM...Thursday through next Monday. The ridge will once
again shift south and east of our area beginning Thursday, and at
the same time and area of low pressure will be approaching the
PacNW coast. The interplay between these two features will result
in a redevelopment of broad southwest flow across the region along
with a return to breezy conditions. The low pressure system and
associated cold front will begin to enter our area late Friday,
and showers and isolated thunderstorms should begin late in the
day, especially across the central Idaho mountains. After several
days of excellent model agreement, agreement and associated
forecaster confidence has begun to dissolve a bit with this
system. The GFS now develops a large precipitation shield ahead of
the front across far SE Idaho Friday night into Saturday morning.
This idea does have some ensemble support, so have followed the
National Blend of Models idea that modestly bumps up precipitation
potential during this time. Showers still look like a good bet
across the central Idaho mountains, Montana border region and
Wyoming border region from late Friday through Saturday. Breezy
to locally windy conditions will accompany the cold frontal
passage on Saturday morning. Temperatures will cool back down to
seasonal averages behind the front on Saturday, with the cooler
temperatures remaining through next weekend.

AD

&&

.AVIATION...Wildfire smoke will continue to be the main story
through the period with respect to aviation. The lowest visibility
will continue to be in and around KSUN, where predominantly MVFR
conditions should persist through the period. Other than KSUN, VIS
should bounce around between MVFR/VFR through the period. A large
and growing wildfire SW of KBYI may spread a smoke plume towards
KBYI today and tomorrow as SW flow thorugh the column becomes more
established. South winds of 20-30kt will be in place this morning
and tonight, causing borderline wind shear issues, especially for
KBYI, KPIH, KIDA and KDIJ. SW/W winds at the surface should
increase into the 10-15kt range by this afternoon once inversions
mix out.

AD

&&

.FIRE WEATHER...A Red Flag Warning will be in effect this afternoon
and evening for zones 475 and 476 resulting from expected wind gusts
in the 25 to 30 mph range and critically dry conditions from 8 to 15
percent; critical fuels remain in place as well. Widespread dryness
are noted across the remainder of forecast zones today; averaging a
Min RH value of 11 percent. Winds and wind gusts will be increasing
this afternoon across much of the region out of the southwest; with
gusts nearing the 25 mph mark across portions of zone 413 and 427.
Otherwise, gusts at or less than 20 mph can be anticipated for 411,
410 and 422. Looking at tomorrow, improved conditions are expected
as a whole though windy afternoon conditions are likely again across
476 and 475. These winds should not be as strong compared to today.
ND

&&

.AIR STAGNATION...A ridge of high pressure will be in control of the
weather pattern the next several days with some stronger winds
across Central Idaho. That may allow for some improvement, or at
least quick transport of smoke into that area later today. Similar
improvements are possible for Southeast Idaho under increasing wind
speeds this afternoon as well. Forecasts after today become pretty
uncertain; depending on how models handle the smoke coming in again
from CA as well as how local fire develop and their smoke impact.
ND

&&

.PIH WATCHES/WARNINGS/ADVISORIES...
Red Flag Warning from noon today to 9 PM MDT this evening for
IDZ475-476.

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