Wildfire in Idaho, July 19 to 21, 2020

Area Forecast Discussion
National Weather Service Pocatello ID
224 PM MDT Sun Jul 19 2020

.SHORT TERM...THIS AFTERNOON THROUGH TUESDAY NIGHT...
Synoptically, a WNW flow will continue through Tues, supporting
continuation of dry, hot, and modestly breezy conditions across all
of SE Idaho. The active storm track still appears to remain shunted
well to our N and E across Canada into the nrn Plains, and this flow
should also generally serve to block any monsoonal moisture from
advecting in from our south. While we can't completely rule out a
shower or dry t-storm creeping into Cassia County late Tues
afternoon or the rest of the srn highlands Tues night, the real
pattern change/shift will hold off until Wed, so confidence remains
high in a dry forecast for everyone else. High temps will generally
reside close to climo norms for mid-July and warm a couple degrees
each day, reaching 90-96 by Tues afternoon across the ern Magic
Valley, Snake Plain, and valleys across the srn highlands (a bit
cooler elsewhere and in the mntns, as usual). These raw values,
combined with decent overnight recovery and slightly lower highs
straddling Tues (Mon/Wed), are not invoking much of a response in
our projected HeatRisk values, so we're not too concerned about a
potential Heat Advisory or significant heat-related issues for most
groups (outside of the standard heat-related risks and associated
safety rules that should always be observed on warmer days). Thus,
no high-impact hazards are expected through Tues night. Overall
forecast confidence is high. - KSmith

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.LONG TERM...WEDNESDAY THROUGH NEXT SUNDAY...
By early Wed, longwave troughing will be building into wrn Canada/
the PacNW, and this will gradually become further established and
build southward across the western US over the latter part of the
week into next weekend. At the same time, a weak low-pressure center
will drift inland over CA (kind of an embedded feature in the
overall trough). These features will work in tandem to back our
winds into the SW. This, folks, changes the whole game. Monsoonal
moisture from the 4-corners region will no longer be blocked and
will instead be advected northward into our forecast area, with
moistening RH values. At the same time, a more pronounced shortwave
will start to rotate through the longwave trough inland across
WA/OR, increasing shear and broad ascent over the region. Taking
advantage of the increased moisture and plenty of instability in
place thanks to continued warm temps, the combination of this ascent
and local orographic lift in the mntns will generate t-storms Wed
afternoon into early Wed night. We are concerned that this first
round on Wed will still lean dry (favoring virga with very little
meaningful precip/QPF reaching the ground) as it will take some time
for the low-level airmass to moisten, and given our recent long
stretch of dry wx, any lightning strikes from dry t-storms will pose
a fire ignition risk. Our fire partners are monitoring this scenario
closely as well, and we are engaged in daily coordination calls with
GBCC and adjacent offices. Should expected t-storm coverage reach
sct, a Fire Weather Watch may need to be considered as early as Mon
for this Wed activity. Nudged PoPs slightly above NBM guidance based
on coverage advertised on the GFS/ECMWF 12z deterministic runs, and
ran with WPC guidance for QPF, which only manages to lay down
isolated pockets of a few hundredths of an inch of rain. The axis of
the shortwave feature mentioned above rotates inland across the nrn
Rockies Tues and even takes on a bit of a negative tilt (sometimes a
severe wx signal), so another round of t-storms is expected Thurs
afternoon/eve with better shear/dynamics and slightly better
coverage. Better moisture too will support more efficient downpours,
with some areas potentially exceeding 0.10 inches if hit directly by
a storm, especially in the Central Mntn region. Strong t-storms will
be a concern with both the Wed and Thurs activity. Additional storms
are possible each afternoon from Fri through the weekend as broad
troughing remains in place. Temps should hold steady pretty close to
norms, or perhaps back off a few degrees over the week due to the
increased clouds/t-storms. - KSmith

&&

.AVIATION...
Will have continued VFR conditions today through the foreseeable
future. Expect slightly breezy wind gusts this afternoon around 14
to 20 knots briefly for KPIH, KBYI, and KIDA. - Wyatt

&&

.FIRE WEATHER...
Expect dry weather through Tuesday. Do not expect winds to be quite
as strong today in Zone 425 with most gusts in the 20 to 25 mph
range this afternoon. There will be some isolated areas from Rock
Lake/Richfield to Potter Butte with gusts of 30 mph with critical
humidity mainly between 3 to 7 pm. Gusts will be much lighter in
all other zones. Expect lighter winds Monday and Tuesday though
expect continued warm and dry conditions. Expect a Haines of 6 by
Tuesday. The next bigger impacts will be monsoonal moisture from
southerly flow bringing a slight chance to chance showers and
thunderstorms Wednesday through next Sunday. - Wyatt

&&

.PIH WATCHES/WARNINGS/ADVISORIES...
None.

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