Area Forecast Discussion
Issued by NWS Pocatello, ID

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FXUS65 KPIH 200947

Area Forecast Discussion
National Weather Service Pocatello ID
347 AM MDT Thu Sep 20 2018

.DISCUSSION...Today Through Next Thursday.
Expectations for the large-scale pattern remain unchanged this
morning, at least for the next several days. A fairly robust
trough/attendant cold front are in the process of crossing SE Idaho
early this AM, with a few residual showers observed on radar in the
Island Park area and NW WY. These should come to an end this AM as
the front continues east, with a W-NW wind ushering in a fresher and
cooler airmass today. Expect clearing skies, plenty of sunshine, and
high temps in the upper 60s/low 70s in lower elevations from the
Snake Plain southward (60s elsewhere and cooler in the mntns).
Breezy westerly winds are expected this afternoon. A real taste of
fall and a gorgeous day for outdoor work or recreation! Speaking of
winds, gusts to 25+ MPH across the Snake Plain and eastern Magic
Valley are expected, but projected minimum RH values of 15-20% look
to hold us shy of Red Flag criteria. See the FIRE WEATHER section
below for more details. Bear in mind that near-record dry fuels and
near-critical fire weather conditions will still favor effective
fire spread even without hitting Red Flag thresholds. Largely zonal
east-west mid- to upper-level flow with perhaps weak ridging is
expected Fri and Sat with continued dry weather, mostly sunny skies,
and warming temps on a SW surface wind. Sat afternoon looks breezy
region-wide, with high temps pushing 75-80 degrees again in many of
our valleys and plains. Went with a forecast/Superblend mix for
temps, but this seemed to be running a bit cool for highs and a bit
warm for lows compared to numerical MAV/MET guidance, so adjusted
high temps up 1-3 degrees and lows down 1-2 degrees for each of the
next 3 days. Forecast confidence is moderate to high through Sat.

Another strong trough/cold front will approach SE Idaho Sun.
NAM/GFS/ECMWF guidance has trended a bit stronger/more pronounced
with this feature, increasing confidence in at least some shower
activity and another cool-down. Potential vorticity/height analysis
continue to favor frontal passage west to east late Sun afternoon
through about midnight Sun night, with the EC lagging by several
hours. Still not seeing much in the way of precip making it down
into the Snake Plain, so few changes were made to the ongoing PoP
forecast. Isolated showers will begin early Sun morning in the
Central Mntns north of Ketchum and Mackay, nudging just a bit south
and spreading east into the Upper Snake Highlands and Teton Valley
during the afternoon with isolated to scattered coverage, tapering
off last in the Island Park area by late Mon AM. This looks like
another nuisance event as far as rain goes, with no model guidance
painting more than one-tenth of an inch of QPF. However, strong
winds are possible along/behind the front Sun afternoon/eve, gusting
in excess of 40 MPH in the Snake Plain/Arco Desert.

Forecast confidence wanes a bit next week. Models still favor a long-
wave trough over the central US, but are now suggesting development
of a blocking ridge over the Rockies as well. This may shift the
more active NW flow zone down the windward side of the trough a bit
further east, and keep shortwaves riding that ridge from having much
of an impact on our forecast area. This trend also suggests warming
by late week. For now, we are keeping the forecast dry Mon through
Thurs with gradually warming temps and no major wind issues. We will
still need to watch for patchy frost in the Snake Plain, especially
Tues and Wed mornings with lows close to freezing and good
radiational cooling under clear skies. - KSmith/RS


Gusty west winds of 15-20 kts are expected this afternoon at
KPIH, a little lower at KBYI and KIDA, otherwise VFR. Lighter
surface winds expected on Friday. - RS


Cooler temperatures and higher humidity is expected to offset
some gusty winds this afternoon. Winds this afternoon around
Aberdeen and Pocatello are expected to gust up to about 32 mph.
Winds may gust 25 to 30 mph around Shoshone as well. Wind gusts
close to 25 mph are expected between McCammon and Soda Springs,
but once again not real widespread. Will stay with the previous
coordination and not go with a Red Flag. Temperatures rebound some
on Friday, but with lower winds. The next front is expected on
Saturday with afternoon gusts at or above 25 mph in the Central
Mountains and the Caribou and the exposed upper slopes of the
Targhee Forest areas. Southerly winds early Saturday morning will
also be breeze on the Southern Sawtooth. A low pressure
disturbance will advance across the area on Sunday, bringing a
chance of showers and thunderstorms roughly north of a Stanley,
Mud Lake, to Driggs boundary. Best chance of any precipitation
reaching 0.10 inch would be north of Shoup and North Fork,
possibly near Driggs. - RS


Widespread smoke/haze and attendant air quality issues are not
expected over the next several days, with moderate to high
forecast confidence. A fresh airmass is on its way in behind this
morning`s cold front on a N-NW mid-level (500mb) flow of 35-45
MPH, and breezy SW surface winds of 10-15 MPH coming up the Snake
Plain. The Idaho Department of Environmental Quality currently
rates air quality as "Good" or GREEN across all of SE Idaho. Winds
will relax a bit and become more westerly Fri and Sat under
largely zonal flow. This will maintain transport of smoke from
major fires in WY and UT away from our area, and both the HRRR and
RAP smoke models do not signal any significant incoming smoke
from current upwind fires in OR. Another strong/pronounced
trough/cold front are progged to sweep across SE Idaho Sun eve
with another round of wind and a wind shift into the NW. LOCALIZED
pooling of smoke/degraded air quality in valleys near local
active fires such as Bible Back, Rabbit Foot, Indian Butte, and
the new Oneida Fire will be possible, especially at night as winds
weaken and decouple from the environmental flow, but the
progressive pattern does not favor widespread issues. - KSmith/RS




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