Area Forecast Discussion
Issued by NWS Pocatello, ID

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FXUS65 KPIH 201526

Area Forecast Discussion
National Weather Service Pocatello ID
926 AM MDT Fri Oct 20 2017

.SHORT TERM...Today and Saturday...A quick update this morning to
message winds and precipitation potential. The latest water vapor
satellite imagery was showing some subsidence coincident with the
upper trough axis over SE Oregon this morning which was making me a
little uneasy especially with the understanding that the upper
trough sweeps through SE Idaho this afternoon. Thankfully the models
were showing mostly stable conditions out ahead of the trough which
would limit deep mixing. Still 9+ 850-700mb lapse rates will mix
down the 700mb 40+kts wind speeds to the surface. Some of the grided
guidance was suggesting the potential for High Wind Criteria across
CNTRL INL while the preponderance of the ERN Magic Valley and Snake
River Plain see advisory level winds. Thus have nudged the winds
higher on this update but kept the advisory. If winds do nose up
into High Wind levels this afternoon, I plan to cover the limited
area with short-term Special Statements. Also, radar and obs were
showing precipitation working into the ERN Magic valley and GFS was
a little more bullish in spreading the love EWRD today so have
nudged the precip probability upwards as well. No real impact on
accumulations as near as I could see. See previous discussion below.

The passage of a cold front today and subsequent trough of low
pressure tomorrow aided by an atmospheric river across the Pacific
Northwest will provide the conditions favorable for rain and snow
over the next 48 hours. Areas of particular concern are the western
facing slopes in the Central Mountains beginning this morning and
continuing through early evening; with another round of snow showers
in this area expected Saturday afternoon and evening. Expect 8 to 10
inches here today and another 6 to 9 inches tomorrow. For those
traveling through highway 75, highway 21, Galena Summit and Banner
Summit, accumulating snowfall will be lower at 1 to 3 inches today
and 1 to 5 inches tomorrow with emphasis placed on highway 21 north
of Stanley. In addition to the snowfall today, winds will be quite
gusty in the 15 to 25 mph range, allowing the risk for blowing snow.

As the cold front progresses to the southeast through the Snake
Plain, very strong winds of 30 to 40 mph are forecast with higher
gusts. A wind advisory will remain in effect from late this
morning through early evening for travel hazards due to cross
winds, particularly to high-profile vehicles. Snow levels this
morning were initially quite a bit higher than forecast models
though they will be dropping today behind the front, it is just a
matter of timing. Snow levels through the Snake Plain are forecast
to drop to 5500 ft later this evening and around 4500 ft
overnight. By this time, much of the precip will have shifted
eastward; the change in this thinking changes the forecast
resulting in very few locations seeing snow in the Snake Plain.
Therefore, expect scattered rain showers this morning and

For the Upper Snake Highlands/Eastern Highland/Southern Highland
locations, precip will start off as rain this late morning and
shift to a rain/snow mix as the front approaches and subsequently
passes through. Increasing winds this afternoon are expected as
well in the 20 to 30 mph range, allowing for areas of blowing
snow. For tomorrow, additional snow showers are likely above 5000
ft, of 2 to 5 inches. NP/RS

.LONG TERM...Sunday through next Friday...A few lingering snow
showers are forecast in the Upper Snake Highland region Sunday
with very little snow accumulations. Otherwise, Sunday will be
transition day with warming temperatures. High pressure remains in
the forecast for next week with continued warm upper 60/low 70
degree temperatures and dry conditions. At this juncture, global
models are hinting at a disturbance nearing our northern borders
on Wednesday/Thursday time frame with slight chances for precip
and increased wind. NP/RS

.AVIATION...A strong cold frontal passage today will bring
sustained wind speeds of 20 to 30 knots with gusts to 45 kts
expected at BYI, PIH and IDA. Expect those speeds to start up
between 15 and 18Z this morning. Rain showers are likely especially
at SUN and DIJ with MVFR ceilings expected and rain could change to
snow at those locations around 17-18Z. Showers at KBYI, KPIH, and
KIDA probably more isolated and difficult to time, as early as 14Z
at KBYI and 16-21Z at KPIH and KIDA. MVFR ceilings possible with
showers. Estimate snow levels over the Snake Plain (KIDA) at 18Z
near 6800 Feet elevation and by 24Z roughly 5700 Feet elevation. RS

.FIRE WEATHER...A surface cold front associated with the low
pressure disturbance moving inland will cross the Snake Plain early
this morning. Southwesterly winds 25 to 35 mph with gusts to near 50
mph will develop this morning and continue through early evening. A
Wind Advisory is in effect. Southwesterly winds will increase over
the exposed upper elevations of the mountains in all areas, gusts to
30 mph pretty common, 35 for the Southern Sawtooth and Caribou
Forests. The disturbance is tracking a little more south which
helped easy the winds on the Lemhi Range down some. Showers will
spread to the Wyoming border by noon today. Through about midnight
Friday night, the west slopes of the Central Mountains, including
Stanley and Galena Summit area could see .10 to .40 inches liquid
equivalent of precipitation, a few west facing slopes possibly a
half inch. Temperatures drop back to the 40s in the mountains and
around 50F valleys Friday. Snow levels after rising to near 10000
feet elevation on Thursday, come back down to around 5500-6000 feet
Friday afternoon and continue to fall to the valley floors by
Saturday morning. Scattered showers will diminish Friday night and
early Saturday before the next round of moisture advances across the
Central mountains and Eastern Highlands Saturday night and Sunday.
This second round may target the west slopes of the Central
Mountains Saturday evening through Sunday evening, thus the wet
facing slopes could see an additional .1 to .3 inches of
precipitation. Banner and Galena Summits may see 1 to 4 inches of
snow from this second round. Then a gradual return to dry high
pressure ridge follows Monday and Tuesday. Models are more in
agreement by Wednesday of next week and bring a chance of showers in
north-northwest flow through the Salmon-Challis and Targhee Forest
area. This is followed by another two day period under a ridge of
high pressure. RS

Wind Advisory continues through 9 PM MDT this evening for IDZ017-020-

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