Area Forecast Discussion
Issued by NWS Pocatello, ID

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000
FXUS65 KPIH 222122
AFDPIH

Area Forecast Discussion
National Weather Service Pocatello ID
222 PM MST Thu Feb 22 2018

.SHORT TERM...Tonight through Sun afternoon. Storm that wrought
so much havoc in the Boise area appears to be slow in arriving
here and is also breaking up somewhat. However, still expect some
light snow with some winds and will let the Winter Weather
Advisory continue for the area with the greatest impacts, the
Sawtooths and Wood River drainage. Light snow will spread across
all zones, with 0.4 to 2 inches expected in other locations, with
1 to 3 inches in the southern highlands between the Snake River
and the Utah border. Cloud cover will keep temperatures warmer
tonight. Fri and Fri night are a break from the moderate and heavy
precipitation with breezy afternoon conditions ahead of an
incoming storm for Sat and Sat night. This will come with
widespread moderate and some high elevation heavy snow, along with
windy to very windy conditions for Sat afternoon/evening and again
Sunday afternoon/evening. A Special Weather Statement has been
issued with forecast amounts. The wind, combined with the moderate
snow, may need more than a Winter Weather Advisory, so that is
the reasoning behind going with a Special Weather Statement, not
locking in future forecasters to just an Advisory. Will take
another look tomorrow on that score. Overnight lows will be
similar to this morning for tonight, but cloud cover Fri night
will keep temperatures somewhat mild for that night and again on
Sat night. With a shift to southerly air flow, afternoon high
temperatures will also warm slightly, offset partially by the
increase in cloud cover. Messick

.LONG TERM...Monday Through Next Friday Morning.
The large-scale pattern remains unchanged for the upcoming week,
with an upper-level trough over the western US. There are signs that
this feature may start to break down next Thurs/Fri, transitioning
toward some mid to upper-level ridging near or just east of our
region. This would result in more of a deep, SW flow and perhaps
finally some real temp modification. Until then, no significant temp
advection is expected, supporting all snow throughout the week.

In the meantime, shortwave troughs will cont to dive SE around the
parent upper trough this week, with the next system on tap for Sun
eve-Tues AM. GFS/EC continue to show good agreement on not only the
track of this system, but also on distribution of precip and on a
deep, pronounced trough with possible cutoff low development over
CA. The latest blend of the GFS/EC/NBM with WPC guidance yields only
minor tweaks to the previous forecast as well. While this storm is
still several days out, all of this does nudge our forecast
confidence up just a bit, and we continue to increase PoPs. This is
looking like yet another potentially major winter storm with
significant snow accumulations and strong winds possible. A very
preliminary look at snow totals suggests a solid advisory event (6+
inches) for the western Sawtooths, the Upper Snake Highlands/mntns
surrounding the Teton Valley, and higher elevations of the Caribou
Highlands, Bear River Range, and Srn Highlands. Up to several inches
will be possible at lower elevations including the Snake Plain.
While we have certainly done worse in terms of storm totals, the
exacerbating factor will be the wind, sustained at 15-25 MPH with
gusts of 25-35 MPH in the eastern Magic Valley, Snake Plain, Island
Park region, and ridges. Where these winds overlap with active
snowfall and even limited accums, blowing/drifting snow could have a
significant impact on travel. Everyone is encouraged to monitor our
forecasts as we evaluate model trends over the next few days.

Tues eve-Fri AM, we should generally trend drier w/ no significant
systems pushing through, but lingering moisture and light snow
showers are possible, especially in the mntns. - KSmith/Messick

$$

.AVIATION...
Our approaching low pressure system will bring some level of
cig/vsby impacts to all terminals over the next 24 hours. Model
guidance develops a band or zone of snow just ahead of the
low/trough axis, translating WSW to ENE this afternoon and tonight.
The majority of model guidance has trended faster with the
progression of this activity, so significant changes were made to
the 18Z TAFs particularly to adjust TEMPO groups to reflect the
latest target period of greatest potential impacts.

At KSUN, expect snow through late eve, with the period of greatest
impacts/best potential for airport closures expected 21Z-01Z, before
VFRs return overnight/clouds thin out. As the snow pushes east, now
expecting the greatest impacts/lowest cigs and vsbys at KBYI 00Z-
04Z, and KPIH 04Z-08Z, with IFR conditions possible. As precip
departs, most of the models suggest lingering low ceilings, but this
is not reflected in the latest statistical MAV/MET guidance, so we
have low confidence in the BKN025 cigs. Greatest impacts at KIDA now
expected 06Z-10Z, but the band seems to break up a bit, so we may
only bottom out at MVFR here. Impacts at all of these terminals
could start earlier if guidance continues to trend faster. The most
uncertainty is at KDIJ, with the overall flavor being less impact
with a longer period of snow showers. MVFR conditions are possible
for much of the night into Fri, with IFR not out of the question if
the Teton Valley sees persistent snow showers. - KSmith/Messick

&&

.PIH WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory until 6 PM MST this evening for IDZ018-
031.

&&

$$


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