Area Forecast Discussion
Issued by NWS Pocatello, ID

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FXUS65 KPIH 172058
AFDPIH

Area Forecast Discussion
National Weather Service Pocatello ID
158 PM MST Sat Feb 17 2018

.NEAR TERM...Through Sunday night. Active period of Winter Weather
in store for SE Idaho for the remainder of the Weekend as a strong
Pacific Low and associated Cold Front drops through the region.
Initial concerns this Afternoon are with Winds. SW winds have
already reached Advisory level (gusts to 45 mph) across much of the
Arco Desert eastward to Idaho Falls, so will begin the Wind Advisory
with the Afternoon forecast package, which will be a couple of hours
sooner than the inherited 5 PM start time.

Snowfall is expected to develop ahead of the Cold front late this
Afternoon across the Sawtooths, Stanley Basin and Wood River
Valley. In addition, Snowfall will redevelop from Palisades
stretching Northwest to the hills East of Idaho Falls and Rexburg
north to Island Park just before Sunset. The combination of Snow
and Wind will make for hazardous travel conditions this Evening
for these areas.

As we progress through the Overnight hours, a couple of impactful
bands of Precipitation is expected to develop. One band will be
associated with the Cold Front itself which may produce locally
Heavy Snow. This band is forecast to stretch from Island Park to
Stanley by 2AM, and will quickly emerge out of the Mountains and
into the Snake Plain by Sunrise, with associated Precipitation
gradually diminishing. A second Precipitation band is expected to
develop by Midnight for higher elevation areas south and East of
Pocatello, stretching Westward along the I-86 corridor. Rain is
initially expected in Valleys, but the combination of wet-bulb
effects in conjunction with the Cold Front should allow for a
changeover to Snow for all areas before Sunrise. There is some
uncertainty in the exact positioning of the band. Currently, it is
expected to remain largely south of the I-86 corridor, but only a
slight Northward nudge could mean impactful Snow for the I-86
corridor after midnight. Snow Showers are then expected to continue
through the day Sunday for areas between the I-86 corridor to the
Utah Border. Other areas where Precipitation is expected to continue
through the day Sunday include the Sawtooths, Island Park area and
Highlands along the Wyoming border south to Bear Lake. Snow Showers
will likely persist through Sunday evening for the Bear Lake region
and surrounding areas so will likely extend the current Winter Storm
Warnings. Will also hoist a Headline for areas between I-86 and the
Utah border to account for impacts associated with Snow and Wind for
these areas.

Colder air will filter into the area in earnest Sunday Evening, with
Temperatures expected to quickly drop below freezing all areas.
Heaviest Snow for Sawtooths, Stanley and Wood River valley expected
tonight. For the Island Park area, heaviest Snow expected tonight
through Sunday morning. For I-86 south to Utah and High Terrain east
of I-15 to Wyoming, heaviest Snow expected from the pre-dawn hours
Sunday morning into Sunday afternoon. Snow Accumulations expected to
range from 1-4 inches for the Southern Wood River valley, up to 6
inches for Stanley with amounts around 12 inches for the Sawtooths.
12-16 for Island Park with 6-12 inches for higher terrain areas east
of I-15 to the Wyoming border with locally higher amounts. 6-12
inches for higher elevation areas around Oakley, Malad and Preston.
AD/Keyes

.MONDAY AND BEYOND...Monday night through next Saturday.  With low
pressure remaining over Idaho and the Rockies/Great Basin next
week, look for snow showers to remain in the forecast. Where
showers occur and subsequent snowfall will be dependent on time of
day and any small feature embedded in the pattern to kick them
off. Same with any gusty winds in this pattern. The one thing that
remains a constant...IT WILL REMAIN COLD! In fact, it looks like
the coldest temperatures in several weeks for most of us will
occur Monday- Wednesday. Highs in the mountains will be in the
single digits and teens with 20s for the valleys. Some lower
elevations may actually NOT make it out of the teens Tuesday. We
will warm up after that as northwest flow remains entrenched over
the West. The difference between then and the past few weeks, we
actually see a couple of storms drop south out of Canada. This
will keep any warm up tempered and not really allowing us to warm
much above average, if at all. The ECMWF is a lot stronger with
these storms and would keep much colder air in place vs the GFS.
There will still be a chance of much need moisture especially in
the mountains. Keyes


&&

.AVIATION...Moisture is quickly developing northwest to southeast
this afternoon. Despite the high level of confidence of this storm
affecting all TAF sites in some way in the next 24 hours, there are
likely some issues that need to be monitored and likely changed with
little lead time. At KSUN, snow should begin before 5pm despite warm
temperatures thanks to drier air in place. We are expecting
MVFR/high end IFR weather as this isn`t really a favored pattern for
worse conditions. Snow should end after midnight with VCSH after
that. The potential for showers and at least VCSH continues Sunday.
Ceilings will likely flirt with MVFR as well. At KDIJ, we are also
expecting snow to begin before 5pm. The problem is we have a decent
south wind working against snow coming in from a favorable northwest
direction. Will the south wind win out and keep conditions MVFR or
will the snow be heavy enough combined with a favorable pattern take
over? The current forecast has the latter win out and quickly. We
have conditions rapidly dropping to IFR (potentially LIFR) along
with blowing snow. This may be too early and actually not occur
until late evening. Once it does, look for poor conditions to last
into Sunday morning before switching to VCSH with IFR/MVFR ceilings.
Moving on to KIDA, strong south winds are there as expected. Those
winds will continue into this evening. The current forecast holds on
to the strong winds throughout the night, but there are indications
once the main band of snow moves in (between 23z and 3z) those winds
could cease. Once the snow moves in here, look for IFR/LIFR weather
including possible blowing snow through 10-12z at the very least.
Once this moves south, gusty southwest winds return along with VCSH
and MVFR ceilings. We left KBYI and KPH to the end because they may
end up being the trickiest sites. Strong west to southwest winds are
ongoing and will continue through tonight, although similar forecast
uncertainty for this exists just like KIDA. We are looking for
showers before 6z at these airports. We may not see a lot of
precipitation and just virga/stronger gusts with the very dry due to
downslope conditions. The main band should hit juts after midnight,
with the current forecast dropping to IFR with possible blowing
snow. The band of precipitation should quickly become heavier
overnight, which may or may not allow things to start out as snow
immediately. It could take a little bit for that to happen. The band
should slip into the southern highlands tomorrow afternoon, with
VCSH and MVFR ceilings. Keyes


&&

.PIH WATCHES/WARNINGS/ADVISORIES...
Wind Advisory until 5 AM MST Sunday for IDZ020-021.

Winter Storm Warning until 11 PM MST Sunday for IDZ019-023-025.

Winter Storm Warning from 5 AM to 11 PM MST Sunday for IDZ022-
024.

Winter Storm Warning until 5 PM MST Sunday for IDZ018-031.

Winter Weather Advisory from 5 PM this afternoon to 5 PM MST
Sunday for IDZ032.

&&

$$



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