Area Forecast Discussion
Issued by NWS Pocatello, ID

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000
FXUS65 KPIH 182056
AFDPIH

Area Forecast Discussion
National Weather Service Pocatello ID
156 PM MST Sun Feb 18 2018

.SHORT TERM...Through Tuesday night. No changes to active
Headlines. Will Highlight the Pocatello area west into the Arco
Desert with a Special Weather Statement for a likely convergence
band and associated locally Heavy Snow for Monday evening`s
commute. Details follow. Active mid-level frontal zone currently
located along the ID/UT border will allow for a of Snow
along/south of the I-86 corridor into the post-Sunset hours. The
most persistent Snow will then shift into the Bear Lake region
before diminishing around/just after Midnight. Additional Snow
Showers are expected to propagate Eastward along the I-86 corridor
through about Sunset. Elsewhere, Arctic surface boundary has
propagated Southward and currently extends from INL to Sage
Junction to the Ashton Hill area. This boundary has advanced
farther South than expected. As a result, the low-level
convergence forced by the boundary combined with daytime heating
has allowed for a consolidated band of Snow Showers, locally
heavy, with embedded lightning, to develop farther South than
expected. This line is expected to remain active into this
Evening. Could see some semblance of Shower activity continuing
well into the night along this boundary, but intensity will likely
wane with the loss of daytime heating.

Going forward, the anomalously strong (heights and temps around
record lows for this time of year) trough will pivot overhead on
Monday. Embedded disturbances will interact with the existing
surface boundary which is expected to meander South across the
Snake Plain Monday into Monday evening. As a result, a convergence
band is forecast to develop Monday afternoon by the entire model
suite. This boosts confidence in the development of the band and
associated impacts to the Monday Evening commute due to Snow. The
exact location of the band is uncertain, but models agree it will
set up somewhere in between an Arco to Coldwater to Arco to
Blackfoot line. Locally Heavy Snow will be possible beneath this
band, which could mean slick driving conditions and low visibility
for the Monday evening commute along portions of the I-86 and
I-15 corridors. The peak Snowfall intensity with the band is
expected to occur during the peak of the Evening commute (5-7 PM).
Further enhancing impacts will be quickly cooling temperatures to
below freezing after Sunset. The loss of daytime heating combined
with cold advection could make for a hazardous commute for areas
where this band sets up. 1-3 inches of Snow with locally higher
amounts are possible with the band. Elsewhere, diurnally enhanced
Snow Showers will be possible across the area Monday, with
mountains favored. Depending upon the extent of cloud cover, low
Temperatures Tuesday morning may make a run at 0 degrees across
much of the Snake Plain. Temperatures will remain well below
climatological averages on Tuesday with the trough overhead.
Scattered Snow Showers will be possible across the area on
Tuesday, with the Afternoon instability boost favoring the late
Afternoon/Early Evening timeframe for the most widespread
activity, with Mountains again favored. In terms of winds, areas
west of the convergence band will see breezy W/SW winds on the
order of 10-20 mph with gusts to 35 mph, strongest This
Afternoon/Evening and again Monday afternoon. North winds with
gusts to 35 mph will be possible on the other side of the
boundary, particularly along the I-15 and HWY 20 corridors from
Idaho Falls north, and where parallel valleys of the Birch Creek
and Little lost River Valley empty into the INL. AD/Keyes

.LONG TERM...Wednesday through next Sunday. Northwest continues with
a lot more active and cooler weather ahead. The next "wave" moves
through Wednesday, followed by another one later Thursday/Friday and
again later next weekend. Although temperatures will climb, we are
NOT expecting temperatures to be as warm as recent days and weeks.
Each of these storms will help to keep cooler air overall in place.
We will have to see how each of the storms develop as they move
ashore. There are some indications that the 2nd storm could actually
pull farther west, which could leave us drier but potentially pull
colder air in to eastern Idaho. At least there is a daily chance of
adding to much needed snowpack and seasonal snowfall.  Keyes



&&

.AVIATION...The main band of snow has shifted into the southern
highlands this afternoon. That leaves the rest of the TAF sites with
VCSH and potential snow or graupel showers through the next 24
hours. There is even the potential for an isolated thunderstorm
through sunset today. As of 115pm, it looks like that might be trying
to form north and west of KIDA and KDIJ, and even west of KBYI
toward KTWF. Any of these showers produce reduced visibility and low
ceilings (IFR or lower). Gusty winds have continued this afternoon
as well. We will eventually see north winds pushing southward
tonight and Monday. We should see this happen at KIDA and KDIJ 10-
13z tomorrow, and KPIH 14-16z. At the moment, the wind shift should
NOT reach KBYI. In areas where showers aren`t occurring tonight and
in the morning, there should be some scattered/broken stratus. Keyes


&&

.PIH WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory until 5 PM MST this afternoon for IDZ032.

Winter Storm Warning until 11 PM MST this evening for IDZ019-
022>025.

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

&&

$$



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