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FXUS65 KPIH 230500
AFDPIH

Area Forecast Discussion
National Weather Service Pocatello ID
1000 PM MST Tue Jan 22 2019

.UPDATE...A very late update to tweak the overnight forecast. In
this case mainly to raise overnight lows based on current trends,
and snowfall just a bit. Light snow has already started in a few
places in the Island Park and Teton Valley areas. Snow will
continue to spread south across eastern Idaho overnight. The bulk
of it should fall from Island Park south through the Teton Valley,
Big Holes and into the Caribou Range. Those area could see
anywhere from 1-3" before sunrise. Along the I-15 corridor south
through McCammon, look for very light snow on the Plain floor with
under 1" along the benches. The higher peaks of the Blackfoot
Mountains, the Marsh Highlands and the Sawtooths could see an
inch or two as well. Otherwise, little to no snow expected by
morning. Gusty winds in some places will lead to some
blowing/drifting. The headlines for tonight and Wednesday remain
in place. The overnight shift will need to look at snowfall
amounts tomorrow afternoon over the eastern highlands, as well as
along 15 and 86 from around Pocatello west toward Raft River south
into the highlands. The models are wanting to produce more snow,
especially east of I-15 and around the benches around Pocatello.
Keyes



&&

.PREV DISCUSSION... /issued 225 PM MST Tue Jan 22 2019/

SHORT TERM...Tonight through Thursday night.

A potent storm system will arrive from the northwest tonight. Expect
snow to spill over from Montana into the central Idaho mountains,
eastern highlands, and possibly into the eastern Snake Plain. This
storm will bring quite a bit of wind to the area, with heavy snow
likely for the eastern highlands. Storm total accumulations of 5 to
8 inches with heavier amounts along the passes are likely across the
eastern highlands on Wednesday. Elsewhere, amounts look light
generally 1 to 3. Temperatures will be warm enough in the Magic
Valley for rain or maybe a rain/snow mix with no accumulation.
Although the Magic Valley may not see much of anything with the bulk
of the system focused on the east. Portions of the Magic Valley and
lower Snake Plain will be flirting with wind advisories on
Wednesday. Blowing and drifting snow where snow does fall may be a
problem as well. Will probably keep the advisories "as is." Not
confident enough that there will be enough impacts, such as road
closures, to justify an upgrade to a warning at this time. This
system should depart the region Wednesday night, with some lingering
light snow showers possible for Thursday across the eastern
highlands.

Valle

LONG TERM...Friday Through Next Tuesday.
At this early vantage point, weather in the long-term portion of the
forecast (Fri into the upcoming weekend and early next week) looks
fairly low-impact compared to the short-term. 12Z model runs lost
some of their consensus with a shortwave trough diving over the
crest of a Pacific Ridge on Fri. The ECMWF remains the wettest but
initiates snow shower activity by Fri AM. GFS holds off on precip
until Fri afternoon. The Canadian is less amplified with the trough,
but seems to fall in between the other two. All three models
continue to target the Island Park region, Teton Valley,
eastern/Caribou Highlands, and possibly the Bear River Range, with
little to no impact west of I-15 and only very light snow showers
possible in the central mntns, so this is a good/consistent sign.
Ran with a blend of the previous forecast with the latest
NBM/GFS/WPC guidance for PoPs and QPF and nudged PoPs slightly
higher in the eastern Highlands/Island Park zone described above,
but didn't go too hardcore here over the blend pending better model
consensus. Overall confidence on this zone bearing the brunt of the
system is high, but holding with moderate confidence in the rest of
the details. ECMWF appears to be an outlier and far too high on QPF,
with the GFS, NAM, and NBM all in good agreement. An early look at
snowfall Fri based on forecast QPF/snow ratios yields 1-2 inches in
this eastern zone well east of I-15, with perhaps some localized 3
inch amounts. A dusting is possible in the central mntns.

Sat and most of Sun cont to look dry across the region with a break
between shortwaves and no significant change in temps, although
there may be some low stratus clouds running around in portions of
the Snake Plain E-NE into the Upper Snake Highlands and eastern
Highlands. PoPs ramp up again by Sun eve across the Island Park
area, expanding across the central mntns and eastern half of the
forecast area into Mon AM. Overall this second shortwave looks a bit
stronger and more moist than Friday's system, and may also bring
breezy conditions to the region. (An increase of 5-10 MPH in
anticipated wind speeds Sun night into Mon eve was noted in guidance
today.) For now models are in very good agreement on the progression
of this system, although have held PoPs in the 20-50% range this far
out. The cold front associated with this trough will bring a colder
northerly flow Mon and Tues, with highs backing off at least 5
degrees and lows falling into the single digits for many Tues and
Wed mornings, assuming mostly clear skies. - KSmith/Valle

AVIATION...Some persistent low stratus has held it's ground
today in southern and eastern portions of the Snake Plain,
dropping KPIH to IFR cigs and even granting KIDA a period of
freezing fog. Satellite trends suggest this deck is breaking up as
temps creep up and increase T/Td depressions, and some upper-
level cirrus wanders overhead. Do not believe these woes will
return for the rest of the afternoon/eve, but confidence is low.
Our attention then turns to our next storm system approaching the
region late tonight. Timed out VCSH and/or - SHSN in the TAFs
based on a combination of high-res HREF ensemble and NAM time-
height plots. Generally everyone is expected to remain MVFR or
better, except KDIJ where there is a better signal for low clouds.
Of course, transient periods of reduced vsbys are possible
anywhere a moderate snow shower can pass over a terminal, with the
best chance of this at KSUN, KIDA, and KDIJ. Winds will gradually
be on the increase Mon, and left in LLWS remark for KIDA
tonight/Wed. If anything, winds at 2,000 feet may be slightly
stronger here than currently indicated...will let the next shift
reevaluate and update as necessary in the next couple TAFs
packages. Decent mixing to the surface reflected in model guidance
may hold the magnitude of LLWS at bay overall with this system. -
KSmith/Valle

&&

.PIH WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory until 11 PM MST Wednesday for IDZ062>066.

&&

$$
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