Area Forecast Discussion
Issued by NWS Pocatello, ID

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268
FXUS65 KPIH 151013
AFDPIH

Area Forecast Discussion
National Weather Service Pocatello ID
313 AM MST Sat Dec 15 2018

.SHORT TERM...Today through Sunday night.
Mountain snow showers will begin to taper down this morning. A
de-amplifying shortwave trough lifting through western MT this
morning will push off to the northeast. Per latest satellite
imagery, overrunning upper-level moisture is already moving east
of the area, leaving only mid-level moisture being squeeze out
over the central mountains. Expecting about 2-4 inches along the
Sawtooths just west of Stanley, otherwise, amounts will drop off
rather quickly through the peaks east of Stanley. Aforementioned
moisture will then scrape through the Yellowstone region, bringing
mountain snow showers of limited consequence through midday. Can`t
rule out a few flurries through the Snake Plane, but nothing
significant is expected.

Strong downsloping winds coming out of the Albion Range, Deep
Creek Mountains and Bannock Range has led to frequent gusts
30-40mph with temperatures fluctuating as much as 8-12 degrees in
the course of an hour. This is expected to subside moving through
the morning but expect temperatures just north of these mountains
to be relatively warm to start the day.

Relatively quiet weather will carry into Sunday with a ridge
building in. This will set up stout inversions in valleys and
promote fog potential during the morning hours. Opted to insert
patchy mention through the upper Snake Plain.
TAX

.LONG TERM...Monday through Saturday. Best chances for impactful
weather during the long-term portion of the forecast are centered on
Monday and the late Tuesday and Wednesday period, with the latter
carrying a greater potential for higher impacts. We`ll begin with
Monday with guidance in good agreement that an approaching weakening
wave will spread snow showers across the Sawtooths and Central
Mountains and adjacent peaks and valleys, with snow showers
extending south across the southern highlands. This activity should
gradually weaken as it expands eastward, so we have less confidence
in impactful snow reaching the Wyoming border region. Across the
Snake Plain and especially the eastern Magic Valley, there is some
potential for light snow showers or a rain/snow mix Monday afternoon
and evening. Overall amounts for these areas should be mainly light,
with the highest amounts likely to occur across the Sawtooths.

After a brief break in the action Monday night, the next wave will
already be on the doorstep. This next wave will likely pack much
more moisture and wind compared to Monday`s event. Snow is expected
to enter the Sawtooths/Central Mountains Tuesday morning and
eventually the Island Park, Palisades and Eastern Highlands region
by late Tuesday. Snow showers for all highland areas will then
likely persist into Wednesday while gradually weakening. Across the
Snake Plain and eastern Magic Valley, the west-northwest flow regime
depicted by guidance tends to result in a shadowing effect, with
places like the Arco Desert and eastern Magic Valley seeing little
Precipitation, and places like the Pocatello east bench and terrain
just east of the I-15 corridor seeing impactful Precipitation. There
is some evidence of this in modeled Precipitation fields. As
mentioned earlier, West wind gusts of at least 30 mph are possible,
especially for mid-upper slopes and possibly across the Snake Plain.
Warmer Temperatures will accompany this wave, so a mix is possible
for the eastern Magic Valley and southern Snake Plain, should
precipitation occur here. A ridge of high pressure is then
represented by all guidance to build into the region Thursday,
ending precipitation and allowing for inversions to briefly develop.
The next wave is then forecast to bring another round of snow
showers to mainly mountain areas Friday, although the Precipitation
looks mainly light at this time. Forecast confidence erodes after
Friday, but from Monday-Friday confidence in expected outcomes has
improved greatly over the last couple of days. AD/TAX

&&

.AVIATION...Thick canopy of mid-level VFR cloudiness expected to
persist across the region for much of the day, with some hints of
MVFR stratus development seen in model cross-sections for IDA, DIJ
and SUN from roughly 18-00Z. The potential for this development is
less compared to previous runs, which makes sense given the lack of
observed Precipitation with this morning`s wave. Will hint at the
potential in official TAF. Meanwhile, strong southerly flow (30-
40kt)just above inversion level has led to LLWS, particularly at PIH
and IDA. For BYI, these winds were able to erode the inversion and
mix to the surface for a bit. With the passage of the mid-level
trough around 12Z, winds should slacken and shift southwesterly
through the first few thousand feet AGL, ending this issue. With the
approach of another trough, LLWS issues may again become apparent
for these same locations Sunday morning as southerly flow again
increases (20-25kt) above inversions. Some potential for Fog at IDA
Sunday morning as well. Will hint at this but allow later shifts to
assess further. Think the threat of any impactful Precipitation
through the period has come to an end. AD/TAX

&&

.PIH WATCHES/WARNINGS/ADVISORIES...
None.
&&

$$



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