Area Forecast Discussion
Issued by NWS Pocatello, ID

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371
FXUS65 KPIH 192033
AFDPIH

Area Forecast Discussion
National Weather Service Pocatello ID
133 PM MST Mon Feb 19 2018

.SHORT TERM...Tonight through Thu afternoon. Arctic air under
northerly flow behind the front will penetrate as far south as
Burley tonight, bringing overnight low temperatures to positive
single digits or even colder the farther north in the Snake River
plain you are. Coupled with a light to moderate wind, effective
chill temperatures will range from minus 20 to minus 35 over
nearly all of Clark County and a solid portion of Fremont County.
So have issued a Wind Chill Warning for Zone 19. Farther to the
south and west, even the eastern Magic Valley will see effective
Chill Temperatures of fifteen below zero. On top of that, the
HRRR, the GFS, and the NAM have all been consistently forecast the
leading edge of the arctic air to act as one side of a convergence
zone, as it advances down the valley. Snow is expected to amount
to widespread 0.5 to 2.0 inches, with more intense snowfall but in
a limited area in the Cold Water to American Falls area, including
the north end of the Rockland Valley, of 3 to 5 inches. Have
issued a Winter Weather Advisory to cover that. The convergence
band of snow should be all wrapped up by morning, which leaves
some weak troughing moving through the mean northwest flow. Expect
only light snowfall mainly along the border with Montana, although
sometimes all of the central Idaho mountains will be affected. Do
not see anything more than light snow, which for the mountains is
up to 6 inches in 24 hours. The most likely period for this
incident is Wed and Wed night. With the cloud cover Wed night, the
chilly air will modify some, with temperatures in the lower Snake
River plain staying in the teens, and positive single digits for
places like Idaho Falls. The wind should be considerably less, so
at this time do not see a repeat of a Wind Chill Warning for Tue
night.  Messick


.LONG TERM...Friday through next Monday. A deepening area of low
pressure over the West will keep some snow in the forecast early on
Friday, but drier air works in during that. There could be some
light snow/flurries remain in the eastern highlands and also between
Stanley and Challis. The next storm slides through later Saturday
and Sunday. This one looks breezy with some snow possible especially
in the mountains. The ECWMF and GFS have their often typical battle
with one another (One is wetter than the other) especially across
the Snake Plain. The trend is there though for a higher chance of
mainly snow (could be a mix at lowest elevations) so we kept pushing
those percentages higher. With the wind/snow combo, we could see
blowing and drifting become an issue especially over high passes and
into the backcountry. The next storm will already taking shape
across the Canadian Rockies southwest along the West Coast by
Monday. There will be some moisture ahead of it, enough for clouds
obviously and some light rain and/or snow across eastern Idaho.
Temperatures will continue a steady climb into the weekend. We`ll
eventually see some places pushing into the mid, maybe upper, 30s.
That is actually at/just below average for this part of February.
Keyes

&&

.AVIATION...A couple of concerns through this evening. One is light
snow continuing across the eastern highlands, producing light snow
and low clouds at KDIJ. What snow that`s there now should end by
late evening, but expect the low clouds to remain and in fact lower
a bit after the snow ends. Expect MVFR/IFR weather during the
period. The other is developing showers around KSUN that are moving
into the Snake Plain, toward KBYI and KPIH. The showers around KSUN
should end also later this evening, becoming VCSH. If showers
directly impact the airport, look for rapid drops into IFR
conditions due to heavier snow/graupel. There could also be
localized erratic winds. The biggest impacts will be at or in
between at least KPIH and KBYI. A convergence band of snow will
develop, the question is where. The area of convergence has been
wavering back and forth between both airports. With the fact the
higher resolution models haven`t quite figured out what`s going to
happen, these TAFs have a question mark or two attached to them. We
did drop ceilings down to IFR and included at the very least VCSH.
The consensus was to add IFR/LIFR conditions at KPIH as there is a
hint the band actually will shift far enough east to affect the
airport. Again, this is definitely NOT a high confidence scenario
and we will likely have to make a change (or even more) to the TAFs
as this develops. KIDA could see a couple of showers, being on the
northern of showers moving off the central mountains. They should be
VFR overnight. For Tuesday, the potential of that band of snow
hanging around will keep the potential for VCSH or -SN in the
forecast at KBYI and KPIH early on. The band should move northeast
away from KBYI mid morning and KPIH around Noon. However, that means
the band will eventually impact KIDA and KDIJ. Expect MVFR/IFR
ceilings and visibility with this. KSUN should see scattered to
broken clouds flirting with MVFR throughout the day with VCSH as
most showers will be over surrounding terrain. Keyes

&&

.PIH WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory from 6 PM this evening to 5 AM MST
Tuesday for IDZ017-021-022.

Wind Chill Warning from 6 PM this evening to 10 AM MST Tuesday
for IDZ019.

&&

$$



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