Area Forecast Discussion
Issued by NWS Pocatello, ID

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FXUS65 KPIH 151055

Area Forecast Discussion
National Weather Service Pocatello ID
355 AM MST Wed Nov 15 2017

.SHORT TERM...Winter Storm System is taking shape for Wednesday
night and continuing into Friday morning for the mountains of
Southeast Idaho. Latest satellite imagery shows pretty significant
band of moisture off the Pacific streaming across North California
to Southern Idaho that will combine with the energy from the low
pressure disturbance advancing into the Northwest Coast. Models have
varied run to run with snow levels and the amount of moisture, but
it is looking reasonably certain for our mountains. Snow accumulation
of 8 to 14 inches in the mountains with higher peaks possibly seeing
16 or 17 inches of snow. Winter Storm Warning will be issued for the
Central Mountains above 6500 feet elevation Wednesday evening
through Thursday evening. A Winter Storm Warning will also be issued
for the Upper Snake Highlands above 6000 feet through Friday
morning. For the Caribou Highlands and Northern Wasatch, a Winter
Weather Advisory will be issued. For the Snake Plain and Eastern
Magic Valley, snow levels lower to about the valley floors Thursday
night and early Friday morning. Much of the precipitation should
fall as rain, but Friday morning up to an inch is possible, and may
be 1 to 3 inches on some bench areas. RS

.LONG TERM...Saturday Through Wednesday.
Significant winter storm will be winding down Fri night, with light
snow showers ending last in higher elevations of the Central Mntns
and along the WY border. Little to no additional accumulation is
expected after dinnertime Fri. Long-range models have all fallen
into agreement on the position of the trough axis as it exits SE
Idaho to the east, and forecast confidence is near the high end of
moderate for the end of this storm. Models then agree on ridging
building across the area with dry conditions and even some sunshine
Sat afternoon into Sun eve. All in all, most of the weekend should
be dry (although cold) across southeast Idaho.

Sun eve, it all falls apart. Some degree of upper level high
pressure ridging is expected to dominate the large-scale pattern
over the western US, but long-range models vary with respect to
strength, with the ECMWF and GFS both allowing mid-level shortwave
troughs to break through this regime (particularly the EC, which
reflects weaker ridging). Digging into the details, models advertise
VERY different solutions into early next week, with run-to-run
consistency also poor. Both the GFS/EC have some form of a closed
low moving across the NW, with track varying from the Canadian
border to NV. In addition to precip implications, this will also
have a major impact on how warm temps end up being through this
period, as reflected by 500mb height differences of 10+ dam by Mon
afternoon. Confidence in any solution remains very low, and we did
not make major changes to this part of the forecast. For now, we do
carry a low chance of showers across the region Sun eve into late
Tues night, which will ultimately depend on the exact track of any
shortwave/low that does impact SE Idaho. - KSmith/RS


Model time-height cross sections this morning advertise somewhat of
a low-stratus signature at all of the terminals, lasting throughout
the morning hours. However, this is not reflected in MOS guidance.
Nothing concerning is noted on satellite, BUFKIT soundings off the
various models do not indicate surface saturation, and early AM
dewpoint depressions have been decent. We will be monitoring closely
as always, but pretty confident that high pressure and clear skies
will dominate this morning with no fog/stratus development.

A mid to high cloud deck will build across the region ahead of our
approaching storm system, reflected in the 17Z-20Z TAF FM groups.
These ceilings will progressively lower this eve. Next FM groups pin
down when we expect earliest precip may begin at each terminal (00Z-
03Z at KBYI/KPIH/KIDA where VFR conditions are expected at least
through 06Z, 20Z at KSUN, and 02Z at KDIJ). This will be a major
winter storm for portions of our area including KSUN and KDIJ, where
major cig and VSBY impacts are expected. KSUN is expected to drop
to MVFR if not IFR by 01Z, and continue deteriorating from there.
Assuming snow is the predominant precip type and our model guidance
is accurate, the current forecast toward the end of the TAF period
at KSUN is likely optimistic. Airport closures are possible. For
KDIJ, BUFKIT profiles after 06Z look eerily similar to KSUN, again
assuming temps stay cold enough to support mostly snow. Airport
personnel and aviation interests are encouraged to monitor TAF
updates closely and expect major impacts at these terminals.
Upcoming 12Z TAFs will update precip start times and try to further
pin down periods of worst conditions now that more of the storm will
overlap into the forecast period. - KSmith/RS


Winter Storm Warning from 6 PM this evening to 6 AM MST Friday
for IDZ019.

Winter Weather Advisory from 6 PM this evening to 6 AM MST
Friday for IDZ023-025.

Winter Storm Warning from 6 PM this evening to 6 PM MST Thursday
for IDZ018-031.


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