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Area forecast discussion
National Weather Service Pocatello Idaho
253 am MDT Thu Oct 17 2019

Short through Saturday night. Early morning satellite
imagery was showing a Pacific trough spreading into the northwest coast
while regional radar returns were noted streaming north through
Pendleton and Spokane. Numerical models show this activity spreading
east into the central mountains this morning and the remainder of southeast
Idaho late this afternoon as the upper trough sweeps east into
Idaho. The central and eastern mountains will receive the bulk of any
precipitation from this initial system with snow accumulations of 1
to 3 inches across the higher elevations and a slight chance of rain
in the Snake River Plain primarily with the frontal passage this
afternoon. Winds will pick up with and behind the front but remain
below advisory criteria. A fairly stiff northwest flow will take shape over
the region Friday in the wake of the exiting trough with decent
mixing enough to support a marginal Wind Advisory event over the
central Snake River Plain. Fully expect day shift to follow-up with a
wind highlight with the afternoon package if the models remain on
track. A second Pacific storm system shears into the region Saturday
morning resulting in widespread precipitation sweeping across the
region with northwest upslope enhanced accumulations across the
central and eastern mountains. Current projections are showing warning
level snowfall in the central mountains and advisory level snowfall
across the eastern mountains with just a dusting in the Snake River
Plain mainly Saturday morning. Drier northwest flow spreads into the region
late Saturday night as the trough exits east into Wyoming. Huston

Long term...Sunday through next Thursday. By sun am, our "main
event" will largely be over as the trough axis continues to shift
east from Wyoming into the Central Plains. In it's wake, light snow
showers are likely to linger for much of the day sun, most
persistent east of I-15/US-20 to the Wyoming border including Fremont
County, the Teton valley, Caribou Highlands, and perhaps Bear Lake.
Additional quantitative precipitation forecast amounts in this corridor look light, less than 0.10
inches, translating into an additional 1-2 inches of snow where
showers are most persistent and in spots that benefit from some
upslope enhancement on a west flow. Sun will be cold/breezy Post-
front, with highs only reaching the 30s across out mntns and lower
40s in our valleys/plains with cold air advection occurring.
Forecast confidence is moderate to high to round out the weekend.

Confidence really remains pretty low next week, although nothing
looks particularly high-impact at this time. Broad, longwave ridging
is forecast by all models to build across the western third of the
US starting Mon, but the "parent high" will be located at a fairly
low latitude out over the central Pacific off the California coast. A steady
stream of moisture appears to take aim at the northern rockies and
continuously ride over the ridge Mon/tues in northwest flow, providing some
chance of showers across most of our mntns ranges, drier across the
snake plain, ern Magic Valley, and srn Highlands. Weak, embedded
waves in the northwest flow may enhance these showers at times, but at this
range it's very hard to pick out anything significant. By Wed, the
GFS depicts a more organized shortwave trough tracking southeast across our
area, but this feature is barely discernible on the European model (ecmwf) and non-
existent on the Canadian. So for now, the general flavor is cool and
partly cloudy, with light mntn showers. Each afternoon should remain
breezy. High temps will reside near or slightly below normal for
this time of year. We let the nbm lead all elements of the forecast
from Mon Onward, with just a few minor tweaks to pops and relative humidity.

Aviation...main aviation concern today will be strong afternoon
winds as a shortwave trough and cold front cross the region (also
bringing at least vcsh). Conveniently, SW flow appears to align
fairly well with major runways and should keep Crosswinds at a
minimum. The exception may be ksun, where winds may quickly turn west-
SW early this afternoon. At this time, guidance does not suggest
cigs/vsbys dropping below VFR. Kdij will have the best potential for
some reductions to MVFR tonight per NAM time-heights, although this
is not yet heavily supported in MOS guidance and will likely be
defendant on persistence of rain/snow shower activity near the
terminal, so confidence is low. Will likely hold off on upgrading
anyone beyond vcsh with the 12z taf package as many high-res models
keep showers focused over the mntns.

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