Area Forecast Discussion
Issued by NWS Pocatello, ID

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FXUS65 KPIH 142020

Area Forecast Discussion
National Weather Service Pocatello ID
120 PM MST Tue Nov 14 2017

.SHORT TERM...Tonight through Friday. Early afternoon satellite
imagery was showing a low pressure trough and associated
precipitation exiting east of the region while the next in a series
of Pacific storms was advancing into the NW coastal waters.
Numerical models were showing a modest ridge of high pressure
building over the region with clearing skies this evening. By
Wednesday afternoon the ridge shifts east of the region while sub-
tropical moisture is entrained into the cyclonic flow around the low
with the leading edge of this moisture advancing into the CNTRL
mountains. The models are much more moist and protracted with the
evolution of the upper low than yesterday`s runs resulting in a warm
moist storm event Wednesday night through Thursday with snow levels
generally running from 6000 to 7500 ft MSL. The latest calculations
continue to show warning level snowfall across the CNTRL mountains
and now the Upper Snake Highlands. Therefore the Upper Snake
Highlands have been added to the Winter Storm Watch. The Caribou
Highlands will likely see advisory level accumulations throughout
the storm event and will probably be added to the mix with the
afternoon package Wednesday. The upper low gradually shifts inland
Wednesday night through Friday exiting the region Friday evening.
Colder temperatures and drier conditions begin to filter into the
region Thursday night and Friday with snow levels falling to valley
floors during that time frame. Snow accumulations within the Snake
River Plain will likely remain below an inch for the entire event.
Strong gusty winds are expected Thursday afternoon and again Friday
possibly requiring an advisory within the Snake River Plain. Daytime
highs will continue to run above normal Wednesday and Thursday
before falling to near normal Friday with the passing storm system.

.LONG TERM...Friday night through next Tuesday night.  Appears as
though the outlying ECMWF has come significantly closer to the GFS
solution for the weekend; that is, it should be dry for most
locations through Sun, except a slight chance for the northwest and
northeast mountain zones. But anything south of Galena summit should
be dry with a warming trend. On Sunday night the GFS is the more
aggressive with the next upstream shortwave already spreading
precipitation through the central Idaho mountains. However, the
ECMWF catches up for a short time Mon with a very similar
precipitation pattern and amounts as the GFS. The sharpest
disagreements occur Mon night as the GFS continues to pour
precipitation into central Idaho especially. By Tue the
precipitation is mostly limited to the Idaho-Wyoming border region.
And even this has cleared by Tue night. So there are only a couple
of periods in question, and for the Mon/Mon Night storm it is a
matter of amounts. Temperatures may be the largest discrepancy. The
500mb heights are considerably lower on the ECMWF, which means
colder and denser air over the Gem State compared to the GFS. This
accounts for the 4 to 6 deg F difference between the two guidance
products. Not really sure who to believe, so perhaps splitting the
difference is the best route for temperatures. As for wind with this
storm, it looks no more than breezy to windy, so the only event
could be a Winter Advisory or Warning for snow amounts in the
central Idaho mountains, possibly the upper Snake River highlands.

.AVIATION...Skies will be clearing all locations during the early
afternoon with the exception of KDIJ (which may take until early
evening). Local wind study indicates early afternoon wind near wind
advisory criteria, so pushed wind to 25G35KT for KPIH and KIDA, and
20G30KT for KBYI, which was out at TAF time due to repair issues,
but should be back. Behind the front, KSUN has northwest wind not
expected to do its usual switch to southeast today, but should
tomorrow. Currently do not see enough precipitation and calm wind to
develop any kind of fog threat tonight. Messick

Winter Storm Watch Wednesday evening through Thursday for IDZ018-

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