Area Forecast Discussion
Issued by NWS Pocatello, ID

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FXUS65 KPIH 152030

Area Forecast Discussion
National Weather Service Pocatello ID
130 PM MST Wed Nov 15 2017

.SHORT TERM...Tonight through Saturday. A deep low pressure system
remained anchored just off the NW coast early this afternoon as sub-
tropical moisture rotated into the NW states. The regional radar
loop was showing an increase in precipitation returns across NRN
California and extending into SW Idaho. Numerical models continue to
target the west-CNTRL mountains with rapidly increasing rain and
snowfall this evening spreading throughout the remainder of SE Idaho
overnight. Rising snow levels are expected through the night before
leveling off near 6500 to 8000 feet by noon Thursday. The rising
snow levels will make for a tricky snow accumulation forecast and
for now we have maintained the Winter Storm Warning above 6500 feet
for the west-CNTRL mountains starting at 6 PM this evening. The ERN
mountains won`t really get started until close to midnight and thus
have pushed the start time on those warnings/advisories back 6
hours. The upper low gradually shifts east through the region
Thursday night and Friday with a cooler and drier airmass filtering
into the area along with the upper trough passage. Snow levels
should lower to valley floors by Friday morning with rapidly
diminishing accumulations from west to east across the region Friday
afternoon. Breezy winds are expected Thursday ramping up toward the
lower end of advisory levels Friday afternoon. Daytime temperatures
will remain above normal Thursday before falling to near seasonal
norms Friday and Saturday with the trough passage. Huston

.LONG TERM...Saturday night through next Wednesday night. 500mb
longwave forecast is for the weak trough in the eastern Pacific and
western Canada to slide inland slightly by Tue. This keeps Idaho in
the storm track with a zonal or nearly zonal pattern. This means
that the hits should keep coming, with a wet period in store for the
forecast area. Factoring in the shortwaves, the forecast does get a
reprieve for Sat night/Sun as a moist shortwave in the Pacific NW
amplifies the pattern and keeps the forecast area dry, but it starts
making its way into the Gem State on Sun night. The GFS used to be
more aggressive with this winter storm, but now the ECMWF has sped
up the timing and pushes the trough through by Mon night. The GFS
now hangs on to this feature with a much slower progression. The GFS
continues to pump moisture into southern Idaho on Tue/Tue night/Wed
as well, much more than the ECMWF. So there is marked disagreement
in the guidance solutions starting Mon night. Will try my best to
split the difference, but there is not much confidence in the
forecast at this time. Messick

.AVIATION...Incoming winter storm will start affecting airdromes as
early as this afternoon/evening. First to have problems will be
KSUN, although it will stay with marginal VFR until at least 16/00Z.
There is potential for airport closure there at that time, but
believe it may hold off until later tonight when the upward vertical
motion is strong enough to bring the heavier precipitation. KSUN,
once they go down, should be problematic all day Thu. KDIJ is in
similar straits, although the moisture does not get there until
tonight in the mid- to late evening. And closure there is much more
likely with both Thu and Thu night problematic. For the three Snake
Plain airdromes, KIDA has the most impact with marginal VFR late
tonight along with gusty wind. KBYI as the furthest south has the
least impact, mainly the wind, but there is a chance of some heavy
precipitation cells during the evening, according to the HRRR, so
have indicated that with a TEMPO group. The most difficult forecast
is what will happen at KPIH. The large amounts of moisture and cloud
could be offset by a strong southerly downslope wind. I believe
eventually the marginal VFR and precipitation should develop, but it
normally takes longer than guidance indicates. One thing about this
storm is that the gusty wind tonight, along with the clouds, will
keep the Snake Plain airports in rain, not snow, at least tonight
and Thu. Messick

Winter Storm Warning from midnight tonight to noon Friday for IDZ019.

Winter Weather Advisory from midnight tonight to noon Friday for

Winter Storm Warning from 6 PM this evening to 11 PM MST Thursday
for IDZ018-031.

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