Area Forecast Discussion
Issued by NWS Pocatello, ID

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FXUS65 KPIH 160910

Area Forecast Discussion
National Weather Service Pocatello ID
310 AM MDT WED Oct 16 2019

.SHORT TERM...Tonight through Friday night. Early morning satellite
imagery was showing a transient ridge shifting east of the region as
the next upstream Pacific trough advanced toward the NW coast.
Numerical models show the trough advancing inland tonight with the
leading edge of the associated precipitation shield working into the
CNTRL Idaho mountains Thursday morning and the remainder of SE Idaho
Thursday afternoon and night as the trough axis and surface frontal
boundary sweep east through the region. The bulk of the
precipitation will be focused across the CNTRL and ERN mountains
where modest snow accumulations of 1 to 4 inches are anticipated
Thursday through Friday night while mainly rain showers are expected
with the Snake River Plain. We may see snow levels lower toward
valley floors by Friday morning as the back edge of the trough works
east into Wyoming and then a moist, unsettled west flow across the
region as the next Pacific trough advances toward the NW coast
Friday night. Wind speeds should begin ramping up ahead of the
system Thursday afternoon and approaching advisory levels across the
CNTRL Snake Plain Friday afternoon behind the exiting trough.
Temperatures will peak well above normal and near record levels in
some places today and then drop below normal by Thursday and Friday
behind the exiting storm system. Huston

.LONG TERM...Saturday through next Wednesday. We are beginning to
see signals that moderate to significant winter wx will impact
PORTIONS of our forecast area this weekend, and this is where we
focused our time/attention in the long-term portion of the forecast.
We open Sat with broad, longwave troughing over the wrn US. Long-
range models bring the axis of a pronounced shortwave onshore across
WA/OR by daybreak Sat, with the leading edge of a developing precip
shield working into the nrn Central Mntns and Sawtooths around that
time. Sat afternoon, GFS/ECMWF/Canadian show excellent agreement in
precip overspreading the CWA, with shortwave axis (and associated
cold front) approaching the western ID border. Sat afternoon/eve
look impressively wet on all 3 models, with frontal passage progged
for the 4 PM to 11 PM timeframe per potential vorticity/700mb temp
analysis. The trough pushes into WY overnight (and begins to acquire
a negative tilt as it approaches the Plains) with a general
decreasing trend in precip for us, although precip continues over
the ern highlands/Bear Lake/Teton Valley/Fremont County into Sun AM.
WPC continues to highlight our Central Mntns and immediate WY border
for heavy snow on their latest Day 3-7 Hazards Outlook. It`s hard to
"feel out" the models on post-frontal convergence zone potential in
the Snake Plain/adjacent highlands with the coarser spatial/temporal
resolution at this range, but given the pattern, this phenomena may
come into play Sat night as well. We generally nudged PoPs 5-10%
higher than NBM guidance, and used an NBM/WPC blend for QPF, with
targeted adjustments made for expected downsloping (and thus
drying/less precip) off the southern highlands from Burley, to
Pocatello, to Idaho Falls, and possible upslope enhancement in the
northern Snake Plain and Clark/Fremont counties.

Snow levels ahead of the front Sat are forecast to reach 5,500-6,500
ft as highs reach the 40s in our valleys/plains, so an elevation-
based rain/snow mix is expected, transitioning to all snow by eve as
snow levels crash to valley floors with the front. This means we will
burn through the heaviest QPF in the Snake Plain/Magic Valley as
rain pre-front, while temps support nearly all snow for most of the
event above 6,000 ft. Early expectations are for storm totals Sat-
Sun AM (just over 24 hours) of 6-12 inches N of Ketchum/W of Challis
in the Central Mntns (over 1 foot near the Sawtooth peaks), 2-4
inches for the rest of the Central Mntns including valleys, 3-8
inches for the Upper Snake Highlands, Teton Valley, and Caribou
Highlands, a dusting-2 inches in the srn highlands, and little to
nothing in most of the Snake Plain due to the initial warmer
temps/later changeover. Accumulation potential in the Snake Plain
will also depend on any convergence precip Sat night. Slick travel
is likely at all mntn passes/highways above 6,000 ft. These storm
totals sit quite comfortably near the best QPF/snowfall clusters
noted in the GFS/EC ensembles, which nudges our confidence higher
yet. Breezy conditions Sat afternoon may linger late into the eve
behind the front, tossing blowing/drifting snow into the mix. Bottom
line, this is looking like an advisory-level event in our mntns, with
warnings not out of the question in the Central Mntns. Stay tuned.

Broad ridging tries to start building in Sun afternoon into the new
week, but models advertise some moisture spilling over that ridge,
perhaps enhanced from time to time by weak embedded waves or
disturbances. Our mntn ranges will see the best chance of showers in
this regime, and nothing looks terribly high-impact at this
juncture. We let the NBM lead the forecast here, with few edits, as
confidence in the fine details is low. High temps look to modestly
warm back into the 50s in the Snake Plain by Tues. - KSmith

.AVIATION...High confidence continues in VFR cigs/vsbys throughout
the TAF period as high pressure remains in control, with our next
round of showers and increased winds initiating Thurs afternoon as
the next trough of low pressure approaches. Some low-level wind
shear remains possible this AM at our Snake Plain terminals


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