Area Forecast Discussion
Issued by NWS Pocatello, ID

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000
FXUS65 KPIH 140202
AFDPIH

Area Forecast Discussion
National Weather Service Pocatello ID
802 PM MDT Tue Apr 13 2021

.UPDATE...

Updated forecast to cancel the lake wind advisory and issue a wind
advisory into the overnight hours for the lower Snake Plain. Winds
have increased notably early this evening as the low pressure
area approaches. Winds may take a while to diminish this evening
and have noted strong gusts late this afternoon across the Arco
Desert into the Magic Valley. This area of wind has expanded
towards the Pocatello region. Blowing dust was observed as well.
Meanwhile, high wind warning continues for the Preston to Bear
Lake region with strongest winds beginning there and expected to
continue through the night. Observations in the low 50s for gusts
have already occurred near Franklin this evening. Band of
light snow will spiral northward during the overnight hours. This
may help to suppress winds some.

13

&&

.PREV DISCUSSION... /issued 224 PM MDT Tue Apr 13 2021/

SHORT TERM...Tonight through Thursday night. Active pattern
remains in the short term with both wind and precipitation a
continuing concern. Satellite imagery places center of upper low
circulation near Oregon/California/Nevada triple point. Broad area
of moisture continues to develop associated with the low while
stiff northeast surface gradient remains in place over East
Idaho. Strong winds in place today through the Snake River Plain,
from the Arco Desert southwest toward the Magic Valley, with
several observation sites noting gusts close to or just above 50
mph today. Models continue to sag center of upper low southeast
into northern Nevada overnight. Gradient remains elevated so winds
likely to remain above Wind Advisory threshold past midnight,
though Lake Wind Adv less of a concern past sunset. Elsewhere,
models remain VERY favorable for strong downslope wind event
across the southeast corner of the forecast area, with continuing
expectations for a rare localized high wind event. Gusts in this
region have already approached 50 mph today, ahead of the peak
event tonight into tomorrow morning. High Wind Warning will
remain in place for these areas, with greatest expectation for
impacts in terrain-aligned down-valley areas, and Bear Lake, both
open water and shoreline especially aligned with east-west
canyons. Moisture begins to develop over northern Nevada and
northern Utah, nudging into southern Idaho after 06Z/midnight.
There is still some uncertainty as to the northern extent of the
precipitation push, with short term model ensembles favoring light
snow reaching the southern stretch of the Snake River Plain from
Fort Hall to Burley between 09Z and 12Z. Temperatures should be
cold enough for snow down to valley floors, and although
precipitation amounts continue to fluctuate slightly, there is
consensus for amounts to remain less than an inch for this first
round through about 18Z/Noon. Highland areas south of this region
are still looking into the 1-3 inch range. Any snowfall that
occurs coincident with strong winds in the southeast corner will
be problematic for spring-growth trees and power lines.

For Wednesday, upper low wobbles in the vicinity of the
Idaho/Nevada/Utah triple point. Moisture continues to spread north
through the forecast area, though the greatest chances remain
across the southeast half. Surface gradient remains elevated, and
the wind headlines may need to be extended through the day over
the Snake Plain (new issue for Lake Wind Adv, depending on speeds
over the Lower Snake Plain). For the southeast corner, surface low
should shift toward/across that region, with the core of the 700mb
jet shifting east as well. Surface gradient remains tight, so
this could signal just a shift in direction. The terrain-funneling
will likely be less of an issue, but the broad north-south Cache
Valley and Bear Lake valley could help locally funnel stronger
winds through the region. For now, guidance does indicate that the
winds will lessen through the day, but caution is warranted. Winds
are expected to decrease through the day elsewhere as the surface
gradient weakens. Morning snow should mix with or change to rain
below about 5500 ft elevation, meaning a changeover is expected
through the Snake Plain and most of the travel corridors.

For Wednesday night through Thursday, upper low fills slightly and
shifts east into Utah/Colorado. Wrap around showers will be the
main impact expectation for the region, generally snow at night
to valley floors becoming a mix or change over to rain for lower
elevation areas Thursday. Most of the precipitation should be
across the Snake Plain and regions east and south, closest to
center of upper low. Surface gradient remains generally northeast,
and daytime winds through the Snake Plain may be strong enough to
support low end Wind thresholds for the reservoir. There is some
uncertainty regarding development of a reinforcing shortwave
Thursday night, which could strengthen potential for showers
across the region overnight. NAM is in favor of this solution,
which remains washed out in the GFS. Blended solution kept chance
showers across mainly the eastern highlands through the night,
which seems to be a fair compromise. DMH

LONG TERM...
Slow moving upper-low will be pushing away from the region at the
start of the day Friday as upper-level ridge starts to build in its
place. Could see a few isolated rain/snow showers in the eastern
Highlands for the first half of the day, with very limited QPF,
before skies gradually clear as the day progresses. Temps Friday
will be near seasonal norms but as we move into the weekend, expect
pleasant weather with mostly clear skies and above normal temps. 60s
seem quite likely in the valleys, perhaps nearing 70 as we get into
early next week. A bit more uncertainty though that far out as some
models show perhaps a shortwave moving through with north to
northwest upper-level flow in place. This would bring back light
precip chances and a bit cooler temps. For now, ran with the NBM
which has low-end PoPs across the highlands and will see how things
trend over the next few days. There remains a decent spread amongst
ensembles members so this seems like the best approach for
now. McKaughan

AVIATION...
VFR expected to continue through the evening and into the early
portion of the overnight as clouds gradually increase in coverage
and lower in height through daybreak. A deepening upper-low moving
into the region will keep winds gusty especially at KPIH, KIDA and
KBYI into the overnight. Precipitation, mainly in the form of snow,
will move into southern Idaho around 10Z or so and gradually push
north through the area impacting all terminals on Wednesday. For
now, have introduced MVFR vsbys associated with -SN at all terminals
tomorrow morning but some localized reductions to IFR certainly seem
possible although confidence isn`t quite high enough yet to introduce
TEMPOs but they will likely be needed in subsequent forecasts.
McKaughan

&&

.PIH WATCHES/WARNINGS/ADVISORIES...
Wind Advisory until 3 AM MDT Wednesday for IDZ051-052-054-055.

High Wind Warning until noon MDT Wednesday for IDZ059>061.

&&

$$


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