Winter Storm Warning 8, NWS Pocatello, ID

Area Forecast Discussion
National Weather Service Pocatello ID
155 PM MST Wed Mar 8 2023

.SHORT TERM...Tonight through Thursday night. Early afternoon radar
and satellite imagery was showing an area of light snow lifting NE
through the Snake River Plain and Southeast Highlands. Numerical
models show a negatively tilted trough ejecting from the coastal low
into the Panhandle overnight (so a glancing blow) with a front
running disturbance shearing NE into Wyoming/Montana this evening.
And thus, the 12z models confine much the more moderate snow
accumulations to the Southeast Highlands this afternoon and early
this evening and then again later tonight with the incoming trough
where we will allow the Winter Weather Advisory to continue. As for
the central mountains (a place that is accustomed to getting snow),
the models have edged away from the previously stronger
precipitation that was expected and have now favored light snowfall
overnight with local areas of snowfall bordering "moderate" over the
Sawtooth crest. By and large, this region fails to clear the lower
end of the Advisory threshold (5 inches) overnight and thus we will
remove that region from the going Advisory. We catch a breather
Thursday before the NW coastal low becomes juiced with sub-tropical
moisture (an Atmospheric River) spreading it inland and across the
region starting Thursday evening in the CNTRL mountains and later
across the remainder of the region by Friday morning. Strong south
winds will overspread the region Thursday night ahead of the
incoming trough which may actually inhibit significant accumulations
along the Interstate 84/86 corridor at least early in the event
while enhancing the upslope potential across the NRN half of the
valley running up into the CNTRL mountains. Thus we will issue a
Winter Storm Watch for the Central Mountains starting Thursday
evening and work it into the northern valley zones after midnight to
include Richfield, Arco/Mackay, and INL. The northern portions of
the Magic Valley and Lower Snake River Plain may yet see Advisory
level snowfall but I am a bit concerned that the downslope winds may
eat away at some of those forecast accumulations with time. Still,
something to watch. And finally, the ERN mountains look to get in on
the action sometime after midnight Thursday night and particularly
Friday (see more on that below) with the complication of warming
temperatures, rising snow levels, and strong winds. Huston

.LONG TERM...Friday through next Wednesday.
The same area of low pressure located off the Washington/Oregon
coast that has brought several waves of moisture to the region will
continue lifting northeastward across the Pacific Northwest and into
Canada Friday. This system will bring impactful precipitation,
especially to our central Idaho and eastern Idaho mountain zones,
Friday into early Saturday. A WINTER STORM WATCH has been issued for
these areas Friday through Saturday morning. While the heaviest snow
is expected to occur prior to midnight Friday night, additional
modest snow will continue through late Saturday morning. It is worth
mentioning again that the operational GFS continues to run
relatively slow in moving the center of the low onshore, so exact
timing of heaviest precipitation could change slightly between now
and the end of the week. Forecast snow totals for the 24-hour period
Friday morning through Saturday morning are currently running one to
one and a half feet of accumulation above 6,500 feet in elevation
and up to 6 inches below in the valleys.  Winds will be an added
impact, creating major blowing and and drifting snow concerns.  We
can expect sustained winds over 20 mph across the central Idaho
mountains and eastern and southeast highlands, with gusts exceeding
50 mph in some areas. A prolonged period of southerly/southwesterly
flow will result in a warm-up through much of the extended period,
allowing for rain to mix in during the daytime hours across the
eastern Magic Valley, lower Snake Plain, and valleys further south,
where daily highs will reach into the 40s. This warm-up will greatly
limit snow potential below 6,000 feet elevation and will also lead
to hydro concerns as rain, snow melt, and re-freeze in certain
locations becomes relevant. Models then feature a weak transient
ridge building in late Saturday through Monday, during which time we
can expect a lull in the active pattern. Meanwhile, the next upper
low will approach the PNW region, with the associated trough moving
inland late Monday/Tuesday morning. This combination of features and
synoptic setup will result in favorable flow for the advection of
moisture into the region and additional snow for our upslope
mountain zones, especially Tuesday into Wednesday morning. Cropp
&&

.AVIATION...As a disturbance lifts through Idaho, snow showers will
continue to impact KDIJ through late morning tomorrow and all other
terminals through earlier morning. While some of the heavier
precipitation has already moved through, hi-res models feature
another relatively organized, heavier period of snow spreading SW to
NE beginning around 7z tonight, which could lead to a reduction in
VIS at all sites. Winds will remain light and variable at times
throughout most of the period but will begin to increase overnight
tonight and even more so during the evening Thursday ahead of the
next major system. Cropp
&&

.PIH WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory continues until 5 AM MST Thursday for IDZ058-
060-062>064.

Winter Storm Watch from late Thursday night through Friday morning
for IDZ051-052-068-069.

Winter Storm Watch from late Thursday night through late Friday
night for IDZ060-063-064-066-067.

Winter Storm Watch from Thursday evening through late Friday night
for IDZ071>075.
&&