Avalanche in Washington, February 19, 2023

Area Forecast Discussion
National Weather Service Spokane WA
949 AM PST Sun Feb 19 2023

.SYNOPSIS...
Significant snow looks to impact travel across the mountain passes
from this evening into early next week. Gusty winds are also
expected at times, especially Monday and Tuesday. Lowland snow
potential increases Tuesday, followed by a blustery blast of cold
temperatures for Wednesday and Thursday. Slowly moderating
temperatures are expected by the end of the week.

&&

.DISCUSSION...
Today through Monday night...The weather during this period will
be characterized by the influx of significant plume of moisture
associated with a weak atmospheric river. The latest satellite
imagery shows this plume of moisture currently pointed toward the
central BC coast. However during the next 36-48 hours this plume
will slowly sag southward which will result in an increasing
chance of precipitation, starting in the north Cascades...later
this morning and then developing over the Idaho Panhandle. This
will become a very mild system with the upper level jet remaining
poised well to our north today and then drifting south overnight.
Consequently snow levels will get quite high, starting around
2500-3000 this am and rising to 3500-4000’ by Monday/Monday night.
Meanwhile the winds in the mid-levels of the atmosphere will
remain out of the west due to a strong low expected to spin up
over the Prairie Provinces later today. As a result there will be
a prominent rain shadow through tonight with most of the
precipitation occurring either near the Cascade Crest over over
the southern half of the Idaho Panhandle. The precipitation
amounts will be quite impressive. For Stevens Pass there is a 60%
chance of seeing over 18” of snow and around a 10% chance of
seeing nearly 30” of snow. For Lookout Pass, the numbers aren’t
quite as impressive, with a 50% chance of 18” or more and a 10%
chance of 24”. We have issued winter weather highlights to address
these two locations.

For the valley locations, the main story through this period will
be mild temperatures and locally strong winds (more on that
later). Highs today will be in the upper 30s in the northern
valleys of Idaho and eastern Washington and over the lower to
middle 50s. By Monday conditions will warm a few more degrees with
40s expected for most valleys and 50s over the Columbia Basin and
LC Valley. We also cannot entirely rule out a stray 60F or warmer
reading over southern Grant County, as the Hi-res ensemble
guidance is showing nearly a 30% chance of hitting that mark. This
should be the warmest day we will see for quite some time due to
the arctic front heading into the area on Tuesday.

As for the wind, we should see speeds pick up today due to
the low forming over southwest Saskatchewan. The peak speeds are
expected to occur over southeast Washington and the Blue Mountains
this afternoon and evening with gusts of 30-40 mph possible.
There will also be a small chance of seeing some mountain wave
winds cresting over the western Waterville Plateau per some of the
hi-res guidance. This is far from certain as the stable layer
over the top of the Cascades isn’t very prominent. Another
stronger period of winds is expected late Monday/Monday night as a
much deeper low pressure system forms over southern BC. The peak
speeds for this period should occur Monday night with some 40-50
mph gusts likely over the Palouse, Blues and Cascade Ridges with
widespread 30-40 mph winds expected elsewhere. We may need to
issue future wind advisories to account for this situation.
fx

...Significant mountain snow early next week, then potentially
record cold temperatures combined with brisk wind chills coming to
the Inland NW mid-week...

Tuesday thru Saturday.

Tuesday: Broad upper level ridge off the coast that has been
providing the prolonged northwest flow over Eastern Washington
and North Idaho this weekend shifts to the west. As earlier
discussions noted the end result is a west to east oriented jet
stream more or less positioned over Oregon and Central Idaho with
most of Eastern Washington and North Idaho to the north of the jet
and under the influence of the cold conditionally unstable
airmass. A passing low pressure system will keep the surface
pressure gradient fairly packed as it moves from west to east and
through the area with high confidence of generally persistent
heavy pass level snow over the Cascade Crest and higher terrain of
North Idaho with widespread light to moderate snow coupled it
between. In addition robust windy conditions as the low pass allow
for mainly westerly winds to prevail but as the upper level trof
over the region digs further to the south/southwest the winds will
transition to blow more from the north/northeast as colder and
drier air works its way down. This robust wind will allow for a
mention of blowing and drifting snow in varying locations. Winter
weather highlights are in place primarily to address the
considerable snowfall and windy conditions for Lookout Pass on I90
and Stevens Pass on US Route 2.

Wednesday: The upper level trof keeps a cold conditionally
unstable airmass overhead with the prolonged northerly flow
allowing the cold air advection to continue. Blowing and drifting
snow will continue to be mentioned as rapid cooling that started
Tuesday night continues on into Wednesday. Any mention of
lingering snowfall remains generally highest on locations that
best utilize this northeast flow as an upslope flow which is
mainly over portions of the East Slopes of the North Cascades and
portions of the the Washington and Idaho Palouse and points to the
south including the Blue Mountains. The persistent cold air
advection provided with this pattern will result in some brisk
wind chill.

Thursday: Generally drier weather expected at this point with cold
trof lingering overhead. The persistent northeast flow and
associated cold air advection allows for temperatures to continue
to drop and brisk wind chill remaining an issue. Thursday may have
the coldest daytime highs may have most of the coldest morning
lows of the seven day forecast if they aren't surpassed on Friday.

Friday and Saturday: The upper level trof shows moderates some
with another low pressure system dropping down from the northwest
favoring Saturday more than Friday at this time. Temperatures show
a gradual rebound/warming. /Pelatti

&&

.AVIATION...
12Z TAFS: Dealing with some pockets of MVFR/IFR cigs this morning
across far eastern WA and northern ID. By later today, increasing
chances for precip across the Cascades and northern ID, and the
potential for mostly virga or sprinkles/flurries elsewhere. For
MWH and EAT, VFR conditions are likely to persist through the 24hr
TAF period. The other issue is winds over SE WA will become
breezy late today and tonight. This will mainly impact KPUW and
the Blue Mountains with gusts of 30-40 mph possible. /KD

&&

.PRELIMINARY POINT TEMPS/POPS...
Spokane        44  36  47  35  40  14 /  10  10  20  40  70  60
Coeur d'Alene  41  35  43  34  38  12 /  30  30  60  70  90  60
Pullman        42  38  44  35  40  19 /  20  40  40  80  80  80
Lewiston       51  42  53  42  48  26 /  20  30  40  60  60  60
Colville       40  28  43  32  39  14 /  10  10  30  40  80  50
Sandpoint      38  34  39  34  37  12 /  50  50  70  90 100  60
Kellogg        37  36  39  34  36   9 /  70  80  80 100 100  70
Moses Lake     51  36  54  36  47  20 /   0   0   0  10  10  50
Wenatchee      47  36  49  36  42  22 /  10  10  10  30  20  30
Omak           40  29  42  31  40  20 /  10  10  10  20  30  50

&&

.OTX WATCHES/WARNINGS/ADVISORIES...
ID...Winter Weather Advisory from 4 PM this afternoon to 4 PM PST
     Monday for Central Panhandle Mountains.

     Winter Storm Watch from Monday afternoon through late Tuesday
     night for Central Panhandle Mountains.

WA...Winter Storm Warning from 4 PM this afternoon to 4 AM PST
     Wednesday for Western Chelan County.

&&