Heavy Snow in Washington, November 9 to 10, 2021

Area Forecast Discussion
National Weather Service Spokane WA
940 PM PST Mon Nov 8 2021

.SYNOPSIS...

Rain and snow arrives Tuesday morning, with wet snow accumulations
possible in places like the Methow Valley, Okanogan Highlands,
and lowland communities north of the Spokane metro. Heavy snow is
expected over the Cascade crest Tuesday morning through Tuesday
night. A warming showery pattern is expected late week into the
weekend.

&&

.DISCUSSION...

...HEAVY SNOW FOR THE CASCADES AND HEAVY WET SNOW FOR THE NORTHERN
MOUNTAIN VALLEYS NORTH OF HIGHWAY 2 TUESDAY...

Tonight through Wednesday: An upper level low pressure system
circulating in the Gulf of Alaska will send across a shortwave
disturbance Tuesday morning. The frontal occlusion will bring
moderate to heavy snow in the mountains, rain in the basin and wet
snow in the northern mountain valleys. Timing of precipitation
will be ideal for the Cascade valleys for snow with precipitation
increasing late tonight when temperatures should be at their
coldest. Places like Leavenworth, Plain, Stehekin, and the Methow
Valley are expected to pick up anywhere from 2 to 5 inches through
the morning on Tuesday. The frontal band will sweep across eastern
Washington and into the Idaho Panhandle as well with rain over the
basin to a mix of rain and snow across the Highway 2 corridor into
Spokane and Coeur d'Alene. Wet snow is expected up into the
northern mountain valleys in communities such as Republic,
Colville, Priest River, Sandpoint and Bonners Ferry. Snow looks to
be light enough with temperatures just hovering around freezing
that snow will have a harder time effectively accumulating on
roads. I kept snow ratios lower because of this and generally came
up with around a half of an inch to up to 1.5 inches and locally
up to around 2 inches possible. The Okanogan Highlands should be a
bit colder with Republic expected to drop down into the mid 20s
for a low tonight. This should result in better snow ratios closer
to 10/1 and have snow accumulations more between 1 to 3 inches.
Valleys away from the Cascade crest and the Okanogan Highlands are
under a Winter Weather Advisory for snow for late tonight through
Tuesday morning.

Heavier snow is expected at the Cascaded crest. The crest will get
a shot of snow with the front Tuesday morning, and then continue
with snow with moisture slopping over the crest through Tuesday
afternoon and Tuesday night. Lapse rates will increase with
convective banding becoming more likely behind the front. This
could lead to periods of heavy snow over Stevens Pass and
Washington Pass. Pass conditions may see improvement in the
afternoon as diurnal heating helps to melt snow, but high
resolution models show periods of heavy snow picking back up in
the evening on Tuesday, which will likely result in the passes
quickly becoming snow covered again. Winter travel is likely and
we have upgraded the Winter Weather Advisory to a Winter Storm
Warning that will start late tonight and extend through late
Tuesday night.

Winds will also become breezy to windy with this incoming frontal
system. Gusty southerly winds will pick up across the Camas
Prairie overnight where 40+ mph winds will be possible. Then
pressure gradients will tighten up behind the front across the
region for Tuesday afternoon with 30-40 mph winds across the
exposed areas of the basin into the Palouse and West Plains.
Precipitation will be decrease through the day on Wednesday with
mainly some lingering showers over the Panhandle and along the
Cascade crest. Winds will also be decreasing through the night on
Tuesday into Wednesday. /SVH

Wednesday night through Monday: Ensembles suggest an intensifying
area of low pressure off the coast digs an inflexion point onto the
backside of a baroclinic band/Moisture feed and allows a portion of
it to pivot northward as a weak warm front of sorts and pass over
Eastern Washington and North Idaho Wednesday night. Minor pops and
light qpf accompany it with along with a possible rain/snow mix for
some lowland/valley locations. An additional disturbance digs into
that same baroclinic band/Moisture feed a bit downstream and allows
a warm surge of moisture into the area Thursday night and Friday
allowing for more of a mountain snow/valley rain mix. This stream
becomes more west to east oriented and drapped over building ridging
going into the remainder of the weekend. This will allow for rain
shadow development over some of the lowland locations between the
lee of the Cascades and the higher terrain of North Idaho while
remaining locations continue to keep good potential going for valley
rain and mountain snow. Warming trend with this scenario looks to
peak with the ridging on Sunday with a hint that the ridge dampens
some thereafter. /Pelatti

&&

.AVIATION...
06Z TAFs: An occluded front will sweep across the Inland
Northwest late tonight pushing into eastern Washington after 09Z.
A tight easterly gradient at the surface with strong southerly to
westerly winds aloft with the front will result in low level wind
shear into the early morning hours. Precipitation will also
increase with the front with a mix of rain and snow across the
Spokane-Coeur d'Alene corridor, at the Pullman Airport and the
Wenatchee Airport. Little to no snow accumulations are expected at
these TAF sites. Airports in the northern mountain valleys such
as in the Methow, Colville, Deer Park, Sandpoint and Bonners Ferry
will see a better chance for heavy wet snow with minor
accumulations. Precipitation will bring lower ceilings with
widespread marginal VFR conditions and IFR conditions with snow.
Winds are also expected to become gusty to between 25-35 kts
across the basin and into KPUW and KGEG for Tuesday afternoon.
The precipitation turns more showery through the late morning to
afternoon, followed by some clearing into the Tuesday evening away
from the mountains. /Cote'

&&

.PRELIMINARY POINT TEMPS/POPS...
Spokane        34  46  30  45  31  44 /  20  90  20  10  20  20
Coeur d'Alene  32  45  31  42  30  43 /  10  90  30  20  20  10
Pullman        35  45  30  43  31  43 /  40  90  20  10  30  30
Lewiston       38  51  36  50  39  49 /  30  80  10  10  30  30
Colville       29  44  23  43  23  43 /  10  90  20  10  30  10
Sandpoint      30  42  30  39  28  40 /  10  90  50  30  30  20
Kellogg        33  44  32  40  31  43 /  10  90  50  40  30  20
Moses Lake     37  50  30  48  33  47 /  60  70   0   0  30  30
Wenatchee      34  48  32  47  36  44 /  70  80  30  10  40  40
Omak           33  46  28  45  30  45 /  30  90  10   0  30  20

&&

.OTX WATCHES/WARNINGS/ADVISORIES...
ID...None.
WA...Winter Storm Warning from 1 AM Tuesday to 1 AM PST Wednesday for
     Western Chelan County.

     Winter Weather Advisory from 3 AM to 10 AM PST Tuesday for
     Okanogan Highlands.

     Winter Weather Advisory from 1 AM to 10 AM PST Tuesday for
     Central Chelan County.

     Winter Weather Advisory from 3 AM Tuesday to 1 AM PST Wednesday
     for Western Okanogan County.

&&

Hazardous Weather Outlook
National Weather Service Spokane WA
458 AM PST Mon Nov 8 2021

IDZ001-WAZ037-038-043-091300-
Northern Panhandle-Northeast Mountains-Okanogan Highlands-
Okanogan Valley-
458 AM PST Mon Nov 8 2021

This Hazardous Weather Outlook is for portions of North Idaho...
North Central Washington and Northeast Washington.

.DAY ONE...TODAY AND TONIGHT

No hazardous weather is expected at this time.

.DAYS TWO THROUGH SEVEN...TUESDAY THROUGH SUNDAY

An incoming system expected to bring accumulating snow down into
the valleys Tuesday morning. Mountain passes will see a better
potential for significant snow accumulations with travel impacts
possible at times.

IDZ002>004-026-027-WAZ031>036-041-044-091300-
Coeur d'Alene Area-Idaho Palouse-Central Panhandle Mountains-
Lewiston Area-Lewis and Southern Nez Perce Counties-
Northeast Blue Mountains-Lower Garfield and Asotin Counties-
Washington Palouse-Moses Lake Area-Upper Columbia Basin-
Spokane Area-Wenatchee Area-Waterville Plateau-
458 AM PST Mon Nov 8 2021

This Hazardous Weather Outlook is for portions of North Idaho...
North Central Idaho...Central Washington...East Central
Washington...North Central Washington...Northeast Washington and
Southeast Washington.

.DAY ONE...TODAY AND TONIGHT

No hazardous weather is expected at this time.

.DAYS TWO THROUGH SEVEN...TUESDAY THROUGH SUNDAY

No hazardous weather is expected at this time.

WAZ047>049-091300-
Central Chelan County-Western Chelan County-
Western Okanogan County-
458 AM PST Mon Nov 8 2021

This Hazardous Weather Outlook is for portions of North Central
Washington.

.DAY ONE...TODAY AND TONIGHT

An overnight system expected to bring another round of several
inches of snow to the Cascade Passes and impacting travel.

.DAYS TWO THROUGH SEVEN...TUESDAY THROUGH SUNDAY

Snow from the system will last through Tuesday before exiting the
region. No hazardous weather is expected after Tuesday.