Heavy Snow in Washington, January 5 to 6, 2021

Area Forecast Discussion
National Weather Service Seattle WA
306 AM PST Tue Jan 5 2021

.SYNOPSIS...The last in a series of storm systems off the Pacific
will move across W WA today, bringing another round of lowland
rain, mountain snow, high seas, and breezy winds. Unsettled
conditions are then expected through much of Wednesday before the
area dries out on Thursday. The forecast then trends a bit wetter
Friday and through the latter half of the weekend as the region
remains open to storms off the Pacific.

&&

.SHORT TERM /TODAY THROUGH THURSDAY/...The upper level pattern
this morning features a trough departing the Pacific Northwest
into the Rockies, with a shortwave ridge building into the region
in its place. Upstream lies a broad area of cyclonic flow,
stretching from roughly the northern BC Coast westward through the
Aleutian Islands. A potent extratropical cyclone can be seen
nicely in water vapor imagery this morning southeast of the
Aleutian Islands, with several perturbations noted to its south.
At the surface, yesterday's cold front has moved well east of the
area into the Rockies, with a ridge of high pressure found in its
wake extending from off the California Coast into the Pacific
Northwest. Another Pacific frontal system can seen upstream, which
is en route to the Pacific Northwest.

It has been a generally tranquil night/early morning across W WA,
with lingering showers continuing within a broad/weak convergence
zone across eastern King and Snohomish Counties. This led to a
brief extension of the Winter Storm Warning thru 2AM across the
Central Cascades to account for travel impacts across the Passes.
This zone will continue to slowly weaken and diminish over the
next several hours. With shortwave ridging/decent height rises,
along with high pressure at the surface, we will see a break in
the active weather/precipitation this morning. As has been said
plenty of times recently, this will be short-lived as a leading
perturbation within the large scale North Pacific trough pushes the
aforementioned Pacific sfc frontal boundary towards the area. The
warm frontal portion of the boundary will lift into the local
area first and should begin to reach the Coast around daybreak.
Strong WAA associated with the front will supply broad ascent over
the area, allowing for another expansive area of stratiform rain.
Winds will also noticeably pick up out of the south over the
offshore waters and along the Coast through the morning hours,
prompting the issuance of a High Wind Warning for the North Coast
(gusts up to 60 mph possible) and a Wind Advisory for the Central
Coast (gusts up to 50 mph). The front will then continue to move
across the area into the afternoon, spreading rain inland through
the late morning (Puget Sound area) and early afternoon
(Cascades). Breezy to windy conditions are also expected across
the North Interior (Admiralty to the San Juans and over into
western Whatcom and Skagit Counties) this afternoon and evening-Refer
to the NPWSEW for details on the winds. At the onset of the
precipitation and through much of the afternoon, snow levels are
expected to hang between 2500-3000 feet before then rising to
between 3000-5000 feet this evening ahead of the cold front. This
has led to the issuance of another round of winter weather
headlines, with a Winter Weather Advisory set for the Olympics
and Central Cascades, including the Passes, and a Winter Storm
Warning set for the North and South Cascades, mainly to account
for new snowfall accumulation of 1-2 feet near ski resorts and
other heavily visited areas (See WSWSEW for more details). In
addition to all of this, waves near the Coast are forecast to
increase to between 20-22 feet this afternoon and evening-A High
Surf Advisory has been issued, with dangerous beach conditions
expected.

The cold frontal portion of the system will then move across the
area this evening and through the overnight hours. Widespread
precipitation will continue along and ahead of the front, becoming
confined mostly to the Cascades by daybreak Wednesday. Expecting
a much drier day on Wednesday, however, the main forecast concern
will the unstable post-frontal environment. The parent trough is
expected to move across the area off the Pacific, providing a
source for lift amidst the advection of colder mid level temps
(near -20C). Model guidance supports the idea of the development
of moisture just east of the upper trough axis. A fairly narrow
band of precip is expected to develop in response to this,
initially offshore and then pushing at least to the Coast and
Olympic Peninsula through day's end Wednesday. A thunderstorm or
two may materialize within this environment, carrying a threat for
lightning and small hail. Precip will then pull east of the area
overnight Wednesday as the trough continues its journey towards
the Rockies.

Another shortwave ridge develops over the local area on Thursday
in response to another digging trough over the Pacific. This ridge
holds more promise in providing the first dry day in some time. It
may even come complete with some breaks for sun during the
afternoon!

Kovacik

.LONG TERM /FRIDAY THROUGH MONDAY/...The long term forecast period is
still expected to feature a very strong ridge of high pressure
extending from the North Atlantic into Canada. This is
characteristic of a -AO and -NAO pattern, which will typically
allow cold air to spill into the southern and eastern US. Ensemble
guidance still suggests this pattern will materialize, with the
development of a trough from the south-central US to the East
Coast. Things are a bit trickier out west, with current trends in
the ensemble solutions pointing towards the development of a weak
ridge over the Rockies. This would act to keep the PNW open to the
deep North Pacific trough and its associated storm parade. Friday
continues to trend wetter so have increased chances for the next
round of rain. Similar to previous systems, it's currently
expected to give way to showers through early Saturday. Shortwave
ridging is then expected to build in the system's wake, likely
keeping most locations dry for much of the daytime hours Saturday.
Most guidance is then not overly keen on keeping the ridge axis
over the PNW, so its possible that another system may approach
late in the weekend. Details on this potential remain a bit fizzy
due to the models' struggle with ridge axis placement. This
uncertainty will carry through the remainder of the long term. It
still appears the overall pattern into mid-Jan will feature a West
US ridge/E US trough, but for us in the PNW the location of the
ridge axis will determine whether we begin to trend drier or
remain open to the storms off the Pacific.

Kovacik

&&

.AVIATION...Westerly flow aloft today as another weather system
moves across the area. A few showers linger this morning in the
vicinity of an earlier convergence zone but otherwise dry conditions
are present. Ceilings are a mix of MVFR and VFR across the area.
Expect some improvement in the morning before rain moves onshore.
Ceilings will lower again with MVFR to IFR in heavier showers. These
conditions will likely continue into tonight. Southerly winds at the
surface today with gusts increasing throughout the day. Strongest
winds expected at the coast and northern interior.

KSEA...Ceilings are bouncing between MVFR and VFR early this morning
and this trend may continue through the morning hours. Ceilings/vsby
will lower again by late afternoon as rain moves in with MVFR likely
into tonight. Southerly winds 8 to 15 kts with gusts of 20 - 25 kts
after 21z.

Carpenter

&&

.MARINE...A strong front will move through the waters today with
high end gales along the coast, northern inland waters and the
eastern Strait of Juan de Fuca. Small Craft Advisory winds elsewhere
into this evening. Winds will taper off tonight. Seas of 16 to 20
feet this morning will build to 20 to 27 feet later tonight into
early Wednesday morning. Winds will be lighter Wednesday into
Thursday with increasing winds Thursday night and Friday as another
system moves across the waters.

Carpenter

&&

.HYDROLOGY...Three points remain in flood this morning: Skokomish
at Potlatch and the Chehalis River near Grand Mound and Porter.
These rivers will remain above flood stage for the next few days.

Another round of rain is expected today into Wednesday. The heaviest
rain will mainly impact the Olympic and Southwest WA basins with
renewed rises on those rivers which are already swollen and running
high. Current forecasts suggest no additional flooding beyond the
aforementioned rivers, but will continue to monitor. In addition,
the landslide threat will remain elevated with this next round of
heavier rain as soils remain saturated. Several landslides have
been reported across the area since this weekend.

Carpenter

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Winter Storm Warning until 2 AM PST early this morning for
     Cascades of Snohomish and King Counties.

     High Wind Warning from noon today to 10 PM PST this evening for
     San Juan County-Western Whatcom County.

     Wind Advisory from noon today to 10 PM PST this evening for
     Admiralty Inlet Area-Western Skagit County.

     Wind Advisory from 10 AM this morning to 7 PM PST this evening
     for Central Coast.

     High Surf Advisory from 2 PM this afternoon to 4 AM PST
     Wednesday for Central Coast-North Coast.

     High Wind Warning from 10 AM this morning to 7 PM PST this
     evening for North Coast.

PZ...Small Craft Advisory until 4 PM PST Wednesday for Grays Harbor
     Bar-West Entrance U.S. Waters Strait Of Juan De Fuca.

     Small Craft Advisory until 6 AM PST early this morning for
     Coastal Waters From Cape Flattery To James Island 10 To 60
     Nm-Coastal Waters From Cape Flattery To James Island Out 10
     Nm-Coastal Waters From James Island To Point Grenville 10
     To 60 Nm-Coastal Waters From James Island To Point
     Grenville Out 10 Nm-Coastal Waters From Point Grenville To
     Cape Shoalwater 10 To 60 Nm-Coastal Waters From Point
     Grenville To Cape Shoalwater Out 10 Nm.

     Small Craft Advisory from 10 PM this evening to 4 PM PST
     Wednesday for Coastal Waters From Cape Flattery To James
     Island 10 To 60 Nm-Coastal Waters From Cape Flattery To
     James Island Out 10 Nm-Coastal Waters From James Island To
     Point Grenville 10 To 60 Nm-Coastal Waters From James
     Island To Point Grenville Out 10 Nm-Coastal Waters From
     Point Grenville To Cape Shoalwater 10 To 60 Nm-Coastal
     Waters From Point Grenville To Cape Shoalwater Out 10 Nm.

     Gale Warning from 6 AM this morning to 10 PM PST this evening
     for Coastal Waters From Cape Flattery To James Island 10 To
     60 Nm-Coastal Waters From Cape Flattery To James Island Out
     10 Nm-Coastal Waters From James Island To Point Grenville
     10 To 60 Nm-Coastal Waters From James Island To Point
     Grenville Out 10 Nm-Coastal Waters From Point Grenville To
     Cape Shoalwater 10 To 60 Nm-Coastal Waters From Point
     Grenville To Cape Shoalwater Out 10 Nm.

     Small Craft Advisory from 5 AM early this morning to noon PST
     today for Admiralty Inlet-East Entrance U.S. Waters Strait
     Of Juan De Fuca-Northern Inland Waters Including The San
     Juan Islands.

     Gale Warning from noon today to 4 AM PST Wednesday for Admiralty
     Inlet-East Entrance U.S. Waters Strait Of Juan De Fuca-
     Northern Inland Waters Including The San Juan Islands.

     Small Craft Advisory from 6 AM this morning to 4 AM PST
     Wednesday for Central U.S. Waters Strait Of Juan De Fuca.

     Small Craft Advisory from 10 AM this morning to 4 AM PST
     Wednesday for Puget Sound and Hood Canal.

&&

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