Coastal Flood Advisory 13, NWS Seattle, WA

Area Forecast Discussion
National Weather Service Seattle WA
331 AM PST Mon Nov 15 2021

.SYNOPSIS...An atmospheric river will continue to bring heavy
rain to W WA today resulting in serious river flooding impacts
along with an increased threat for landslides. Strong winds will
also lead to power outages and downed trees given saturated soil.
Heavy rain will push east this afternoon, with very strong west
winds down the Strait and rapidly falling snow levels. A PSCZ and
mountains snow will then give way to a drying trend on Tuesday,
which will continue through Wednesday before the next system
moves in for Thursday.

&&

.SHORT TERM /TODAY THROUGH WEDNESDAY/...An atmospheric river event
continues across the Pacific Northwest early this morning as
deeper subtropical Pacific moisture continues to be channeled
northeastward as cyclonic flow continues over the Gulf of Alaska
and adjacent Pacific and high pressure remains centered offshore
southern California. The lower levels continue to depict a nearly
stationary boundary draped from southern BC through the NW tip of
the Olympic Peninsula and into the open Pacific. A shortwave
trough rounding the base of the broader cyclonic flow over the
Pacific is helping induce a sfc wave along the stationary frontal
boundary this morning out to our west and this will have important
forecast implications for today.

A look at current MRMS and local radar imagery around 230AM this
morning shows very heavy rainfall along the frontal boundary
extending from the coastal waters through the NW portion of the
Olympic Peninsula and into the San Juan Islands and Whatcom
County. Observations within this general vicinity just over the
last 6 hours have reported 1-2 inches of rainfall. Elsewhere,
light rain and heavy drizzle is noted and has been persistent most
of the night, only totaling a few hundredths of an inch to
a few tenths of an inch.

Going forward today, a busy and very impactful weather day should
be anticipated. As the aforementioned mid level shortwave trough
continues to approach the local area, the sfc wave along the
stationary front will continue to strengthen and will help turn
the stationary boundary into a more progressive cold front.
Deepening of the sfc wave will help draw better Pacific moisture
into the area, which is already characterized by over 1 in PW
values, per SPC mesoanalysis. This will make for a heavy rainfall
threat that will push NW to SE across the area through the day. As
a general guide, rainfall totals from midnight Monday through
this afternoon are generally expected to range from 3 to 5 inches
in the Olympic and N Cascades to 1-2 inches from the Coast and
North Interior to around an inch around Puget Sound and remaining
Cascade zones. This will likely lead to significant river
flooding across northern counties and will likely impact many area
roadways. See hydrology section for better details.

As the sfc wave/low deepens, south winds will also strengthen
across the entire area. Generally expect gusts to 30-35mph through
much of the day, so power outages and downed trees will be highly
likely. A Wind Advisory in place for much of the area through the
afternoon.

As the cold front passes through the area this afternoon, the
bulk of the heavy precipitation will follow it, eventually
reaching the Cascade crest by mid to late afternoon. River
flooding will continue even after the rain has largely passed, and
another major concern is the very strong west wind anticipated in
its wake. Model guidance is highly suggestive of very strong
winds channeling through the Strait of Juan de Fuca this afternoon.
This has prompted the Storm Warning for the East Entrance of the
Strait and also a High Wind Warning for the Everett, Admiralty
Inlet, and Western Skagit County areas. West winds of up to 60 mph
will be possible and have the potential to produce significant
tree and powerline damage. In addition to this, strong post-
frontal west winds will allow for the development of a PSCZ this
evening, with rapidly tumbling snow levels (to about 1500-2000ft).
This will lead to additional rainfall across portions of the
area, potentially heavy, but will more so begin to cause mountain
snow issues, particularly at the Passes. Expect quick
accumulations at both Stevens and Snoqualmie Pass as snow levels
drop quickly-this could cause travel disruptions later
today/tonight.

Scattered showers will likely continue overnight tonight and into
Tuesday as cold air continues aloft. A secondary CZ is expected a
bit further north than the previous one and will likely continue
to bring snow to the north and central Cascades. To account for
all of additional precip behind the front, a Winter Weather
Advisory has been issued for the Cascades from 4pm Monday-10am
Tuesday. A drying trend will commence through the day on Tuesday
and winds will also ease. River flooding impacts are likely to
linger, but rivers will at least finally have the chance to
recede. Dry, but much cooler weather is expected Tuesday thru
Wednesday, with highs in the mid to upper 40s and lows in the 30s.
May lowland areas have the potential to see freezing temperatures
Wednesday morning, especially over the East Puget Sound lowlands,
the SW Interior, and Lower Chehalis Valley.

Kovacik

.LONG TERM /THURSDAY THROUGH SUNDAY/...The next weather producer
will be moving towards the area at the start of the long term
forecast period. Current guidance is suggesting that the storm
will move into northern Oregon, but will still likely result in
breezy winds and rain for W WA. There may be a break in the
active weather on Friday but another frontal system may be
approaching for the weekend. A ridge will be attempting to build
into the region this weekend, per ensemble means, but will have to
see where exactly this will set up in relation to troughing over
Canada.

Kovacik

&&

.AVIATION...A stalled frontal boundary lies just north of the
Canadian border with strong westerly flow aloft and gusty southerly
winds near the surface. The air mass remains very moist and stable.
Area cigs mostly MVFR with some isolated IFR in spots. Portions of
Puget Sound are shadowed from the rain this morning, mainly from BFI
south and west across the Kitsap Peninsula, so brief VFR conditions
have developed with gusty winds ongoing at the surface. Rain will
likely fill in across these areas by 17-18Z with gusts increasing to
between 25-30 kt. Very gusty surface winds will continue today
before easing late this afternoon and evening. Low level winds just
above the surface will remain quite strong...S/SW 45 to 60 knots
between 2000 and 5000 feet. This is expected to continue tonight and
into early Monday morning. After the frontal passage, the airmass
will be much cooler.

KSEA...Currently holding VFR at the terminal early this morning,
with gusty SW winds to 30kt. VFR conditions may not linger through
the morning as rain is expected to fill in across Puget Sound
between 15-17Z where a reduction to MVFR is likely. Brief periods of
IFR are possible as the moderate rain showers and windy conditions
impact the terminal. Surface winds to remain gusty out of the S/SW
15 to 25 knots before speeds increase early Monday morning ahead of
an approaching cold front with some gusts of 30 to 40 knots possible
12Z-18Z. Strong low level winds are likely to continue, generally
between 45-60kt between 020-050. SW winds upwards of 70kt are
possible between 050-080. Winds will generally start to ease late
this afternoon and evening but may still be gusty at times.

Kristell

&&

.MARINE...Gale warnings in effect for all waters today as confidence
remains high that winds will become rather strong in the vicinity of
a cold front as it moves onshore around midday. Models remain
consistent that a strong westerly surge will follow the
front...which has prompted a storm warning for the east entrance to
the strait. In addition, the formation of a meso-low in the lee of
the Olympics on Monday followed by impressive pressure rises will be
a driving force for widespread gale force winds over the interior
waters Monday afternoon.

Seas will be between 10-15ft through the day today as the frontal
system passes through, subsiding to below 10 ft by early Tuesday
morning. High pressure will rebuild across the waters Tuesday into
Wednesday allowing winds to relax and seas to subside.

Kristell

&&

.HYDROLOGY...Cold front sliding south and east of the area today
with the front mostly in the Cascades by early to mid afternoon.
This morning will be the last round of heavy precipitation over
the Olympics and North and Central Washington Cascades. Another
one to two inches is possible in the mountains with slightly lower
amounts over the lowlands. Snow levels will drop significantly
behind the front turning the precipitation in the Cascades from
rain to mostly snow. Hydrologically significant precipitation will
come to an end this afternoon into this evening.

Some 24 hour rainfall totals ending at 2 am include 6.24 inches at
Sumas, 5.21 inches at Mount Baker, and 3.22 inches at Bellingham.
Rainfall amounts drop off quickly with just a quarter to a half an
inch at NAS Whidbey Island and at Skagit Regional airport.

Another one to two inches of rain is forecast this morning over
the northern portion of the area before the rain rates decrease
quickly behind the front this afternoon.

Record breaking flooding is forecast for the Skagit river at
Mount Vernon. Current forecast calls for a crest of 39 feet at
Mount Vernon with the record being 37.34 inches set on Nov. 25,
1990. This number is lower than what should have happened at Mount
Vernon due to levy breaks upstream of Mount Vernon during the
1990 event. The current forecast crest for the upper reaches of
the Skagit at Concrete would put it in the top 6 crests on
record.

Minor to moderate flooding occuring on most of the other rivers
from north King County northward today and along the north coast
and north slopes of the Olympics. The upper reaches of the rivers
will be cresting today with the flood crests making their way
downstream later today into Tuesday. Felton

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Flood Watch through Tuesday afternoon for Admiralty Inlet Area-
     Bellevue and Vicinity-Cascades of Pierce and Lewis Counties-
     Cascades of Snohomish and King Counties-Cascades of Whatcom
     and Skagit Counties-Central Coast-East Puget Sound Lowlands-
     Eastern Strait of Juan de Fuca-Everett and Vicinity-Hood
     Canal Area-Lower Chehalis Valley Area-North Coast-Olympics-
     Seattle and Vicinity-Tacoma Area-Western Skagit County-
     Western Strait of Juan De Fuca-Western Whatcom County.

     Wind Advisory until 4 PM PST this afternoon for Bremerton and
     Vicinity-Hood Canal Area-Seattle and Vicinity-Southwest
     Interior-Tacoma Area.

     Winter Weather Advisory from 4 PM this afternoon to 10 AM PST
     Tuesday for Cascades of Pierce and Lewis Counties-Cascades
     of Snohomish and King Counties-Cascades of Whatcom and
     Skagit Counties.

     Wind Advisory from 10 AM this morning to 7 PM PST this evening
     for Eastern Strait of Juan de Fuca-San Juan County-Western
     Whatcom County.

     High Wind Warning from 10 AM this morning to 7 PM PST this
     evening for Admiralty Inlet Area-Everett and Vicinity-
     Western Skagit County.

     High Wind Warning until 4 PM PST this afternoon for Central
     Coast.

     Wind Advisory until 10 AM PST this morning for Admiralty Inlet
     Area-Everett and Vicinity.

     Wind Advisory until 1 PM PST this afternoon for North Coast.

PZ...Small Craft Advisory until 7 PM PST this evening for Grays
     Harbor Bar.

     Gale Warning until 4 AM PST Tuesday 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.

     Storm Warning from 10 AM this morning to 7 PM PST this evening
     for East Entrance U.S. Waters Strait Of Juan De Fuca.

     Gale Warning until 10 AM PST this morning for East Entrance U.S.
     Waters Strait Of Juan De Fuca.

     Gale Warning until 4 PM PST this afternoon for Admiralty Inlet-
     Northern Inland Waters Including The San Juan Islands-Puget
     Sound and Hood Canal.

     Gale Warning until 7 PM PST this evening for Central U.S. Waters
     Strait Of Juan De Fuca-West Entrance U.S. Waters Strait Of
     Juan De Fuca.

&&

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