Areal Flood Advisory 13, NWS Seattle, WA

Area Forecast Discussion
National Weather Service Seattle WA
308 PM PST Sun Nov 14 2021

.SYNOPSIS...Precipitation through Monday as an Atmospheric River
impacts the area. River flooding, landslide, and power
outages/downed tree threat will exist through Monday. Snow levels
will lower Monday night into Tuesday with scattered lowland
showers and mountain snow. Drier weather Tuesday night into
Wednesday with the potential for another system late week.

&&

.SHORT TERM /TONIGHT THROUGH WEDNESDAY/...The 230 PM radar shows
the heaviest rainfall focused along Whatcom County and into BC
with upslope precipitation for the Olympics and King County
northward across the Cascades. The pattern will be similar into
this evening, with the heaviest rain focused Skagit County
northwards as a front slowly slides south across the northern
areas. The pattern will be active through Monday night, with
the expected weather impacts:

Rain: The plume of moisture with the AR is focused along the
North Coast and near the BC border this afternoon into this
evening. A stationary front will be located along the BC border
into tonight with the heaviest precipitation in these areas.
Scattered showers south of Skagit County, and upslope
precipitation for the Olympics/Cascades. A secondary low will
strengthen offshore into Monday morning, and a cold front will
slide south across the area during the day. For most areas, the
most impactful (outside of Skagit/Whatcom/North Coast) will be
during the day on Monday. Heaviest rain expected to move through
between 9AM-3PM for most of the interior. Urban flooding will be
possible for low-lying areas with poor drainage during this with
rates of 0.25-0.50" possible. Winds will also be a significant
concern for more widespread impacts with the saturated soils
across the area (refer to Wind section). Rain will taper off late
Monday with a potential convergence zone expected Monday night
into Tuesday with strong west winds for Admiralty Inlet/Snohomish
(refer to marine section). Additional 50th Percentile Amounts:

Cascades/Olympics: 5-9" - heaviest rainfall amounts expected
north of King County, particularly across Whatcom/Skagit Counties.

Lowlands: 0.50-1.50" - an additional 2-3" for Whatcom County
lowland areas and 2-5" for the coast.

Winds: Windy period to continue overnight into Monday across the
whole of Western Washington. Models are consistent with the
development of a mesoscale surface low on the lee side
of the Olympic Mountains on Monday morning. This will act to enhance
southerly winds across the interior lowlands. Strong gradients and
pressure rises associated with the cold front could lead to a
period of high winds along the coast Monday morning. A strong post-
frontal westerly surge down the Strait of Juan de Fuca is expected
on Monday afternoon which could acutely lead to wind related impacts
for the Admiralty Inlet area, Whidbey Island, and shoreline areas of
Snohomish County. Advisory level winds of 25-35 mph with gusts 40-45
mph are expected across nearly all lowland areas of the region.
Given saturated soils in place, the potential for power outages and
tree damage are elevated. Winds will gradually ease Monday night.
High Wind Watches for the Central Coast and Admiralty Inlet have
been issued with Wind Advisories for most other areas. (JD/27)

Mountain Snow: Snow amounts for the Cascades have increased. Snow
levels will lower to 1500-2000 feet Monday night into Tuesday.
Convergence Zone activity expected during this period with
potentially additional banding across the Cascades. Amounts of
4-6" possible for Snoqualmie Pass with 6-8" for Stevens Pass.
Heavier amounts will be possible across areas of King/Snohomish
Counties with convergence zone activity into Tuesday.
Precipitation tapers off on Tuesday.

Dry weather for Wednesday with high pressure across the area.
Cooler weather expected midweek.

.LONG TERM /THURSDAY THROUGH SUNDAY/...Another weather system will
move through the area Thursday into Friday. Lowland rain, breezy
winds, and additional mountain snow is expected. Ensembles have
potentially drier weather on Saturday as ridging builds into the
area, but some slight precipitation in the forecast with
uncertainty for the area. Temperatures will be cooler, with chilly
overnights, for the area. JD

&&

.AVIATION...A stalled frontal boundary lies just north of the
Canadian border with strong westerly flow aloft and gusty southerly
winds near the surface. Flow aloft will become more SWerly overnight
tonight and into Monday. The air mass remains very moist and stable.
Area cigs generally MVFR to IFR with light to moderate rainfall
expected to continue off and on throughout the day today before
becoming more persistent tonight and throughout the day Monday. That
said, no improvement to cigs expected during the TAF period. Very
gusty surface winds will also persist into Monday. 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.

KSEA...Low-end MVFR conditions expected to persist tonight and into
Monday with light rain at times. 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. LLWS will again be an issue for the
remainder of the day and into the overnight hours with S/SW winds of
40-45 knots down to 2000 feet. 27/18

&&

.MARINE...Gale warnings in effect for all waters into Monday.
Confidence remains high that winds will become rather strong
in the vicinity of a cold front as it moves onshore around midday
Monday. 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.

High pressure will rebuild across the waters Tuesday into Wednesday
allowing winds to relax and seas to subside. Surface ridging will
settle over the interior of British Columbia by midweek for offshore
flow.  27

&&

.HYDROLOGY...Persistent moderate to heavy rain has spread across
the region at this hour with area rivers beginning to respond to
additional excess runoff. Excessive additional rainfall over
saturated ground coupled with snowmelt portions of the Cascades
and Olympics due to wind friction and high snow levels will drive
some rivers up to Major Flood Stage on Monday. These include:
South Fork of the Nooksack, the Skagit, Dungeness and the Elwa
Rivers.

High resolution guidance is in agreement in keeping the band of
heaviest rainfall generally from Snohomish County/northern Jefferson
County northward this evening through early tomorrow morning
before the rainband pivots and pushes across all of Western
Washington late morning through the afternoon. This will yield
higher rainfall amounts in these areas, generally on the order of
2 to 4 inches in the lowlands and 4 to 8 inches, locally higher in
the mountains through late Monday in these areas. Rainfall totals
will be less south of these areas, with rivers not expected to
reach flood stage outside of a few points in northeastern King
County and the Skokomish River in Mason County. Rivers reaching
flood stage are generally expected to crest Monday afternoon and
recede Monday evening, with larger rivers expected to crest on
Tuesday and recede into the day Wednesday.

Heavy rainfall rates approaching 0.5-1"/hr at times will make urban
flooding a threat through Monday evening. In addition, the risk of
landslides will once again remain high beginning tonight and
persisting through at least Tuesday due to heavy rainfall and
strong, gusty winds that will make area bluffs unstable.

Pullin

&&

.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 Monday for Bremerton and Vicinity-
     Hood Canal Area-Seattle and Vicinity-Southwest Interior-
     Tacoma Area.

     Wind Advisory until 7 PM PST Monday for Everett and Vicinity.

     Wind Advisory until 10 PM PST this evening for Central Coast.

     High Wind Watch from 10 PM PST this evening through Monday
     afternoon for Central Coast.

     Wind Advisory until 1 PM PST Monday for North Coast.

     High Wind Watch from Monday morning through Monday evening for
     Admiralty Inlet Area.

     Wind Advisory until 10 AM PST Monday for Admiralty Inlet Area.

     Wind Advisory from 10 AM to 7 PM PST Monday for Eastern Strait
     of Juan de Fuca.

PZ...Small Craft Advisory until 7 PM PST Monday 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 to 7 PM PST Monday for East Entrance
     U.S. Waters Strait Of Juan De Fuca.

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

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

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

&&

www.weather.gov/seattle

Special Weather Statement
National Weather Service Seattle WA
327 PM PST Sun Nov 14 2021

WAZ503-504-506-507-509-511>517-555-556-558-559-567>569-160100-
Western Whatcom County-Southwest Interior-Western Skagit County-
Everett and Vicinity-Tacoma Area-Hood Canal Area-
Lower Chehalis Valley Area-Olympics-
Eastern Strait of Juan de Fuca-Western Strait of Juan De Fuca-
North Coast-Central Coast-East Puget Sound Lowlands-
Bellevue and Vicinity-Seattle and Vicinity-Bremerton and Vicinity-
Cascades of Whatcom and Skagit Counties-
Cascades of Snohomish and King Counties-
Cascades of Pierce and Lewis Counties-
327 PM PST Sun Nov 14 2021

...CONTINUED HEAVY RAINFALL AND GUSTY WINDS WILL CAUSE AN ELEVATED
THREAT OF LANDSLIDES TO CONTINUE IN WESTERN WASHINGTON...

Rainfall amounts of 6-12 inches over the past 7 days has led to
high soil moisture levels across western Washington. Additional
rainfall totals of 2-4 inches across the lowlands and 5-10 inches
across the mountains is expected with through Tuesday morning.
Rainfall rates could reach 0.5" to 1"/hr at times in the heaviest
pockets of rain. This, coupled with strong southerly winds
through Monday afternoon will put extra pressure on soil
instability and lead to an elevated threat of landslides.

For more information about current conditions, visit
www.weather.gov/seattle, select Hydrology, and then scroll down
for the links to the landslide information pages.

For more information on landslides, visit the website for the
Washington State Department of Natural Resources landslide
geologic hazards at: http://bit.ly/2mtA3wn

www.weather.gov/seattle

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