Areal Flood Warning 4, NWS Seattle, WA

Area Forecast Discussion
National Weather Service Seattle WA
849 AM PST Sun Nov 28 2021

.UPDATE /0900 AM/...Multiple river flood warnings are in effect
this morning as persistent rainfall occurred during the
overnight. This band of precipitation is still situated over the
North Olympic Peninsula as well as Whatcom and Skagit counties.
This band is expected to be shifting southward throughout the day.
Rain is still expected to be persistent into Monday afternoon,
especially for the Central Cascade region. This rain will taper
off just in time for the next weather system to make its way to
the area Tuesday.

The river forecast as well as river flood warnings will change
throughout the day. Everyone is urged to stay aware of changing
conditions not only around them but throughout the region. And
remember do not drive through flooded streets, not only is the
depth of the water uncertain but the stability of the ground below
the surface is as well.

The previous forecast discussion with updated aviation and marine
sections is below.

Butwin

&&

.SYNOPSIS...Persistent rainfall will continue across Western
Washington through today, yielding river and aerial flooding, as
well as the threat for landslides. Following additional rainfall
during the day Monday, another AR will reach Western Washington by
midweek bringing additional excessive rainfall to the region.
Drier and cooler conditions possible Friday into next weekend.

&&

.SHORT TERM /TODAY THROUGH TUESDAY/...The persistent moderate
rainband has shifted northward with the warm front at this hour
with the heaviest rain occuring north of the border in SW British
Columbia. That said, rainfall continues across the northwestern
Olympic Peninsula, as well as northern Skagit and Whatcom counties
at this hour. In the past 6 hours, points in these locations have
seen anywhere from 1 to 1.5 inches of rainfall, with amounts
already in the 2 to 3 inch range at elevation in the past 24
hours. Rainfall efficiency remains high at this hour in this band
as a tongue of PWAT values ranging from 1 to 1.25" is draped
across these areas. The potential for impacts associated with
areal flooding and landslides will remain high across these areas
through much of the day. Excess runoff is already causing rivers
in these areas to rise, with additional rainfall expected through
this afternoon.

To the south of this corridor, rainfall has tapered off at this
hour, with higher winds aloft mixing down to the surface. Southerly
winds will generally gust between 30 to 40 mph at times across Puget
Sound, with a few isolated gusts to 45 mph possible - particularly
from the Admiralty Inlet area, Whidbey Island and into coastal
portions of Snohomish County. Expect winds to remain elevated
through the mid-morning hours before steady rain returns to the area.

Confidence is high in the band of persistent rainfall sagging
southward across the region, starting after daybreak and continuing
through the day as the cold front pushes southeastward across the
region. Most individual members of the HREF stall the front
somewhere between Tacoma and Chehalis this evening with steady
rain remaining focused in this area. Short range ensemble guidance
shows a shortwave trough axis swinging through the region between
12 and 18Z and amplifying the band of rain on Monday before rain
tapers off Monday evening. As a result, QPF in the central and
southern Cascades is higher on Monday with the forecast package
tonight. Given the environment in place, expect short-fused urban
and small stream flooding to be a threat along with landslides and
rising rivers through the day. See the hydrology section below
for specific details.

Also associated with the aforementioned trough will be the potential
for an isolated thunderstorm or two across Whatcom and Skagit
Counties. Mid-level (H500) temperatures will cool to around -25
degrees C as the trough axis swings through. This, coupled with a
speed max crossing the region will provide an environment capable of
briefly supporting thunder Monday afternoon.

Expect temperatures to remain steady across the region today, with
only minimal dynamic cooling in areas of rain as the front comes
through. High temperature readings have likely already occurred this
morning as warm air advection was still notable at midnight and has
since leveled off. Temperatures will remain relatively warm and
steady on Monday, with highs in the mid 50s and lows in the low
50s.

Pullin

.LONG TERM /TUESDAY THROUGH SATURDAY/...Wet and active weather
will persist through the long term period as several systems will
bring additional rainfall and periods of gusty winds to Western
Washington that will exacerbate impacts region wide. Another AR
event of similar strength as the one currently affecting the
region will reach the area on Tuesday with rainfall persisting
through Thursday. New ensemble guidance brings the plume of
moisture further south than the 00Z and 12Z guidance from
yesterday. Freezing levels look similar as well - generally
ranging from 9,000 to 10,000 feet at event onset before settling
between 8,000 to 9,000 feet through the event. Rainfall amounts
look similar to the current event, with the interior lowlands
generally seeing 1- 2" of rainfall, the Olympics and
Central/Northern Cascades seeing 2- 4", locally up to 6" in spots,
and the Olympic coast seeing 1-3", with the highest totals on the
north coast. This additional rainfall will cause another response
on area rivers, with additional flooding possible. Saturated
soils will also keep the threat for landslides and aerial flooding
high through this event.

As has been the case with this system, a period of rain shadowing
will be possible across the central and northern Puget Sound, which
would promote a period of gusty winds. Given saturated ground, more
notable impacts with respect to power outages and tree damage are
not out of the question. This will also likely add extra pressure to
area bluffs, potentially being a trigger for landslides.

Current model guidance has precipitation tapering off on Thursday
with Western Washington remaining in a quasi-zonal flow regime with
high pressure remaining stout over the southwest coast. This will
keep the door open for additional systems bringing periods of rain
Friday into next weekend.

Temperatures wise, expect 50s for highs and upper 40s for lows
through Thursday before temperatures cool into the 30s and 40s for
the weekend.

Pullin

&&

.AVIATION...Westerly flow aloft will continue today and tonight. A
cold front draped across the northwest part of the state this
morning will slide southeast through the area this afternoon and
evening. The air mass will be moist and stable today. It will
become slightly unstable across the far north part of the area
tonight.

Ceilings have lifted a bit over the central and south part of the
interior this morning after a warm front lifted through the area
last night. Ceilings should lower back down as the cold front
slides through later today.

KSEA...Ceilings should lower again late this morning or early
afternoon as rain returns. It looks to stay pretty soupy in the
wake of the front tonight with lower clouds lingering into Monday
morning. Gusty south winds 10-20 knots should ease today. 16

&&

.MARINE...A front will slide southeast through the area today.
Small craft advisory strength winds for parts of the area will
gradually ease today. West swell around 9 feet at 9 seconds will
linger over the Coastal Waters and West Entrance Strait of Juan de
Fuca today then subside tonight.

There will be lull in the active weather on Monday in between
weather systems. A second frontal system will move through the
area Tuesday and Tuesday night for another round of small craft
advisory strength winds most waters. Winds will be lighter
Wednesday and Thursday with weak onshore flow.

West swell over the Coastal Waters and West Entrance Strait of
Juan de Fuca will build to 10-13 feet Tuesday night and remain in
that general range through Thursday. 16

&&

.HYDROLOGY...A wet and warm frontal system will continue to bring
rain to the area through the day today, possibly into early Monday.
The focus of precipitation went further north and for longer than
forecast, and this bigger break will delay, and to some not-
fully- known extent, lower the river crests. Still, river flooding
remains an issue into the start of the new week = especially for
rivers flowing out of the Olympics and the northern and central
Washington Cascades. A flood watch remains in effect for much of
the area, with flood warnings also in effect for several
locations. The extent of expected river flooding varies throughout
the region, with minor to moderate flooding the general rule.
Major flooding no long looks likely. The current forecast
continues to reflect most locations cresting sometime today
-
however - the Skagit River at Mount Vernon and the Nooksack River at
Ferndale will likely not crest until sometime much later - likely
Monday afternoon.

A second frontal system will affect the region Tuesday into
Wednesday, and although confidence is still lower with this system -
ensembles now suggesting a shift in the precipitation further south.
This scenario starting to look very similar to what is expected
through the day today - that is, the heaviest precipitation expected
over the Olympics, and North and Central Washington Cascades. This
certainly would complicate the impacts on area rivers as it would be
following on the heels of a very similar event. The ongoing
precipitation will keep the risk of landslides elevated well into
next week.

Cook/JBB

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Flood Watch through Monday morning for Admiralty Inlet Area-
     Bellevue and Vicinity-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-
     Western Skagit County-Western Strait of Juan De Fuca-
     Western Whatcom County.

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

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

     Small Craft Advisory until 6 PM PST this evening for West
     Entrance U.S. Waters Strait Of Juan De Fuca.

&&

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