Small Craft Advisory 283, NWS Seattle, WA

Area Forecast Discussion...corrected typo
National Weather Service Seattle WA
916 AM PST Mon Nov 20 2017

.SYNOPSIS...A cold front will move east across the Western
Washington interior this morning. Convergence zone showers will
develop behind the front between Seattle and Everett through this
afternoon. A strong warm front will bring periods of moderate to
heavy rain Tuesday and Tuesday night, with the snow level rising
above 8000 feet. Wednesday will be less wet, then another front
moves through Wednesday night and Thanksgiving Day. Friday looks
relatively dry, then another front should arrive around Saturday.

&&

.SHORT TERM...RADAR shows a cold front moving across the W WA
interior this morning was moving over the Cascades at 9 AM. Some
areas of light rain were still present over the interior lowlands
but these should be done with by 11 AM. Onshore flow behind the
front will produce a convergence zone, likely between Seattle and
Everett, that should finally dissipate early this evening. The rest
of the W WA lowlands should remain dry this afternoon. There will be
a brief break in the precipitation this evening through most of
tonight.

A large deep upper level longwave trough is forming over the NE
Pacific around 145W. This is far enough offshore for moist SW flow
aloft to develop over W WA. A strong warm front will begin
spreading rain northward over W WA before 12Z/4 AM Tuesday with
the rain becoming heavy at times late Tuesday morning through
Tuesday afternoon before slowly tapering off. Snow levels are
expected to rise to 8000-9000 feet Tuesday afternoon through
Wednesday night. The warm front will bring heavy rain to the
Olympics and Cascades, which could drive some of the rivers
flowing off the mountains above flood stage. Refer to the
Hydrology section below.

At least the warm front will push temperatures up above normal
Tuesday and Wednesday, highs Tuesday should be in the mid to upper
50s, warming to the upper 50s to lower 60s on Wednesday.

Moist SW flow aloft will continue on Wednesday with a little light
rain around for wet warmth. Accumulations over the lowlands will
be light. However this will be just a short break since another
wet system will arrive later Wednesday night. Kam

LONG TERM...The next frontal system will bring another round of
rain to Western Washington late Wednesday night and Thanksgiving
Day. The rainfall doesn't look as heavy with this feature as with
the Tuesday and Tuesday night system, but the snow level will
still be 6000 to 7000 feet. Fairly moist southwest flow aloft will
continue over the region Friday and Saturday; it doesn't look dry
for these couple of days, but rather less wet. Models agree
generally that another frontal system - also with fairly high snow
levels - will arrive Saturday night or Sunday. McDonnal

&&

.AVIATION...Westerly flow aloft will continue today and tonight as
an upper level low moves east and weak high pressure builds. A cold
front will move southeast out of the area today. At the surface,
light to moderate northwest flow will develop. Flow will become
offshore later tonight as a strong warm front approaches the area
from the southwest. The air mass is moist and stable.

Over the interior, ceilings are still MVFR or even IFR as the cold
front remains nearby. West of Puget Sound there has been significant
improvement, to good VFR mainly, as skies start to clear. A clearing
trend will be apparent over all areas today, except a Puget Sound
convergence zone will likely form around the King-Snohomish county
line this afternoon. Conditions are expected to be VFR tonight over
all areas.

KSEA...IFR until 18Z or so, then improvement to MVFR and then VFR
this afternoon. Northerly wind less than 6 KT this morning will
become more variable this afternoon, then northeasterly tonight.
CHB

&&

.MARINE...A cold front will move east of Puget Sound this morning,
with light to moderate northwesterly wind behind the front. Small
craft advisory westerlies are forecast today in the strait, along
with residual swells above 10 ft on the coast.

A strong warm front will move north through the waters on Tuesday.
Gale force easterly winds are likely on Tuesday at the west entrance
and adjacent coastal waters. High end small craft advisory strength
winds are likely over most other waters, with gales possible on some.

The warm front will be followed by southerly flow on Wednesday.
Strong low pressure over the northeast Pacific will maintain
southerly flow on Thursday and Friday. CHB

&&

.HYDROLOGY...In the very short term, the only river above flood
stage is the Skokomish River in Mason County. It should crest
this morning just above flood stage then fall below this
afternoon.

The threat of flooding will become more widespread, though, as
warm wet frontal systems move through Tuesday/Tuesday night and
again Wednesday night/Thursday. The first system looks more
significant with 2 to 5 inches of rain over the Olympics and Mount
Rainier and 1 to 4 inches further north over the Cascades, and
with the snow level 8000 to 9000 feet. The second system won't
bring as much rain, but the snow level will be 6000 to 7000 feet.
We issued a flood potential outlook yesterday, and I will update
it this morning.

The Skokomish will almost certainly flood Tuesday or Tuesday night.
Northwest River Forecast Center guidance also shows several other
forecast area rivers cresting near or just above flood stage
including the Cowlitz, Nisqually, Snoqualmie, Skagit, Nooksack,
and Dungeness.

Landslides could also become a threat with the lowland rains
Tuesday and Tuesday night. The USGS landslide guidance shows one
or two stations near or just above the threshold of concern, and
of course the additional rain with that front will drive that
higher. McDonnal/Kam

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Winter Storm Warning until 10 AM PST this morning for Cascades
     of Whatcom and Skagit Counties.

PZ...Small Craft Advisory for hazardous seas until 3 PM PST this
     afternoon 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 for rough bar until 3 PM PST this afternoon
     for Grays Harbor Bar.

     Gale Warning from late tonight through Tuesday afternoon for
     Coastal Waters From Cape Flattery To James Island 10 To 60
     Nm-Coastal Waters From Cape Flattery To James Island Out 10
     Nm-West Entrance U.S. Waters Strait Of Juan De Fuca.

     Small Craft Advisory until 3 PM PST this afternoon for Central
     U.S. Waters Strait Of Juan De Fuca-East Entrance U.S.
     Waters Strait Of Juan De Fuca.

     Small Craft Advisory for hazardous seas until 9 AM PST this
     morning for West Entrance U.S. Waters Strait Of Juan De
     Fuca.

&&

www.weather.gov/seattle

You can see an illustrated version of this discussion at
www.wrh.noaa.gov/sew/gafd/latest_webafd.html