Small Craft Advisory 286, NWS Seattle, WA

Area Forecast Discussion
National Weather Service Seattle WA
917 PM PST Sat Nov 26 2016

.SYNOPSIS...A front will weaken over Western Washington tonight
for decreasing showers. A vigorous cold front will bring rain and
mountain snow Sunday afternoon and night. Showers will decrease
Monday, and an upper ridge should bring dry weather Tuesday.
Another front will arrive Wednesday. Dry weather is possible
Thursday and Friday, ahead of the system due in on Saturday.

&&

.SHORT TERM...A front is moving slowly eastward across Western
Washington this evening, with its upper trough along the coast.
The stronger part of this frontal system is digging southeast over
northern California and southern, with the weaker part of the
system affecting us. It's bringing light rain to the interior
lowlands, with spotty post-frontal showers at the coast.
Precipitation in the mountains is light, falling as light snow
above 4000 ft.

The front and upper trough will weaken further tonight as they
continue their slow eastward progression across Western
Washington, with the main upper support heading southeast into the
Sierra Nevada of California. Precipitation will taper off as light
showers, becoming limited mainly to the Cascades overnight.
Additional snow accumulations in the mountains will be light.

A flat dirty upper ridge will weaken as it moves quickly across
Western Washington Sunday morning. The leading edge of a frontal
system will reach the area in the afternoon, and move across the
area and east of the Cascades Sunday night. It will produce breezy
to windy conditions across the forecast area, but winds will
probably remain below advisory level. This feature will move into
the forecast area from the northwest, and it is associated with a
cooler air mass. The snow level will fall to around 2500 ft as it
moves through, and another 7 to 14 inches of snow is likely in the
mountains. A winter storm watch is in effect, though that looks
rather marginal. In the lowlands, there will probably be 0.25 to
0.75 inches at the coast and generally less 0.25 inch in the
interior lowlands.

Some showers will follow the front Monday -- especially in the
mountains and probably a Puget Sound convergence zone -- in
northwest flow aloft ahead of a large upper ridge building well
offshore. Showers will decrease Monday night, with dry weather
expected Tuesday as the upper ridge moves into the Pacific
Northwest. McDonnal

.LONG TERM...Here is the long term section from the afternoon
forecast discussion -- Models are in good agreement showing yet
another system arriving in cold northwest flow. This system looks
weaker than the Sunday night storm, but it should still bring some
rain and accumulating mountain snow to the area Wednesday and
Wednesday night.

Global models all seem to agree on a more amplified flow pattern
toward the end of the week. A lull or break in precipitation appears
possibly Thursday and Friday. If the flow is light enough, fog could
be around and colder low temperatures, especially in the prone cold
pockets like Olympia. There is still some uncertainty on timing
systems toward the weekend. Decided not to undercut the MOS min
temps too much as any quicker timing could bring clouds in faster.

Long range models are hinting at retrogression of the long wave ridge
starting around next Saturday. If this shift were to occur, colder
systems could arrive from the north with even lower snow levels than
we have seen so far this year. Just something to watch at this
point. Mercer

&&

.AVIATION...An upper trough will move through the area tonight. Flow
aloft will be northwesterly on Sunday. At the surface, light flow
tonight will give way to stronger onshore flow as a cold front or
trough arrives late Sunday afternoon and evening. Scattered showers
will give out overnight and then showers or rain will develop late
Sunday afternoon with the cold front.

KSEA...The northerly breeze this evening will turn back to southerly
around daybreak on Saturday. A cold front will reach the metro area
late Sunday afternoon or in the evening.

&&

.MARINE...Weak high pressure briefly builds over the area tonight.
A front will arrive late Sunday afternoon or evening. Stronger
onshore flow will develop later Sunday with the front and a gale
watch is in effect for that.

In the wake of the front Sunday night, westerly swells of 15 to 20
feet are possible on the coast Monday night. Another front will
arrive around Wednesday. This one has been weaker in recent model
runs and does not show gales now.

&&

.HYDROLOGY...The Skokomish River remains above flood stage this
evening, but it crested late this afternoon and is falling slowly.
It will likely remain above flood stage until Monday morning. The
Chehalis River is below flood stage near Grand Mound and falling,
and the flood warning for that point will end this evening. The
Chehalis at Porter will rise slowly tonight, but remain just above
flood stage with only minor flooding. The flood warning will
remain in effect tonight as the flood crest works its way
downstream into Grays Harbor. The Chehalis surge plain may
experience minor flooding around the time of high tide at 11 PM
tonight.

Landslides remain a concern. USGS landslide indices show a continued
threat from the recent heavy rain. A minor landslide was reported
near Quilcene today. A special weather statement addresses this.

The systems Sunday night and Wednesday appear to be fast moving.
Though more rain will fall, it is not expected to produce flooding.
A cooler and drier period is forecast later in the week. After the
current flooding episode has ended, more flooding is unlikley over
the next seven days. Burke/McDonnal

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Winter Storm Watch from Sunday afternoon through Monday morning
     for Cascades of Pierce and Lewis Counties-Cascades of
     Snohomish and King Counties-Cascades of Whatcom and Skagit
     Counties-Olympics.

PZ...Gale Watch from Sunday afternoon through Monday afternoon for
     Central U.S. Waters Strait Of Juan De Fuca-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-East Entrance U.S. Waters Strait Of
     Juan De Fuca-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.weather.gov/seattle/gafd/latest_webafd.html