Small Craft Advisory 289, NWS Seattle, WA

Area Forecast Discussion
National Weather Service Seattle WA
905 PM PST Fri Nov 24 2017

.SYNOPSIS...A frontal system over Vancouver Island should bring a
little light rain to mainly the north part of Western Washington
tonight. After short lull with mostly dry weather Saturday
morning, another frontal system will arrive Saturday afternoon and
night, with wet and locally windy weather through Sunday. An
upper level ridge will cross the area Monday but will be too weak
to end the chance of showers. Another frontal system will move
through the area Monday night and Tuesday. An upper level ridge
could bring some dry weather and a little sunshine Wednesday and
possibly Thursday.

&&

.SHORT TERM...An east-west oriented frontal system will remain
quasi-stationary over Vancouver Island tonight. W WA is in the
warm sector and some light rain has been reaching the WA coast and
the north interior. The front will continue to bring a little
light rain to mainly the far N part of W WA through tonight.

Snow levels will be between 4000 and 4500 feet tonight, which could
bring a little snow to the northern passes. However, amounts will
be light.

A large longwave trough deepening offshore along roughly 145W
tonight will amplify the flow offshore allowing stronger SW flow
to develop downstream over W WA on Saturday. A shortwave trough
embedded in the SW flow will spread rain northward over W WA
Saturday afternoon through Saturday evening. At the same time an
associated deep surface low will move N offshore along 130W. This
low will generate windy conditions along the coast and possibly over
the Cascade foothills late Saturday and Saturday night. There is
a chance the winds could reach advisory levels along the coast and
possibly near the Cascades Foothills.

There is also potential for another period of heavy rain over the
mountains Saturday night. The warm and wet system embedded in the
SW flow will push snow levels up to 6000 to 7000 feet Saturday
night, before lowering back down to 3500 feet Sunday afternoon. A
number of the 12Z mesoscale models show up to 2 inches of rain,
but if the models are low, then potentially up to 3.5 inches in
12 hours over the Olympics, with a little bit less over the north
Cascades Saturday night, which could push river levels back up a
little. However, widespread flooding is not expected.

The pattern offshore becomes a little more complicated and tricky
Saturday night and Sunday morning. The models have a small cold
front moving inland Sunday morning. N-S pressure gradients could
become strong enough for at least windy conditions over the
interior lowlands. Advisory level S winds are also a consideration.
However, this is a small scale feature a few days out, so it
wouldn't take much for the models to change it.

The remnant of the large longwave upper level trough offshore
will move E across the area Sunday night into Monday morning for
showery weather. Kam/19

.LONG TERM...The GFS and ECMWF have regained some consensus for
Monday night through Wednesday. The latest solution is for a warm
front to roll through the area Monday night, followed closely by a
cold front Tuesday morning. The trailing upper level trough will
be right behind, crossing the area Tuesday afternoon and early
evening.

A moderate amplitude upper level ridge will follow with
potentially dry weather and some sunshine on Wednesday and
possibly Thursday. Bear in mind that this fog season, so morning
low clouds and fog could mess up any sunshine Wednesday and/or
Thursday mornings. Looks like a weak front may follow either
Thursday afternoon (ECMWF) or Thursday night (GFS). Kam

&&

.AVIATION...A frontal system will stall and dissipate over
southern British Columbia tonight. Showers across Western WA
will mainly affect the north coast and mountains. The flow aloft
is westerly. Ceilings are mainly VFR. A warm front will spread
rain north into W WA Saturday afternoon and evening. 33

KSEA...VFR conditions tonight with S wind 5-10kt. 33

&&

.MARINE...A frontal system will stall and dissipate over southern
British Columbia tonight. However, will still see moderate
southerly flow across Western WA with Small Craft Advisory winds
over most waters. A stronger system will track north over the
Offshore Waters Saturday night with gales possible - a Gale Watch
is in effect. Strong onshore flow will follow on Sunday. Another
strong frontal system will arrive Monday night or Tuesday with
seas building over 20 feet. 33

&&

.HYDROLOGY...Rivers have crested and are receding at this time.
The White River will remain above flood likely into Sunday as they
drain the pond - Mud Mountain Dam. The Skokomish river in Mason
county is at flood stage, it will drop a bit below then come back
up with the next rain Saturday night. And the Skagit at Mount
Vernon will be above flood stage most of the night.

For a list of current flood watches and warnings, please
check: www.wrh.noaa.gov/sew/get.php?pil=RVA&sid=SEW

More fronts will move through Western Washington this weekend and
early next week, but rainfall will not be as heavy and snow levels
will be mainly in the 3000 to 5000 foot range, except 6000 to 7000
feet Saturday night. 2 to 3.5 inches are expected over the
Olympics and most will be rain over the Skokomish Basin, so that
presents a threat of further flooding there.

With the heavy amounts of rain we have had and continued storms for
several more days, the landslide risk remains elevated and any of
the rainstorms have the potential to trigger one or two. JBB

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Flood Watch in effect for Mason County from Saturday
     afternoon through Sunday afternoon.

PZ...Gale Watch from Saturday evening through Sunday 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-East Entrance U.S. Waters Strait Of Juan De Fuca-Northern
     Inland Waters Including The San Juan Islands-West Entrance U.S.
     Waters Strait Of Juan De Fuca.

     Small Craft Advisory until 6 PM PST Saturday 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 3 PM PST Saturday for Admiralty Inlet-
     East Entrance U.S. Waters Strait Of Juan De Fuca-Northern
     Inland Waters Including The San Juan Islands.

     Small Craft Advisory until 4 AM PST Saturday for West Entrance
     U.S. Waters Strait Of Juan De Fuca.

     Small Craft Advisory until 10 PM PST this evening for Puget
     Sound and Hood Canal.

&&

www.weather.gov/seattle

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