Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 160512
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
912 PM PST Mon Jan 15 2018

.SYNOPSIS...A weakening cold front will bring rain across Western
Washington tonight, followed by some trailing showers on Tuesday.
A wetter and windier frontal system will move through Western
Washington Wednesday and Wednesday night, followed by cooler
showery weather Thursday and Friday. Another frontal system should
keep the area cool and wet this weekend.

&&

.SHORT TERM...Strong southerly flow aloft is pushing a negatively
tilted cold front into Western Washington this evening. Rain
spread onto the coast a few hours ago and is now spreading into
the interior. South Sound locations such as TCM, TIW and OLM have
already picked up a couple hundredths of an inch of rain. Light
rain will continue to spread N-NE for the rest of tonight. Behind
the front, the air mass will cool quite a bit. After tying the
all-time January record high temp at SEA today, 850 mb temps will
fall quickly overnight to near 0C by sunrise on Tuesday. Behind
the front, showers will continue as a lagging upper trough crosses
the area later Tuesday.

The trough will exit on Tuesday evening. A flat shortwave ridge
axis will cross late Tuesday night, bringing a brief period of
mostly dry weather late Tuesday night.

A deep closed 500 mb low center will develop around 140W near our
latitude on Wednesday, with a vertically stacked surface
reflection in the form a large and expansive 965 mb surface low.
Around the E-SE periphery of the low, strong southwest flow aloft
will develop over Western Washington, though such low pressure
well offshore will lead to moderate to strong E-SE pressure
gradients at the surface. At first, the southwest flow aloft will
push a warm front across the area Wed morning, with rain most
likely over the Olympic Peninsula.

By Wednesday evening, the whole pattern will shift slowly east,
pushing a cold front across Western Washington. The frontal band
will be accompanied by heavy precip, but the progressive nature of
the front will limit the duration of heavier rain to just a few
hours. Only the flood-prone Skokomish River will bear watching in
this pattern.

The jet stream will shift SE of the area on Thursday, putting us
on the colder, more showery and unstable poleward side of the jet.
Instability brought by cold 500 mb temps of -32C to -30C on
Thursday and southwesterly cyclonic flow aloft will lead to
plenty of convective showers. Owing to lingering low pressure west
of northern Vancouver Island, southeast gradients will continue to
bring breezy weather to the forecast area.

Along the coast, will be keeping an eye on Thursday for coastal
impacts from a W-SW swell that is currently forecast to reach
28-29 feet over deep water beyond the surf zone. The swell is
forecast to be generated out of the expansive wind field around
the deep surface low well offshore. With a 20-second period, there
could be a lot of energy in the swell. If the forecast holds, this
would support beach erosion and long wave run-ups on the beaches.
Swell of this magnitude can support coastal flooding.  Haner

.LONG TERM...From Previous Discussion: Friday will be similarly
cool and showery as Thursday, thanks to the continuation of an
unstable air mass from cold air aloft.

The remnants of the offshore low and a trailing occluded front should
move onshore over southern B.C. late Friday and early Saturday,
with showers and breezy weather continuing.

Without delay, another compact and energetic upper low will
develop offshore on Saturday. Models agree in pushing another
progressive, wet cold front across the area next weekend, followed
by cold-air instability and showers for next Sunday and Monday.

Hope you enjoyed today. There are no more dry 24-hour periods in
the forecast for the next week. Haner

&&

.AVIATION...Moderate southerly flow aloft will continue tonight as a
front moves inland. Flow aloft will turn westerly Tuesday. At the
surface, moderate southeasterly winds will develop. The air mass is
moist and stable. Light rain has already developed under VFR
ceilings. As rain continues through the night, ceilings are likely
to fall to MVFR 2-3k ft.

KSEA...Discussion above applies. Southeast wind 8-12 kt tonight will
become more southerly Tuesday. CHB

&&

.MARINE...A front will move inland tonight. Southeast winds will
increase to gale force on the coast and at the east entrance. Small
craft advisory strength winds are likely over the other waters.
Advisory level winds will probably persist through Tuesday.

A stronger system on Wednesday will probably also bring gales, and
a gale watch is now in effect for Wednesday and Wednesday night.
Westerly swells of 20 to 25 ft are possible Thursday. CHB

&&

.HYDROLOGY...Minor flooding is possible but unlikely on the
Skokomish River on Wednesday night and Thursday. Elsewhere in
Western Washington, river flooding is not expected for the next 7
days.

A brief period of heavy rain is possible in the Skokomish Basin on
Wednesday evening. This will cause the river to rise on Wednesday
night and Thursday morning. Current rainfall forecasts support
bringing the Skokomish to within a foot of flood stage. This will
bear watching, as forecast rainfall amounts are prone to change.
Haner

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Gale Watch from late Tuesday night through late Wednesday night
     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.

     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.

     Small Craft Advisory for rough bar until 4 AM PST Wednesday for
     Grays Harbor Bar.

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

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

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

&&

$$

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