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Forecast Discussion for Seattle/Tacoma, WA

000
FXUS66 KSEW 170537
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
937 PM PST Tue Jan 16 2018

.SYNOPSIS...A strong frontal system will bring rain Wednesday and
Wednesday night. A broad upper trough will traverse the area
Thursday and Friday, keeping a cool, showery pattern in place.
Large waves are likely to impact the coast on Thursday. Unsettled
weather will continue into this weekend, with a front enhancing
rainfall and mountain snow around Saturday night or Sunday. An
upper ridge may build around next Tuesday for a drying trend.

&&

.SHORT TERM...Currently in the midst of a brief dry lull. This
will not last.

A large, expansive low pressure system is currently in the process
of fully maturing way out between 140W and 145W. This feature is
reflected both at the surface and aloft. At the surface, a 960-965
mb low center will move to near 49N 137W late Wednesday. At the
500 mb level, a 510 decameter low center will be superimposed over
the surface low by late Wednesday, meaning it will become
vertically stacked and barotropic at that time. In advance of this
broad deep upper low and trough, strong southwest flow aloft will
develop over Western Washington Wednesday. At first, this will
push a warm front across the area early in the day. Southeast
gradients will become strong. Have expanded the Wind Advisory to
cover the standard 4 north interior zones and the two coastal
zones. The current expiration time of 6 pm Wednesday may
eventually need to be extended into Wednesday evening. The
Bellingham to Port Hardy pressure gradient is forecast to reach
near +14 mb Wednesday evening, which is a good indicator for the
classic southeast-sucker wind pattern.

As the entire pattern shifts a little further east, a cold front
will be able to move onshore on Wed evening. It will come with a
quick burst of heavier rain lasting a couple hours.

The front will be followed by a lot of cold air aloft, with 500 mb
temps around -33C on Thursday, then -30C on Friday. With the
ensuing instability and cyclonic southwest flow aloft, expect
plenty of convective showers. On Thursday, stronger instability,
large-scale ascent, and a more strongly sheared wind environment
could support a few thunderstorms. By Friday, the upper trough
axis will be quickly weakening and de-amplifying as it moves
across Western Washington, resulting in a net modest rise of upper
heights.

The associated upper trough and the strong surface wind field
will cover a large expanse of the northeast Pacific on Wednesday,
generating swell that will bring coastal impacts on Thursday. A
Coastal Flood Watch is in effect. See the Marine section below for
more insight.  Haner

.LONG TERM...A more progressive and zonal pattern will set up from
Saturday through Monday. A quick-moving front will sweep through
late Saturday and early Sunday with an enhancement to precip. In
all likelihood, snow levels will not rise above 3000 feet during
this period, this could mean moderate to heavy snow for the
mountains on Saturday and/or Sunday.

By Monday, upper heights start to rise, and precip chances start
to diminish. Next Tuesday appears to feature a daylong respite
between weather systems. Haner

&&

.AVIATION...Moderate westerly flow aloft tonight will become more
southwesterly Wednesday as a warm front moves from south to north
through the area. At the surface, southeasterly winds will increase
tonight and Wednesday. The air mass is moist and stable. Dry weather
is expected overnight in advance of a frontal system Wednesday.

Conditions are good VFR. Patchy fog is possible tonight. High
ceilings will develop tonight and will fall Wednesday, though VFR
conditions will probably remain in place during the day in most
places. The coast will probably become MVFR in the afternoon as rain
develops.

KSEA...Discussion above applies. Southeast wind 10-15 kts tonight
becoming more southerly Wednesday. CHB

&&

.MARINE...Small craft advisories are in effect over most waters
tonight as a warm front approaches the coast as it moves south to
north. By Wednesday most of these convert to gale warnings as the
front nears and gradients tighten. The offshore coastal waters will
have the strongest winds; they may approach storm force. After a
brief lull Wednesday night, a cold front will move through the area
that may generate gales again on Thursday.

Strong southwest winds well off the coast will generate large swells
that will reach the Washington coast Thursday. Models have been
consistent in bringing westerly swells of 25-30 ft to the coast,
with even larger swells offshore. Seas have not reached 30 ft
at Buoy 41 since December 2015. A coastal flood watch is in effect
for the central coast and will probably expand it to the north
coast later tonight. This sort of coastal flooding mainly affects
the beaches as waves run far up beyond where they usually do.
Exposed towns along the beaches, such as Ocean Shores and Westport,
could have waves crash over their levees, and sea water could
collect in low areas. CHB

&&

.HYDROLOGY...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, but it is still not expected to flood.

The next round of heavier rain will come around Saturday night or
Sunday. This rainfall should cause most rivers to rise, but only
the Skokomish appears to be at risk of flooding at this time. An
inhibiting factor will be that snow levels could remain at or
below 3000 feet, which would reduce runoff down the river.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Wind Advisory from 10 AM to 6 PM PST Wednesday for Admiralty
     Inlet Area-Central Coast-North Coast-San Juan County-
     Western Skagit County-Western Whatcom County.

     Coastal Flood Watch from late Wednesday night through Thursday
     evening for Central Coast.

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

     Small Craft Advisory until 3 AM PST Wednesday 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.

     Gale Warning from 3 AM Wednesday to 4 AM PST Thursday 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.

     Small Craft Advisory until 3 AM PST Wednesday for East Entrance
     U.S. Waters Strait Of Juan De Fuca-Northern Inland Waters
     Including The San Juan Islands.

     Small Craft Advisory until 11 PM PST Wednesday for Central U.S.
     Waters Strait Of Juan De Fuca-West Entrance U.S. Waters
     Strait Of Juan De Fuca.

&&

$$

www.weather.gov/seattle

NWS Seattle/Tacoma, WA (SEW) Office Area Forecast Discussion

Discussion Script by Ken True at Saratoga Weather - WFO Script by SE Lincoln Weather