Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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636
FXUS66 KSEW 170001
AFDSEW

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

.SYNOPSIS...A strong frontal system will bring rain Wednesday and
Wednesday night with showers late in the week. It will also
generate large waves which may impact the coast on Thursday.
Unsettled weather will continue into this weekend, with a front
enhancing rainfall around Saturday night or Sunday. An upper ridge
may build around next Tuesday for a drying trend.


&&

.SHORT TERM...Ongoing showers will diminish quickly this evening.
An ever so brief period of shortwave ridging tonight will bring a
lull in precip late this evening and overnight, especially over
the interior.

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 issued a Wind Advisory for
three of the interior zones up north, and this may need to
eventually be expanded to the coast and a little further south.
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 of -30C to -35C on Thursday and Friday. With the ensuing
instability and cyclonic southwest flow aloft, expect plenty of
convective showers. Could even be some thunderstorms, especially
Thursday. A veering wind profile with vertical speed shear could
even lend itself to some convective organization. By Friday, the
upper trough axis will be quickly weakening and de-amplifying as
it moves across Western Washington.

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. The
18z ENPWave model shows swell of 28 feet over deep water just
beyond the surf zone mid-day Thursday. This would be the biggest
swell to reach the coast in a few years. Such large, energetic
waves can push water onshore and into harbors, causing coastal
flooding issues around the time of high tide. Will therefore soon
be issuing a Coastal Flood Watch for coastal Grays Harbor County.
Besides coastal flooding, the high surf along the beaches will
cause long wave run-ups that will be dangerous for beachgoers.
Keep your distance from the water! Haner

.LONG TERM...A more progressive and zonal pattern will set up from
Saturday through Monday. A quick-moving front will sweep through
on Saturday night and early Sunday with an enhancement to
precipitation.

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 will continue tonight
becoming more southwesterly Wednesday. At the surface, moderate
southeasterly winds will ease tonight before increasing again by
Wednesday afternoon. The air mass remains moist and slightly
unstable. Any precip this evening will be showery in nature...with
dry conditions expected overnight in advance of a frontal system
expected for Wednesday morning. Precip Wednesday afternoon and
evening will become more stratiform.

Cigs generally VFR with isolated MVFR in showers. VFR conditions
expected to remain in place overnight...although cloud bases are
expected to dip into low-end VFR for most locations early Wednesday
morning. OLM...HQM and CLM may find cigs dipping down into MVFR
conditions during this time frame.

KSEA...Discussion above applies. Southerly wind 10-15 kts this
afternoon shifting to more southeasterly at 6-11 kts. SMR

&&

.MARINE...Winds have subsided over most waters with the exception of
the coastal ones. Will cancel most of the standing SCAs with
afternoon forecast package. Next round will start to fire up late
tonight with SCAs going up for the Northern Inland Waters and East
Strait before the gales are expected to kick in there early Wed
morning. Remainder of strait looks to join in the fun with similar
timing. No gales expected there at this time although current models
suggest the possibility of some bleed over into the central strait.
Will leave that for next shift to evaluate. More fronts later in the
week will keep the weather vigorous...so additional headlines are
likely.

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 ft or more to the
coast, with even larger swells offshore. These have the potential to
be the highest seas in at least several years. Will issue a Coastal
Flood Watch for the Central Coast zone with the afternoon package to
provide plenty of lead time to resources in Ocean Shores. Similar
swells are expected for the North Coast...so expansion of this watch
is entirely possible.  SMR

&&

.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. This will bear watching, as forecast rainfall amounts are
prone to change.

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.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Coastal Flood Watch from late Wednesday night through Thursday
     evening for Central Coast.

     Wind Advisory from 10 AM to 5 PM PST Wednesday for San Juan
     County-Western Skagit County-Western Whatcom County.

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

     Small Craft Advisory from 10 PM this evening to 4 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 from 11 PM this evening to 11 PM PST
     Wednesday for Central U.S. Waters Strait Of Juan De Fuca-
     West Entrance U.S. Waters Strait Of Juan De Fuca.

&&

$$

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