Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 191115

Area Forecast Discussion
National Weather Service Seattle WA
310 AM PST Thu Jan 19 2017

.SYNOPSIS...A broad upper level trough will remain over the
offshore waters through the weekend. A series of systems rotating
around it will bring showers and breezy conditions at times to
western Washington along with seasonable temperatures. By early next
week, a ridge of high pressure will build into the Pacific
Northwest for the possibility of several dry days.


.SHORT TERM...The steady rainfall of yesterday evening has
turned over to showers across western Washington early this morning
as a slightly unstable post-frontal air mass moves into the region.
Showers will continue today before decreasing tonight as another
upper level trough digs into the offshore waters. Another impulse
rotating around the trough will push into the area on Friday for an
increase in shower activity, but the QPF with it will be very light.
A surface low pushing into the coastal waters will serve to tighten
easterly gradients for some breezy conditions in the east Puget
Sounds lowlands and Cascade foothills Friday morning. Additional
disturbances rotating around the trough will continue to keep
periods of shower activity in the forecast into the weekend.

.LONG TERM...A slightly more organized frontal system rotating
around the offshore trough looks to arrive on Sunday. However, the
jet stream remains well to our south over northern California and
the system will likely be losing steam as it arrives. The models
bring the trough onshore over Oregon on Monday as upper level
ridging begins to build offshore. This looks to set the stage for
some drier weather by late Monday into Tuesday and persisting
through much of the remainder of next week. 27


.AVIATION...An upper level shortwave trough just offshore at
11Z will move across W WA through 00Z maintaining moist and somewhat
unstable SW flow aloft and areas of showers. Conditions should be a
mix of VFR CIGS BKN-OVC030-060 with areas of MVFR CIGS around and
after shower activity. The shortwave trough will weaken by this
evening and the air mass will become more stable, allowing showers
to diminish and VFR CIGS to predominate. An occluded front will
approach the coast tonight then move NE across the area on Friday.

KSEA...CIGS should be predominantly VFR BKN-OVC040-060, but will
probably drop to BKN020-030 as showers move across the terminal.
Surface winds will remain southerly 8-14 KT. Kam


.MARINE...A surface trough over the coastal waters this morning will
move inland across W WA today. This will maintain moderate S-SE flow
with SCA winds expected over the north interior waters. The UWWRF
model was a little weaker on the coastal winds today, keeping them
below SCA levels. However, seas will remain above SCA levels at 12
to 15 feet through tonight and winds will rise to SCA levels late
tonight so I opted to cram both seas and winds into one SCA
headline. The SCA winds over the north interior will be spotty and
are expected to begin easing during the afternoon.

An occluded front will approach the area tonight, then move NE
across the coastal waters on Friday. E-SE flow will strengthen with
a gale probable over the outer coastal waters on Friday. At this
point the models are keeping winds over the near shore waters below
Gale levels although a gale is not out of the question. A Gale Watch
has been issued for the outer waters, and for now, SCA winds have
been advertised for the near shore waters. Easterly winds will
develop over the inland waters with SCA E winds possible in the
central strait.

After the Friday front, another weaker occluded front will probably
move NE across the area on Saturday. A third occluded front should
follow on Sunday. At this point, the models agree that the parent
low for the Sunday front will deepen offshore to around 960 MB near
45N/131W. That central pressure is really low and seems a little
overdone. Fortunately, the models take the low SE toward SW Oregon
while filling it quickly Monday afternoon, keeping it far enough
away from W WA for little impact. The models are probably not done
evolving with this system.

A large W swell approaching 20 FT may still reach the coast on
Saturday. Kam


.HYDROLOGY...A flood warning remains out for the Skokomish River in
Mason county. With precip rates continuing to diminish...the
threat of additional flooding is similarly decreasing. The threat of
river flooding across the area will become low for the remainder of
the forecast period. An extended period of dry weather appears
likely to develop early next week.


PZ...Gale watch coast 10 to 60 NM offshore on Friday.
     Small Craft Advisory coastal waters through tonight.
     Small Craft Advisory East Entrance, Admiralty Inlet, and
      North Inland waters through this afternoon.
     Small Craft Advisory West Entrance for Hazardous seas
      through tonight.
     Small Craft Advisory for Rough Grays Harbor Bar thru Friday.



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