Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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967
FXUS66 KSEW 200522
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
923 PM 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...Some patches of showers still present on radar this
evening with activity trending downward. Echoes indicate mostly
light showers...although some isolated patches of moderate rainfall
appear to be present along the coast and slightly inland on the
Olympic Peninsula.

As an upper level trough remains over the coastal waters...will not
be too much variation in the forecast for the weekend as a pair of
upper level lows will impact the area as they rotate around the
center of the trough. The first looks to impact the area by Friday
afternoon bringing...to little surprise...showers with occasional
breezy conditions. Precip coverage with this system looks to be more
scattered as the Low itself is progged to stay well offshore and
thus...limiting the extent of its reach and influence. Saturday sees
the Low weaken into a shortwave with a front passing through the
area late in the afternoon and in the early evening with conditions
drying quickly afterward for the overnight period and into very
Sunday morning. By mid morning Sunday...low number two brings the
next slug of precip with a frontal band that looks well organized
for the late morning and into the afternoon. Activity looks to
scatter out by Sunday evening as this low...like its
predecessor...will remain out over the waters keeping some distance
between it and the coast.

Gradients with each of the aforementioned systems appear to bring
breezy east winds to the area both Friday and Sunday...although the
gradients Sunday look more impressive. Inherited forecast keeps wind
speeds Sunday just below advisory criteria...but with values being
that close...advisory level winds are certainly a possibility future
shifts will have to consider.

Inherited forecast covers the above matters well. No evening updates
needed.  SMR

.LONG TERM...From previous discussion...Monday through much of next
week, models are in agreement that an upper ridge will form over the
area for a change to a drier weather pattern. Temperatures are also
likely to cool by a few degrees. JSmith

&&

.AVIATION...SW flow aloft will persist over W WA the rest of
tonight. Showers are expected to gradually diminish with decent VFR
conditions developing overnight. CIGS BKN040-060 are already present
over central Puget Sound and should rise to around BKN080-100 12Z-
18Z. Spotty CIGS BKN020-030 were still present over the N interior
but should lift to match the VFR conditions elsewhere.

An occluded front approaching from the SW will spread light rain and
CIGS OVC030-050 over the SW interior by 18Z, pushing N across Puget
Sound and the N interior 18Z-03Z.

KSEA...Showers seem mostly over for KSEA so CIGS have been rising.
CIGS should be SCT-BKN040-050 or better, eventually rising to BKN080-
100 by 12Z. Rain from the approaching occluded front should reach
the terminal after 18Z. SE surface winds 5-10 KT will become more
easterly after 10Z. Kam

&&

.MARINE...A large surface low will form offshore near 45N/130W-140W
tonight. An associated occluded front was about 300 NM SW of Grays
Harbor at 04Z/8 PM, heading NE toward the WA coast. SE winds will
rise tonight ahead of the front, reaching gale force over the
coastal waters on Friday as the front move NE across the area. SCA E
winds are expected in the central and E strait. Periods of SCA E-SE
winds can be expected over the rest of the inland waters.

After the occluded front moves N over B.C., a surface trough
wheeling around the offshore low will move NE across the coastal
waters Friday night. Another front will follow along the same path
on Saturday. Both features should generate SCA winds over much of
the area but the Saturday front looks like the stronger of the two.

On Saturday night a deep 960 MB low will form about 350 NM offshore.
The associated occluded front will once again follow the same path,
moving NE across the area late Saturday night and Sunday morning.
This front will generate gales over the coastal waters with gales
likely over the north interior waters.

The deep surface low is expected to depart to the S on Monday,
allowing a surface ridge to build over the area.

A large westerly swell approaching 18-20 FT is forecast to reach the
coast on Saturday. Kam

&&

.HYDROLOGY...A flood warning remains out for the Skokomish River in
Mason county. Showers tonight and into the overnight hours will slow
the fall of the water level on the Skokomish River. No additional
river flooding is expected for the remainder of the forecast period.
An extended period of dry weather appears likely to develop early
next week. JSmith/SMR

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Small craft advisory coast tonight.
     Gale warning coast on Friday.
     Small craft advisory all other waters.
     Small craft advisory for rough Grays Harbor Bar.

&&

$$

www.weather.gov/seattle

You can see an illustrated version of this discussion at
www.weather.gov/seattle/gafd/latest_webafd.html



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