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Area Forecast Discussion (AFD)

Issued by Seattle/Tacoma, WA (SEW)

FXUS66 KSEW 251040

Area Forecast Discussion
National Weather Service Seattle WA
340 AM PDT Fri May 25 2018

.SYNOPSIS...A low pressure system offshore will move over the area
today before exiting Saturday morning. Weak high pressure aloft 
will prevail this weekend for near or slightly above normal 
temperatures. A low pressure system will approach the region on 
Memorial Day; however, it should be dry for outdoor activities.


A stratocumulus deck blanketed much of the interior at this time.
Overall, not anticipating clouds to be as extensive as yesterday
morning. Highs today will be about the same as yesterday.

Meanwhile, an upper level trof offshore will move over the CWA
this afternoon and will be east of the area Saturday morning. The
main impact this system will have on our weather will be to
tighten the low level onshore pressure gradient and deepen the
marine layer. Therefore, expect patchy drizzle late tonight thru
Saturday morning.

A flat upper level ridge or near zonal flow aloft will prevail
this weekend. The low level onshore pressure gradient will be
weaker on Sunday; therefore, temperatures on Sunday will be 
several degrees warmer than on Saturday.

Memorial Day is still expected to be dry, and temperatures should
be about the same or slightly lower than on Sunday. So, it still
looks like a nice day for outdoor activities. 

The GFS and Canadian models were now close to the ECMWF solution 
in regards to the timing of the midweek system. Thus, the ECMWF 
was the model of preference for this period (Memorial Day thru 
Thursday). Expect onshore flow to strengthen on Memorial Day in 
response to the approaching upper level trof. This trof will move 
the area on Tuesday for cooler and cloudier conditions. The models
were drier with this system; therefore, decided to confine the 
mention of showers to the Puget Sound Convergence Zone and the 
north Cascades. 

The Pacific Northwest will remain under the influence of low 
pressure aloft Wednesday through Thursday for below normal temps. 
There will be a chance of showers over mainly the mountains and 
within the Puget Sound Convergence zone.


.AVIATION...An upper level ridge to the east of the area combined
with troughing offshore will give southwest flow aloft through 
today. The trough will move east across the area tonight deepening 
the marine layer over the area Saturday morning. At the surface, 
high pressure offshore combined with lower pressure east of the 
Cascades will give low level onshore flow today. The onshore flow 
will increase tonight as the upper trough moves across the area.

Wide variety of ceilings over the area this morning. For the central 
Puget Sound ceilings near 3000 feet. From about Whidbey Island 
northward ceilings are around 7000 feet. Along the coast ceilings 
are near 2000 feet. Little change in the ceilings this morning with 
the cloud deck becoming scattered around midday across the entire 
area. MVFR ceilings will develop again along the coast tonight with 
the stratus spreading inland after midnight. 

KSEA...Ceilings around 3000 feet scattering out midday. Ceilings in 
the 1000-2000 foot range with possible visibilities 3-5sm in light 
drizzle Saturday morning. Variable wind 5 knots or less becoming NW 
5-10 knots in the afternoon. Felton


.MARINE...A ridge of high pressure sitting along 130W with a
trough over Eastern Washington will give varying degrees of
onshore flow for the next several days.

The onshore flow was strong Thursday night and will relax 
somewhat early this morning. Onshore flow will again become 
strong this afternoon and tonight as a weak upper level 
disturbance moves eastward across the waters. A small craft 
advisory is in effect for the coastal waters from Point Grenville 
to Cape Shoalwater out 10 nm through this evening. Small craft 
advisories have also been issued for later this morning through 
this evening for the western portions of the Strait of Juan de 
Fuca. A gale warning has been issued for mid afternoon today 
through tonight for the eastern two thirds of the Strait of Juan 
de Fuca, and small craft advisories have been issued for Admiralty
Inlet and the Northern Inland Waters for that period as well with
the strongest winds near the east entrance to the strait.

Near gale to possibly gale force winds are possible each overnight
period for the eastern two thirds of the strait through the
Memorial Day weekend and into next week as the onshore flow
pattern continues. Albrecht


PZ...Small Craft Advisory until midnight PDT tonight for Coastal 
     Waters From Point Grenville To Cape Shoalwater Out 10 Nm.

     Small Craft Advisory until 3 PM PDT this afternoon for Central 
     U.S. Waters Strait Of Juan De Fuca-East Entrance U.S. 
     Waters Strait Of Juan De Fuca.

     Gale Warning from 3 PM this afternoon to 3 AM PDT Saturday for 
     Central U.S. Waters Strait Of Juan De Fuca-East Entrance 
     U.S. Waters Strait Of Juan De Fuca.

     Small Craft Advisory from 2 PM this afternoon to 3 AM PDT 
     Saturday for Admiralty Inlet-Northern Inland Waters 
     Including The San Juan Islands.

     Small Craft Advisory from 11 AM this morning to 8 PM PDT this 
     evening for West Entrance U.S. Waters Strait Of Juan De