Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 192126
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
226 PM PDT Sat Aug 19 2017

.SYNOPSIS...Light onshore flow caused by a weak system passing by
to the north will give patches of late night and morning clouds
through Sunday. High pressure offshore will shift eastward and
across the area Monday and Tuesday giving warmer temperatures and
dry conditions. A low pressure trough will bring a chance of
showers late Wednesday and Thursday, along with cooler high
temperatures. High pressure will build over the area Friday
through next weekend for a return to dry and warmer weather.

&&

.SHORT TERM...Skies have cleared across the region today and
temperatures are quite nice for this time of year, in the 60s to
lower 70s. Expect areas of clouds to form along the coast and into
portions of the interior late tonight through Sunday morning.
After a cool morning, light northwesterly onshore flow will give
another nice day on Sunday with highs into the 70s across much of
the interior.

Ridging amplifies offshore Sunday night then progresses eastward
across the region Monday and into Western Montana by Tuesday
afternoon. This ridge will give dry conditions to the area for
Monday and Tuesday with marine clouds confined to the immediate
Pacific coastline. It will warm to around 80 from Seattle
southward on Monday. The eclipse viewing should be fine in the
interior of Western Washington with only the immediate coast
having a threat of obscuring clouds. With the solar eclipse
occurring during the mid to late morning period at a maximum of
over 90 percent across most of the forecast area, high
temperatures could be held down a degree or two from where they
would normally reach given the air mass.

Tuesday will see a shift to onshore flow as the upper ridge
progresses well to the east. High temperatures on Tuesday will
cool over the coastal areas as onshore flow kicks in during the
afternoon. The warmer interior locations, like the Central Puget
Sound area, will see another day of lower 80s with onshore flow
not kicking in until later in the afternoon.

With only minor adjustments toward the new model consensus, the
short term will remain quite similar to the previous package.
Albrecht

.LONG TERM...Onshore flow later Tuesday will usher in a marine
push Tuesday afternoon and evening that will result in increased
clouds. The push will be followed by a frontal system that will
move across the area Wednesday night and a rather deep upper level
trough Thursday through Thursday night. Long term models appear to
be converging on a solution (between the GFS and ECMWF) where
there will be a threat of showers Wednesday afternoon through
Thursday evening. Thursday high temperatures will likely not reach
the 70 degree mark across most of the area with the clouds, some
showers, and the cooler air mass. Snow levels are expected to fall
to 6000 to 7000 feet in the North Cascades Thursday or Thursday
night, and may impact campers and backpackers at higher
elevations.

As high pressure builds into southeastern British Columbia Friday
through the weekend, offshore flow will give a good warming and
drying trend. Albrecht

&&

.AVIATION...Northwest flow aloft over Western Washington this
afternoon will continue tonight and Sunday, as an upper ridge
remains over the offshore waters. At the surface, onshore flow
will continue with high pressure offshore and lower pressure east
of the Cascades. The air mass will be dry and stable, except for
moisture in the marine layer mainly near the coast.

Moisture in the lower part of the atmosphere is giving SCT030-040
over much of Western Washington at 2 pm. These clouds should
dissipate around sunset. Coastal stratus will move inland tonight
through the Strait of Juan de Fuca into parts of the northwest
interior and through the Chehalis Gap into the south Puget Sound
region and southwest interior; however, ceilings are unlikely over
the greater Seattle area.

KSEA...Northwest wind 6-12 kt becoming northeast 4-10 kt after
sunset, then backing to northwest again Sunday. There could be
scattered low stratus at KSEA for a few hours Sunday morning
roughly 13Z-16Z, but a ceiling looks unlikely. McDonnal

&&

.MARINE...A typical August pattern will prevail through the
middle of next week. Expect onshore flow of varying strength to
persist due to high pressure offshore and lower pressure east of
the Cascades. A weak system will approach the coastal waters on
Wednesday and move inland Thursday. Small craft advisory west
winds are likely in the central and eastern Strait of Juan de
Fuca in the evening and early morning hours each night. McDonnal

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Small Craft Advisory from 6 PM this evening to 5 AM PDT Sunday
     for Central U.S. Waters Strait Of Juan De Fuca-East
     Entrance U.S. Waters Strait Of Juan De Fuca.

&&

$$

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You can see an illustrated version of this discussion at
www.wrh.noaa.gov/sew/gafd/latest_webafd.html


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