Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 222217
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
315 PM PDT Tue Aug 22 2017

.SYNOPSIS...Dry and relatively warm weather will continue
tonight. A weak storm system approaching the region on Wednesday
will bring cooler conditions and a chance of showers. Cool weather
with spotty light showers will linger over Western Washington
Wednesday night and Thursday. Dry and warmer weather will return
Friday through Monday with above normal temperatures expected
especially through the interior.

&&

.SHORT TERM...Low clouds have burned back to the coastal
waters and west entrance to the Strait of Juan de Fuca this
afternoon leaving clear skies. High temperatures have warmed into
the mid 80s over the interior with a few spots over the south
interior reaching into the upper 80s. The upper level ridge that has
been over the Pacific Northwest will continue to shift east into the
northern Intermountain area this evening ahead of an upper level
trough digging South along the British Columbia coast. Increasing
low level flow this afternoon and evening will result in a solid
marine push this evening, with stratus spilling through most of the
interior lowlands by Wednesday morning. The marine stratus will lift
through midday while mid and high level clouds begin to work into
the area ahead of the approaching weak system. Showers are possible
along the north coast by late morning then through the Olympic
Peninsula through the afternoon and the remainder of the interior
through the evening. Expect rainfall to be light and spotty with the
best bet remaining the coast, north interior and mountains. Light
showers may linger into Thursday morning, especially over the
Cascades and in a Puget Sound Convergence Zone, during the day
Thursday. Expect some clearing with sun breaks Thursday afternoon
with high temperatures likely rising into the low 70s across the
interior and mid to upper 60s along the coast.

.LONG TERM...The upper level trough will slowly lift into
Central Canada Friday and Saturday leaving the Pacific Northwest
under a dry, westerly pattern with mostly sunny skies. Expect high
temperatures to gradually Warm Saturday and Sunday into the low to
mid 80s over the interior and 70s along the coast. Upper level
heights will rise Monday as the upper level ridge builds along the
west coast into British Columbia. This will boost high temperatures
into the upper 80s with a few of the warmer locations in the
southwest interior close to 90. The upper level ridge will shift
east Tuesday with warm and dry conditions continuing.

&&

.AVIATION...Southwest flow aloft over Western Washington this
afternoon will continue tonight and Wednesday as an upper level
trough approaches the Pacific Northwest from the offshore waters.
At the surface, high pressure remains offshore with lower pressure
east of the Cascades. Onshore flow will increase late today and
tonight, with the marine layer deepening and the air mass becoming
moist below 5000 ft.

There is no significant cloud cover over Western Washington below
12,000 ft, except where marine stratus covering much of the
coastal waters is intruding into immediate coastal areas. Stratus
will push inland tonight, and conditions will likely deteriorate
to widespread low-end MVFR and local IFR 09Z-12. Improvement to
widespread VFR conditions is likely across the Western Washington
interior midday to mid-afternoon.

KSEA...West to northwest wind 6-12 kt, backing to southerly 4-10
kt after sunset. There is high confidence that stratus will move
into KSEA late tonight, probably 10Z-12Z, with ceilings
OVC009-015. Stratus should scatter out to VFR conditions around
21Z. McDonnal

&&

.MARINE...A weak upper trough will approach British Columbia and
the Pacific Northwest tonight and Wednesday, then continue inland
Thursday, resulting in a period of strong onshore flow across
Western Washington. Gale westerlies are likely both nights in the
central and eastern Strait of Juan de Fuca, with small craft
advisory winds in the adjacent Admiralty Inlet and Northern Inland
Waters. Weaker onshore flow will continue Friday through this
weekend, as high pressure remains off the coast with lower
pressure east of the Cascades. McDonnal

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Gale Watch from Wednesday afternoon through late Wednesday night
     for Central U.S. Waters Strait Of Juan De Fuca-East
     Entrance U.S. Waters Strait Of Juan De Fuca.

     Gale Warning until 6 AM PDT Wednesday for Central U.S. Waters
     Strait Of Juan De Fuca-East Entrance U.S. Waters Strait Of
     Juan De Fuca.

     Small Craft Advisory from 6 PM this evening to 6 AM PDT
     Wednesday for Admiralty Inlet-Northern Inland Waters
     Including The San Juan Islands.

&&

$$

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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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