Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000 FXUS66 KSEW 211033 AFDSEW Area Forecast Discussion National Weather Service Seattle WA 333 AM PDT Thu Jun 21 2018 .SYNOPSIS...
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Moist marine air pushed inland overnight, but the clouds should give way to a partly to mostly sunny but cooler afternoon across Western Washington. Onshore flow will continue Friday, and an upper trough will bring a chance of showers. An upper ridge will cross the area this weekend for sunny warmer weather, followed by another upper trough early next week.
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&& .SHORT TERM...
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Strong onshore flow developed Wednesday evening, and cool moist marine air has been pushing inland overnight. By 2 am, much of Western Washington was covered by marine stratus with overcast ceilings generally between 800 and 1800 ft; the only area remaining uncovered by the clouds was the interior from around Everett northward, but it should be a matter of time there. One of the more dramatic signs of the marine push is the westerly gale at Race Rocks (on the Canadian side of the central Strait of Juan de Fuca) where the wind was sustained 49 mph gusting to 59 mph at 2 am. Early morning temperatures were in the 50s to lower 60s. A small, weak upper trough over the offshore waters will weaken further and be shunted northward today, as a minor upper ridge builds over the Pacific Northwest. At low levels moderately strong onshore flow will continue, and the air mass will remain fairly moist in the lowest 5000 ft or so. The day will start cloudy in most places, but the strength of the summer solstice sun should bring some clearing in the afternoon. For most of the interior lowlands, the forecast will be cloudy in the morning then becoming partly sunny. It will be considerably cooler with highs in the 60s at the coast and in the upper 60s to 70s inland. There could be patchy drizzle at the coast and not much afternoon clearing, and there could be some showers in the Olympics and North Cascades. With moderately strong onshore flow continuing tonight, marine stratus should again start covering Western Washington during the night. Another and somewhat sharper upper trough will move into the forecast area Friday and east of the Cascades late in the day. The result of that should be less clearing, as the moist layer deepens somewhat during the day. There will probably be some morning drizzle at the coast and some mountain showers, especially later in the day into the evening. A weak Puget Sound convergence zone will likely develop late in the day as well. Friday will be cooler yet, with highs in the lower to mid 60s at the coast and the mid 60s to lower 70s inland. Northwest flow aloft will prevail Friday night and Saturday as an upper trough builds over the offshore waters, moving eastward and reaching the Pacific Northwest coast Saturday afternoon. There should be considerable cloud cover Saturday morning -- and maybe some residual spotty showers -- but the air mass will dry and the afternoon should be mostly sunny, with highs in the 60s at the coast and upper 60s to 70s inland. McDonnal .LONG TERM...The upper ridge will shift east of the Cascades Saturday night or Sunday for a sunny and warmer day, with highs in the 70s to lower 80s over much of the interior lowlands; an exception to this is the north coast, where the leading edge of the next system might arrive in the afternoon. A cold front will move across Western Washington Sunday night, followed by an upper trough on Monday. The upper trough will probably keep a chance of showers over parts of Western Washington into Tuesday, with another small upper ridge arriving on Wednesday. McDonnal
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&& .AVIATION...
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Upper level trough moving through the area this morning with southwesterly flow aloft becoming westerly. Low level onshore flow continuing into Friday. Air mass stable with low level moisture. Ceilings in the 1000-1500 foot range with isolated ceilings below 1000 feet from about Whidbey Island southward and along the coast early this morning. The stratus will spread north reaching KBLI 12z-14z. Ceilings slowly improving after 16z to 1500-2000 feet. Lower cloud layer scattering out over the interior 20-22z with just mid/high level moisture late afternoon into the evening hours. MVFR ceilings will continue along the coast. The stratus will spread back into the interior late Thursday evening/early Friday morning with ceilings in the 1000-2000 foot range by 09z-12z Friday. KSEA...Ceilings around 1000 feet rising to near 1500 feet 16z and to near 2000 feet around 19z. Stratus layer scattering out 20z- 22z with mid level moisture. Ceilings lowering back down to near 1500 feet around 07z. South southwesterly wind 5 to 10 knots. Felton .MARINE...Onshore flow will continue. The westerly 40kt gales in the Strait of Juan de Fuca early this morning will ease today, but then pick up again this evening. There should be a pretty good push of westerlies in the Strait each evening for the next several days-- probably 20-30kt.
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&& .SEW WATCHES/WARNINGS/ADVISORIES...
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WA...None. PZ...Gale Warning until 3 AM PDT Friday for Central U.S. Waters Strait Of Juan De Fuca-East Entrance U.S. Waters Strait Of Juan De Fuca.
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