Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000 FXUS66 KSEW 170511 AFDSEW Area Forecast Discussion National Weather Service Seattle WA 930 PM PDT Sat Jun 16 2018 .SYNOPSIS...
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A weak upper level low will stall over southern Oregon and northern California tonight with a chance of showers or thunderstorms over the Cascades and foothills. High pressure will build offshore Sunday and Monday with ridge shifting inland Tuesday night and Wednesday. High temperatures will trend around 10 to 20 degrees warmer than normal. Increasing onshore flow will result in cooler temperatures over the interior on Thursday and Friday.
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&& .SHORT TERM...
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IR and WV satellite imagery shows the upper level low circulation centered over Oregon this evening. This is producing light northeast flow aloft over Western Washington with weak instability over the Cascades and foothills triggering a few showers and isolated thunderstorms. Lightning detection indicates the last strike was a couple hours ago near Sultan as a weak thunderstorms shifted southwest off the Cascades and over the lowlands. Showers and isolated thunderstorm cells have all weakened and mostly diminished once the flow steered activity into the lowlands. Only a few isolated showers made it to the I-5 corridor before dissipating with a brief shower reported at Renton and Arlington in the past few hours. The upper low appears to be far enough south allowing and drying to begin over Western Washington. Radar shows a rapid decrease in showers and most activity will likely be over with loss of diurnal instability later this evening. The forecast indicates showers ending after midnight for most zones except some lingering showers over the Cascades of Pierce and Lewis county overnight given the closer proximity to the low. The short term forecast looks good and no updates are planned tonight. High pressure aloft builds into British Columbia Sunday and persists through the middle of next week. The low level flow will remain northerly or offshore, with diurnal onshore westerly winds along the coast and in the Strait during the afternoon and evening hours at times. Some models are showing a marine push Monday night into Tuesday with stratus clouds possibly pushing into the interior. The stratus deck will be shallow and will burn back to the coast during the early afternoon. Models are showing a few showers developing over the Cascades where the air mass will be warm and unstable. Isolated thunderstorms are possible. However, a quick look at some newer 00z model data suggests perhaps just a partial weak push. This may allow the warm air mass to remain in place over much of the interior until a stronger push develops Wednesday or Thursday. Highs will trend warmer Sunday through at least Tuesday and possibly Wednesday, running about 10 to 20 degrees above average. MOS guidance is showing a lot of variability and most guidance seems a bit on the warm side given the 500 mb heights only build to around 5800M by Tuesday. If the thermally induced trough remains parked over the interior of Western Washington with lighter winds early next week and limited sea-breeze effects, highs might be able to reach the rather warm MOS guidance giving highs in the mid 80s to around 90. Guidance for Sunday is warmer than would typically be expected with the lack of strong offshore flow and heights slow to build. Forecast highs in the low to mid 80s may be a bit high for areas along the water including Sea-Tac which should get some decent northerly breeze. This often limits potential highs but the range of MOS have remained consistent in showing mid or upper 80s. Forecast temperatures are in the low to mid 80s which is probably a good call given the north flow. Dry weather should prevail the remainder of the weekend into at least the early part of next week. Mercer given the .LONG TERM...Previous discussion...Wednesday will be the last day of the warm spell as the ridge moves overhead. Temperatures in the interior will peak in the mid to upper 80s with a few low 90s possible. Showers and isolated thunderstorms are possible in the mountains again, especially the Cascades. A strong onshore push will bring cooler conditions as we move into Thursday with highs mainly in the mid 60s to 70s. An upper level ridge will cross the Pac NW Friday and Saturday for more dry weather. 33
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&& .AVIATION...
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An upper level low has shifted south over Oregon this evening. Light northeast flow aloft will gradually bring drier air into the area as high pressure aloft builds into British Columbia and low level northerly or offshore flow develops. Air mass is generally dry except somewhat unstable over the mountains, with a stable air mass developing tonight with loss of daytime warming. Mostly VFR conditions with residual Sct-Bkn clouds around 6-8k ft and in the mid levels above 10k ft. Residual showers mostly ending after sunset and the threat of thunderstorms will be east of the Crest. KSEA...VFR. Expect north to northeasterly winds 4 to 7 kt, then northerly winds increasing to 8 to 15 kt Sunday afternoon. Winds will ease below 10 kt by midnight Sunday night. DTM
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&& .MARINE...
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Surface high pressure will gradually build over southern B.C. through early next week. This will provide increasing northerly flow at the surface, peaking Sunday afternoon with near small craft conditions over the inter waters including Puget Sound. The typical diurnal onshore wind shift is likely over the coastal waters and central/east entrance to the Strait of Juan De Fuca at times next week, mainly during the afternoon and evening hours. A partial marine push may develop Tuesday night but most models hold off on a solid onshore push by Thursday. Expect westerly small craft winds in the strait at times. DTM
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&& .SEW WATCHES/WARNINGS/ADVISORIES...
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WA...None. PZ...Small Craft Advisory until midnight PDT tonight for the coastal waters, excluding the inner north coastal zone. Small craft advisory until midnight PDT for the central and east entrance to the Strait of Juan De Fuca.
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