Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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626 FXUS66 KSEW 140340 AFDSEW Area Forecast Discussion National Weather Service Seattle WA 840 PM PDT Wed Jun 13 2018 .SYNOPSIS...
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Showers will taper off tonight and Thursday, except in the Puget Sound convergence zone, which should persist through tonight. An upper level low will bring showers to mainly the mountains on Friday. The upper low will remain nearby, keeping a chance of thunderstorms in the Cascades through the weekend into next week with generally dry conditions elsewhere.
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&& .SHORT TERM...
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An upper level trough is over Western Washington this evening, reflecting the cool and unstable air mass that followed today`s cold front. Most all shower activity over the lowlands is confined to a classic Puget Sound convergence zone, which extends eastward from the extreme northeast corner of the Olympic Peninsula across the north end of the Kitsap Peninsula and along the King/Snohomish county border to the Cascades crest. There are spotty showers elsewhere, mainly along the west slopes of the mountains. Temperatures at 8 pm ranged from the mid 50s to the lower 60s. Northwest flow aloft will continue tonight, with onshore flow at low levels. The air mass will remain fairly moist, especially in the lower 7000 ft or so, and it will become more stable overnight. Most of the showers will continue to be along the west slopes of the mountains and in the convergence zone, which should gradually weaken as it remains nearly stationary or shifts slowly southward. Lows tonight will range from the upper 40s to mid 50s. Northwest flow aloft will continue Thursday, as a weak upper shortwave digging over the area keeps the axis of the main upper trough over Western Washington. The air mass will remain somewhat moist and slightly unstable with light scattered showers, especially at the coast and over the mountains. The showers should end Thursday night, as the upper trough axis moves east of the Cascades. There will be some sunshine as well on Thursday, especially over the Western Washington interior lowlands. Highs will be in the upper 50s and 60s. Another upper shortwave will dig southward across Washington Friday and Friday night from southwest British Columbia, with the upper flow backing from northerly to northeast late Friday night and Saturday. The combination of some low level moisture and weak instability should produce scattered light showers over the mountains Friday through Saturday, but the lowlands will remain mostly dry with partly to mostly sunny skies. Highs Friday will be close to normal, in the 60s to lower 70s; it will be a little warmer Saturday with highs in the upper 60s and 70s. McDonnal .LONG TERM...Here is the long term section from the afternoon forecast discussion -- The upper level low will wobble around in the vicinity of Oregon Sunday and Monday, then slowly move east (in the GFS) or perhaps slowly move west (in the Euro). This will result in a continued threat of scattered showers and isolated thunderstorms over the Cascades Sunday and Monday. The weather will be generally dry over the rest of the area. Have kept Tuesday and Wednesday dry like the Euro; the GFS keeps spawning showers every day in the Cascades. Temperatures will be in the low to mid 80s each day, Sunday through Wednesday. Burke
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&& .AVIATION...
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Northwest flow aloft over Western Washington this evening will continue through Thursday, as the upper trough over the area is weakly reinforced by a shortwave digging into its base. Moderate onshore flow at low levels will weaken later tonight and Thursday. The air mass will remain moist in the lower levels, especially below about 7000 ft. The Puget Sound convergence zone -- extending east from the extreme northeast corner of the Olympic Peninsula across the north end of the Kitsap Peninsula and along the King/Snohomish county border to the Cascades crest -- is producing areas of MVFR to local IFR ceilings and visibility in showers at this time; it should gradually weaken overnight as it remains nearly stationary or shifts slowly southward. In addition, the Cascades should remain generally obscured tonight. Otherwise VFR conditions prevail with ceilings mainly 045-070. Moist onshore flow should cause conditions to lower tonight across the forecast area, with widespread MVFR to local IFR ceilings in marine stratus. We can expect gradual improvement late Thursday morning through Thursday afternoon. KSEA...Variable wind this evening should become south 5-10 kt around midnight and veer to southwest Thursday afternoon. The PSCZ should remain just north of Seattle, gradually weakening during the night. The lower broken deck -- currently at 028 -- could scatter at times this evening, but then likely lower to around 016 overnight. Conditions should improve to low-end VFR by late Thursday morning. McDonnal
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&& .MARINE...
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Moderate onshore flow this evening will ease tonight, with small craft advisory westerlies of 20 to 30 kt likely in the central and eastern Strait of Juan de Fuca. Weak onshore flow will continue Thursday through Monday. McDonnal
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&& .SEW WATCHES/WARNINGS/ADVISORIES...
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WA...None. PZ...Small Craft Advisory until 3 AM PDT Thursday for Central U.S. Waters Strait Of Juan De Fuca-East Entrance U.S. Waters Strait Of Juan De Fuca.
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