Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000 FXUS66 KSEW 231014 AFDSEW Area Forecast Discussion National Weather Service Seattle WA 315 AM PDT Sat Sep 23 2017 .SYNOPSIS...
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A weak north to south moving weather system will brush the coast today with clouds and a threat of light rain for the coast and Olympics. Another weak system Sunday night and Monday moving into British Columbia should bring more clouds and a chance of light rain to mainly northern portions of the area. Higher pressure aloft is expected to build offshore Tuesday into mid week for a return to sunshine and warmer temperatures.
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&& .SHORT TERM...
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Current radar shows some showers along the coast this early morning...along with a little blob of showers passing over the San Juans at the time of this writing. Models have been pretty consistent in keeping this activity west of Puget Sound and aside for the minor deviation that is the aforementioned blob over the San Juans...would expect activity today to be limited to the coast and the Olympics. Current satellite reveals nothing in the way of surprises as the area is pretty socked in with clouds. Feels weird to say that this is all associated with a slow moving upper level ridge nudging its way into the area...but there it is...a dirty ridge if there ever was one. Enough moisture looks to be over the area for a generally cloudy weekend...although some breaks of sun look to emerge this afternoon east of the Sound. Sunday will see a return of widespread clouds as an embedded shortwave within the ridge looks to move through the area on Monday...bringing with it a returning chance of showers for most of the area. But a ridge is still a ridge...which means the steady warming trend that models have been advertising the past several runs looks to remain on track. Widespread highs in the 60s today...with some locations possibly hitting 70...will continue to gradually increase a degree or two for Sunday. The aforementioned shortwave looks to knock temps down slightly for Monday...but even then high temps in the mid 60s will be pretty widespread. Although this is slightly below normal...all in all a pretty cozy start to fall. SMR .LONG TERM...It is pretty safe to say that Tuesday is where the ridge gets its groove back and sheds the dirty moniker. While clouds will gradually erode throughout the day...the standout will be the high temperatures as they rise once more to around the 70 mark for many locations. With clear skies holding court over W WA for the remainder of the work week...conditions will continue to warm with temps peaking on Thursday as many locations find themselves in the upper 70s. The ridge exits soon afterward and W WA as models start to coalesce on a solution for Friday and into the weekend of an upper level trough starting to enter the area. This will bring temps back down to around 70 in most spots. Models still disagree with whether or not this will result in precip though...as the GFS proves the wetter solution bringing precip area-wide while the ECMWF limits any POPs to the northernmost portions of the area. Leaning toward ECMWF solution at this time. SMR
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&& .AVIATION...
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Western WA lies in between a trough over the Intermountain West and a ridge offshore. The flow aloft is northerly. The air mass is dry and stable with mainly mid-high level clouds over the region. Patchy fog or low clouds may form early this morning, mainly in the valleys and along the coast. Light rain is possible along the coast where there is weak warm air advection. The interior will remain dry. The ridge will shift inland tonight for dry conditions. 33 KSEA...Patchy low clouds or stratus may form early this morning for brief MVFR conditions. Otherwise, light south winds and VFR with dry conditions. Surface winds will flip to light northerly 03-06z tonight. 33
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&& .MARINE...
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High pressure over the NE Pacific will maintain light onshore flow this weekend. A warm front will clip the area on Monday with possible Small Craft Advisory winds over the Coastal Waters. The flow will turn offshore by midweek as a thermal trough forms along the coast. 33
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&& .SEW WATCHES/WARNINGS/ADVISORIES... WA...None. PZ...None. && $$ www.weather.gov/seattle You can see an illustrated version of this discussion at www.wrh.noaa.gov/sew/gafd/latest_webafd.html

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