Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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085 FXUS66 KSEW 211635 AFDSEW Area Forecast Discussion National Weather Service Seattle WA 935 AM PDT Thu Sep 21 2017 .SYNOPSIS...
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An upper level trough will maintain a chance of showers and cooler temperatures over Western Washington today, mainly over the mountains and over the lowlands from Seattle southward. Drier north flow aloft from an offshore upper level ridge will bring sunshine and a little warming Friday and Saturday. The weakening ridge will move over the area Sunday and Monday but could allow transient weak weather systems to bring some spotty light rain. The ridge will build Tuesday or Wednesday for mostly sunny weather.
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&& .SHORT TERM...
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A large upper level trough centered over E WA and NE Oregon this morning will maintain somewhat moist N-NE flow aloft over W WA today. RADAR is picking up small showers forming over N Puget Sound which are drifting S over the SW interior. This could be just left over activity from earlier this morning as the new 12Z NAM, 06Z GFS, and 12Z NAM MOS guidance are all dry for the north part of Puget Sound and the N interior today. 15Z IR imagery suggests a little reduction in the cloud cover over Vancouver B.C. and parts of the NW interior, so will go with the latest guidance and hold off on expanding lowland showers today. Otherwise, guidance agrees that the Cascades and Olympics will have scattered showers today as will the lowlands S of the Olympics, The Oregon upper level trough and the upstream upper level ridge over the NE Pacific will both slide a bit E tonight and Friday. The flow aloft should dry enough to end the chance of showers for everywhere except the Cascades. The upper level ridge will tilt E over southern B.C. by Saturday. There will be a lot of flow through the ridge both Friday and Saturday which will allow mid and high level moisture to move across W WA at times. There will be more sunshine than today, but still periods of cloudiness. The warming and drying from the ridge should be enough to push area high temperatures into the lower to mid 60s, which would be just a little below normal. The weak upper level ridge remains in place over S B.C. on Sunday but the models differ on whether a system moving through the ridge will be strong enough to bring rain to W WA. The 06Z GFS is the wettest, bringing light rain to just about all of W WA. The 00Z ECMWF and 00Z Canadian models keep all the precip up over B.C. The current forecast compromises between the two solutions with low chance POPs over the N part of W WA. Kam .LONG TERM...Previous discussion from the 335 AM AFD...The upper level ridge persists Monday and into midweek keeping the area relatively dry...although each model shows a bit of moisture and thus the potential for showers to dip south enough to be right on the doorstep...but never really panning out. Not helping matters is that there is really not much in the way of agreement as to which systems or which days this will occur. Feels like a bit of a cop out...but best course of action is to keep the forecast painted in broad strokes with slight chance pops for much of the extended until models can gel on something resembling a singular solution. Regardless of precip...should continue to see temperatures gradually climb into the 70s for many locations for the first half of next week. SMR
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&& .AVIATION...
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A broad upper level trough over the interior Pacific Northwest will slowly migrate to the southeast through Thursday. High pressure centered along 140w will persist, maintaining light northerly flow aloft. The air mass is generally stable and somewhat moist, mainly in the lower levels. Northerly flow will allow the air mass to dry tonight and Thursday. Cigs are variable across Western WA this morning. Areas of IFR cigs and vis are mostly affecting the southwest interior and south/central Puget Sound. Residual showers are also causing some IFR conditions at the KSEA/KBFI terminals and PWT (Bremerton). Conditions should improve to mostly VFR by midday and scatter out this afternoon. KSEA...Light southwest flow to 5 kt will become north-northwest by early this afternoon. Northeast flow just above the surface and low level moisture will cause IFR cigs w/ showers as late as 18z. conditions should improve to VFR by midday and clouds will scatter later in the day.
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&& .MARINE...
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Weak surface low over interior Western WA will move east of the Cascades today. Another weak low will move south through the coastal waters later tonight and Thursday. High pressure building on Friday with high pressure remaining over the area into the early part of next week. Winds are forecast to remain below 20 knots across the waters. dtm
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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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