Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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124 FXUS66 KSEW 212240 AFDSEW Area Forecast Discussion National Weather Service Seattle WA 340 PM PDT Thu Sep 21 2017 .SYNOPSIS...An offshore upper level ridge approaching Western Washington will bring more sunshine and a little warming Friday and Saturday morning. The weak ridge will remain over the region Sunday through Tuesday but will probably allow transient weak weather systems to bring more clouds and some spotty light rain to mainly the coast and north part of Western Washington. The ridge will build Tuesday or Wednesday for mostly sunny and warmer weather. && .SHORT TERM...A large upper level trough centered over E Oregon will drift slowly E tonight allowing an offshore upper level ridge to tilt eastward over southern B.C. Drier N flow aloft ahead of the ridge has already allowed clouds to diminish over especially the north part of W WA. However, RADAR is still picking up scattered showers over mainly the mountains this afternoon. Clearing overnight should set up a mostly sunny day on Friday, however the air mass will still be a little cooler than normal with highs in the 60s. The upper level ridge will be over southern B.C. on Saturday, but it will be weak enough to allow transient weather systems to pass through the ridge and brush mainly the N part of W WA with some light rain. Mid level clouds from the first system will probably spread over the area from the NW Saturday morning, but this system should be pretty much dry. The models have been inconsistent with the system on Sunday, especially with the coverage of light rain, ranging from zero to some spotty light rain over the N part of the area. What is more certain is that it will be cloudy with highs remaining a little below normal. Kam .LONG TERM...The weak upper level ridge over southern B.C. will remain in place Sunday and Monday, still wide open to weak transient weather systems. The GFS brings another system Monday through Monday night with a bit more coverage of light rain. The ECMWF is in better agreement but still a little slower, bringing some light rain Monday afternoon through Tuesday morning. Despite the better model agreement, confidence is still low. Models are in better agreement that the upper level ridge will build enough by Wednesday for mostly sunny weather Wednesday and Thursday with temperatures near or slightly above normal. Kam && .AVIATION...A broad upper level low over the interior west will migrate further eastward through Friday. High pressure centered along 140w will persist, maintaining light northerly flow aloft. The air mass will become stable tonight and somewhat moist in the lower and mid levels. Northerly flow will allow the air mass to become mostly dry Friday into Saturday except at the surface. Patchy fog or low stratus is possible late tonight and Friday morning, mainly in the most prone valleys such as the southwest interior, lower chehalis, coast, and river valleys. Any fog or status should mix out before noon. Otherwise VFR conditions. KSEA...Light or west-northwest wind to 5 kt becoming northeast to 5 kt this evening. Scattered clouds tonight. Patchy low stratus or fog will develop in place around parts of the central/south Sound. A few hours of IFR cigs/vis mainly between 12-16z Friday morning cannot be ruled out (20-25 percent chance). Most likely, stratus or fog will just be in the vicinity of the KPAE/KSEA/KBFI terminals and will indicate this in the TAFs. && .MARINE...High pressure over the NE Pacific will build over the waters later tonight and Friday. Winds should remain fairly light, less than 15 kt or so through the weekend. A weak front could bring a little increase in southerly wind Monday. && .SEW WATCHES/WARNINGS/ADVISORIES... WA...None. PZ...None. && $$ You can see an illustrated version of this discussion at is the U.S. government's official web portal to all federal, state and local government web resources and services.