344 FXUS66 KSEW 092315 AFDSEW Area Forecast Discussion National Weather Service Seattle WA 330 PM PDT Tue Oct 9 2018 .SYNOPSIS...A ridge of high pressure building into British Columbia on Wednesday will expand over Washington Thursday. The ridge will shift west just offshore on Friday as a trough drives south into the interior Pacific Northwest. High pressure strengthens over the entire region this weekend into early next week. Light offshore or northerly flow will maintain dry weather with enough moisture at the surface for areas of fog during the night and morning hours. Highs will be near to slightly below average. && .SHORT TERM...Satellite imagery shows areas of stubborn stratus across much of Western Washington this afternoon with most the clearing occurring from the north coast to the north interior. The rest of the area including greater Puget Sound will only have sunbreaks prior to sunset with abundant low level moisture trapped across the area as high pressure builds aloft. Models appear to be drying out the lower levels too quickly, with satellite imagery implying stratus is likely to fill in overnight with areas of fog also developing in prone valleys. Low clouds and fog will be fairly widespread to start off the day, but more clearing during the afternoon should allow highs to reach the low 60s over the interior. The ridge shifts more directly over the region Thursday with residual moisture trapped near the surface. Subsidence will result in less stratus and more fog due to radiational cooling Wednesday night and Thursday morning. Most exposed areas should break out of the fog by afternoon but sheltered valleys including the southwest interior could hold on to fog longer into the afternoon hours. This may cause some of the colder valleys to stay noticeably cooler during the afternoon while areas with clearing reach the mid 60s. The ridge will amplify and shift westward just off the British Columbia and WA/Oregon coast Friday as a trough drives southward east of the Cascades. This is a dry trajectory for Western Washington with no precipitation expected. Weak onshore flow bring slight influx of moisture and continue the pattern with morning low clouds and fog, and partial afternoon sunshine. Highs should be near normal in the low to mid 60s. .LONG TERM...Global models are in good agreement that the ridge of high pressure will continue to dominate the pattern through the weekend into the early to middle part of next week. Offshore flow may strengthen and dry out the low level enough to inhibit widespread fog. Sheltered prone valleys will still get fog, but more of the area should see sunshine with better mixing. MOS shows only slightly warmer temperatures, with mid or even upper 60s possible Monday and Tuesday of next week. Models do not show any significant fronts by this time next week. Expect a prolonged period of dry weather and seasonable temperatures. Mercer && .AVIATION...Moderate northwest flow continues into Wednesday as high pressure builds offshore. As expected, even though showers have ended, clearing around the Sound has been very gradual this afternoon with many sites still reporting MVFR ceilings as air mass remains moist. May be tough to completely scatter out by this evening. Improvement will also be short lived though as light surface gradients, recent rainfall, and longer nights will allow for fog development after 09-10z. Widespread IFR or LIFR conditions expected for much of the region through Wednesday morning. Improvement expected throughout the day. Lighter winds through the period. KSEA...Above discussion applies. MVFR ceilings continue this afternoon but northerly winds have dropped below 10 kts. Some scattering taking place nearby in latest satellite imagery but low confidence in how much additional improvement (return to VFR) will take place this evening before low clouds/fog develops. IFR to LIFR conditions a good bet after 10z with gradual improvement to VFR on Wednesday after 18z. Light and variable winds overnight into Wednesday morning. CEO && .MARINE...Gusty north to northwest winds continue this afternoon across the coastal waters so small craft advisory will remain in place through this evening. High pressure will build into the waters tonight and continue through Thursday. This will result in lighter winds and areas of fog. Winds should be 20 kts or less. High pressure remains in control for the end of the week into the weekend with northerly low level offshore flow. CEO && .SEW WATCHES/WARNINGS/ADVISORIES... WA...None. PZ...Small Craft Advisory until midnight PDT tonight for Coastal Waters From Cape Flattery To James Island 10 To 60 Nm- Coastal Waters From James Island To Point Grenville 10 To 60 Nm-Coastal Waters From Point Grenville To Cape Shoalwater 10 To 60 Nm. && $$ www.weather.gov/seattle