530 FXUS66 KSEW 130348 AFDSEW Area Forecast Discussion National Weather Service Seattle WA 848 PM PDT Sun Sep 12 2021 .SYNOPSIS...Weak upper level ridge moving over the area Monday and remaining in place Monday night. The ridge will weaken Tuesday as a frontal system moves into the northern portion of the area. The front will dissipate over Western Washington Tuesday night. Dry conditions Wednesday and Thursday. The first substantial front of the season arrives Friday with widespread rain. Unstable post- frontal environment Saturday featuring the potential for thunderstorms. && .SHORT TERM /TONIGHT THROUGH WEDNESDAY/...Satellite imagery shows cloudy skies along the west slopes of the cascades and over the central Puget Sound with just some high clouds over the remainder of the interior. Along the coast stratus beginning to form along the coastline. Temperatures at 8 pm/03z were in the 50s to mid 60s. Low level onshore flow continuing into Monday. Weak convergence this evening will keep some cloud cover over the central Puget Sound. With the light onshore flow areas of low clouds developing before sunrise Monday morning. Upper level ridge building into the area will help dissipate the stratus with mostly sunny skies in the afternoon. Highs will be in the 60s and lower 70s. Mostly clear skies continuing Monday night with the upper level ridge over the area. The ridge shifts east Tuesday as another frontal system approaches the area. Rain developing along the coast and north of Seattle. As the front sags south and weakens Tuesday night rain chances will spread south. Current forecast has the trends covered. No update this evening. Felton .LONG TERM /THURSDAY THROUGH SUNDAY/...Previous discussion follows. The long term forecast period begins with W WA located within zonal flow between a deep trough to the north and ridging to the south, per ensemble cluster solutions. Conditions should remain dry but a glance upstream shows rapidly deepening troughing over the Pacific. Conditions will begin to noticeably change on Friday as a pattern change sets up. Ensemble guidance remains in excellent agreement with the deep trough setting up over the far NE Pacific. A robust shortwave rotating within the larger scale flow will push the the first decent front in quite some time across the local area. While general timing of this frontal system will be fine tuned over the next several days, expect an overall wet day to cap out the week, with current rainfall estimates of around a half an inch in the lowlands and 3/4th to around 1 inch of precipitation over the higher terrain into Saturday morning. Saturday will be the day of this forecast period that will need to be monitored closely. The local area will remain in a post- frontal environment with more substantial troughing developing aloft. Mid level temps may drop to below -20C amidst low-mid 50s dewpoints, potentially creating an unstable environment. Widely scattered showers are likely at this point in time, but will go ahead and introduce the possibility of thunderstorms. In addition to instability, very strong winds in the mid levels will allow for a highly sheared environment. Of course there is a fair amount of uncertainty with this setup-mainly in the degree of instability. Guidance is suggestive of overcast skies most of the day, which could limit instability, along with generally cool temperatures near 60. Will continue to watch this pattern develop in the coming days. Wet conditions will then carry into Sunday. Kovacik && .AVIATION...Westerly flow aloft becoming northwesterly Monday as upper level ridge moves over the area. In the lower level light onshore flow will continue through Monday. Ceilings 5000-6000 feet along the west slopes of the Cascades and the central Puget Sound into the early morning hours with ceilings 1000-2000 feet along the coast. For the remainder of the area partly cloudy skies. Stratus spreading over most of the interior 12z-15z with ceilings 1000-2000 feet. Ceilings improving in the afternoon with the cloud deck scattering out by 23z. KSEA...Ceilings near 6000 feet lowering down to 1000-2000 feet 12z-15z. Ceilings improving around 19z with clouds scattering out around 23z. Light north wind becoming variable after 12z. Winds becoming northwesterly 4-8 knots after 20z. Felton && .MARINE...Onshore flow continuing through Monday. Small craft advisory westerlies easing early Monday morning. Southerly winds will increase ahead of the front Tuesday morning, with increasing northwest winds later Tuesday afternoon into Wednesday during the day with SCA winds possible for the coastal waters area. Northwest swell of 5 to 7 feet through tonight, will decrease to 4 to 5 feet on Monday. Increasing swell possible again midweek with the front moving through the area. Felton/JD && .SEW WATCHES/WARNINGS/ADVISORIES... WA...None. PZ...Small Craft Advisory until 2 AM PDT Monday for Central U.S. Waters Strait Of Juan De Fuca-East Entrance U.S. Waters Strait Of Juan De Fuca. && $$ www.weather.gov/seattle