FXUS66 KSEW 241600 AFDSEW Area Forecast Discussion National Weather Service Seattle WA 900 AM PDT Wed Jul 24 2024 .SYNOPSIS...Upper level trough over Western Washington through Thursday. Weak system moving through today will be dry with increasing onshore flow behind the system tonight. The trough will move off to the northeast Thursday night. A weaker trough will take its place over the weekend into the first part of next week. && .SHORT TERM /TODAY THROUGH FRIDAY/...The current forecast remains on track with no planned updates this morning. 33 Previous discussion...Weak shortwave spinning around the trough moving through later today. This feature is dry with just some middle level clouds moving through the area. Increasing onshore flow this afternoon will keep temperatures from warming too much even with the late July sunshine. Highs in the 70s for the interior and mid and upper 60s for the coast. Low level onshore flow tonight with a convergence zone setting up over Snohomish county. Could see light showers/drizzle under the zone overnight. Also a slight chance of showers over the North Cascades as what is left of the shortwave exits the area. Under mostly cloudy skies lows will be in the 50s. Onshore flow weakening Thursday with the low level flow turning northerly in the afternoon. Upper level trough axis moving through combined with the onshore flow keeping skies mostly cloudy in the morning. Skies clearing Thursday afternoon with the weakening onshore flow and trough axis over the Cascades. Cloud cover in the morning plus a little cooler temperatures aloft with the trough axis overhead will keep highs in the mid 60s to mid 70s. Upper level trough moving off to the northeast Thursday night. Weak low level onshore flow developing overnight. The onshore flow will not be strong enough to drive the marine layer very far inland leaving skies mostly clear overnight. Lows in the upper 40s to mid 50s. Friday a transition day with the upper level trough well to the northeast and a weaker upper level trough beginning to develop late in the day. Once again the low level onshore flow will not be strong enough to push the marine layer inland. Highs near normal, 70s and lower 80s inland and mid 60s to lower 70s coast. .LONG TERM /SATURDAY THROUGH TUESDAY/...Extended models in good agreement with a weak upper level trough over the area for the weekend and low level onshore flow. This will give the area a couple days of normal late July weather. Onshore flow could be strong enough to get the marine layer inland Sunday morning with skies clearing in the afternoon. Highs in the 70s with lows in the lower to mid 50s. Model consistency falls apart Monday and Tuesday. The operational ECMWF has an upper level trough moving down the British Columbia coast Monday with an upper level low closing up off the coast Monday night and Tuesday. The operational GFS has a shortwave moving through Monday with a flat upper level ridge building Tuesday. While the ECMWF ensembles are not as strong with the low there is still a very wet signal for Monday afternoon through Tuesday with over half of the ensemble solutions indicating measurable rain. The GFS ensembles have a much weaker signal with less than 20 percent of the ensemble solutions indicating measurable precipitation. Fun fact, there has not been measurable rain in Seattle between July 27th and August 1st in the last 15 years. In addition using the combined record between the Federal Building in downtown Seattle and Seattle-Tacoma airport there has been measurable rain on July 30th only 9 times since 1894. Given the climatology will go more with the GFS solution and only have slight chance pops for Monday and Tuesday. Highs remaining in the 70s inland and 60s along the coast. Felton && .AVIATION...Upper level trough over the area with southwesterly flow aloft. Trough axis moving through tonight with flow aloft becoming west northwesterly behind the trough axis. In the lower levels onshore flow increasing late this afternoon into tonight then weakening Thursday. MVFR ceilings along the coast with some high clouds over the Northwest Interior. Clear skies over the remainder of the area this morning. Increasing high level clouds over the remainder of the area this afternoon and evening with MVFR ceilings continuing along the coast. MVFR ceilings developing over Snohomish County tonight with weak convergence zone. MVFR ceilings continuing along the coast with VFR ceilings elsewhere. Ceilings improving to VFR all areas mid to late Thursday morning. KSEA...Clear skies with increasing high clouds later this afternoon. Ceilings lowering overnight but staying VFR through Thursday morning. West southwest winds 4 to 6 knots becoming northwesterly 4 to 8 knots around 00z. Winds switching back to southwesterly after 09z. Felton && .MARINE...High pressure over the coastal waters with lower pressure inland will continue through the weekend with varying degrees of onshore flow. Strongest onshore flow tonight with gales in the central and east Strait of Juan de Fuca and small craft advisory northwesterlies in Admiralty Inlet. The seas will range from 4 to 6 feet through the rest of the week. Felton && .SEW WATCHES/WARNINGS/ADVISORIES... WA...None. PZ...Gale Warning from 6 PM this evening to 5 AM PDT Thursday for Central U.S. Waters Strait Of Juan De Fuca-East Entrance U.S. Waters Strait Of Juan De Fuca. Small Craft Advisory from 6 PM this evening to 5 AM PDT Thursday for Admiralty Inlet. && $$