FXUS66 KSEW 241035 AFDSEW Area Forecast Discussion National Weather Service Seattle WA 335 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/...Satellite imagery shows upper level trough over Western Washington with the trough axis offshore this morning. Mostly clear skies over the area with stratus just off the coast at 3 am/10z. Temperatures were in the mid 50s to lower 60s. 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...A low tracking across southern British Columbia is expected to bring west/southwest flow aloft as it comes ashore. Satellite imagery is showing some remnants of a dissipating frontal system off the Washington coast. Areas of marine stratus is visible off the coast, and will continue to advance to as far as the Southwest Interior/Olympics early this morning. Widespread MVFR CIGs are expected with this marine layer with pockets of IFR, lifting to VFR by late morning. Remaining areas east of the Olympics will see mostly clear skies to start, with increased VFR cloud coverage by Wednesday afternoon as the weak front boundary passes through. Surface winds light and variable this morning, becoming westerly by early afternoon, and northerly by early evening 4 to 8 kt. Gusty winds to 20 kt are possible for areas along the Central/East Strait of Juan de Fuca and Admiralty Inlet (marine push this afternoon). KSEA...Clear VFR skies to start, with the weak frontal boundary adding a scattered VFR deck this afternoon and evening (from the northwest). Variable winds early morning will become southwest/westerly by 17Z at 5 kt. Expect further shifts to the northwest at 00-03Z 4 to 8 kt, shifting to the northeast/east late evening. HPR && .MARINE...A deep (but weakening) low system will track inland over southern British Columbia today. A dying frontal boundary will move across all waterways today with this low. High pressure is also building behind the low, well off in the Pacific. All in all, various degrees of onshore flow are expected (the strongest of which is expected Wednesday afternoon and evening). Confidence is increasing in the chance of gale gusts in the Central/East Strait of Juan de Fuca. Expectation from HREF guidance is for west winds to reach 15 to 25 kt (with periodic gusts to 35 kt). The gale watch was upgraded to a gale warning for the East Strait of Juan de Fuca, and expanded to include the Central Strait of Juan de Fuca. Admiralty Inlet was also included with this pattern with a small craft advisory. Gusty winds may start as soon as 21Z Wednesday and last through 12Z Thursday. Widespread gusty winds are not expected in the Northern Inland/Puget Sound Waters (max gusts 10 to 15 kt). Additional pushes of onshore winds will continue through the weekend, but are expected to remain below small craft advisory criteria at this time. Coastal waters: winds will become northerly behind the low. Winds next few days remain light with peak gusts 10 to 15 kt (highest winds in the outer coastal waters). By Friday night/Saturday, winds may reach 20 kt (with gusts to 25 kt) in the outer coastal waters. The seas will range from 4 to 6 feet through the rest of the week. HPR && .SEW WATCHES/WARNINGS/ADVISORIES... WA...None. PZ...Gale Warning from 2 PM this afternoon 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 2 PM this afternoon to 5 AM PDT Thursday for Admiralty Inlet. && $$