Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA
Issued by NWS Seattle/Tacoma, WA
718 FXUS66 KSEW 200332 AFDSEW Area Forecast Discussion National Weather Service Seattle WA 832 PM PDT Fri Jul 19 2024 .SYNOPSIS...Warm and dry conditions will persist into the weekend as upper level ridging amplifies over the region on Saturday. A shortwave will skirt the area on Sunday and may bring some light showers to portions of western Washington. An upper level trough moving into British Columbia early next week will cool temperatures closer to seasonal norms. && .SHORT TERM /TONIGHT THROUGH MONDAY/...The forecast largely remains on track this evening. Stratus will push inland overnight, but to a lesser extent than previous days with weaker onshore flow. Overnight lows will be in the 50s across much of western Washington. The remainder of the previous discussion follows below, with updates made to the aviation section. 14 Upper level ridging will amplify over the region on Saturday with 500 mb heights building into the 590 dms. Flow aloft will transition to southerly and temperatures aloft will warm to 20C. Onshore flow will weaken and expect a shallower marine layer to allow for afternoon temperatures on Saturday to reach the upper 80s and low 90s across the interior. High temperatures near the coast will be in the 70s. The upper level ridge will slowly shift east Saturday night into Sunday as an upper level trough approaches the area. Another upper level shortwave looks to precede the trough and may bring some light precipitation to the coast on Sunday. Latest high-res guidance indicates roughly a 10-15% chance of isolated thunderstorms across the mountains Sunday afternoon, so will need to keep an eye on this trend over the next day. Temperatures on Sunday will cool a few degrees along the coast, and generally look to top out in the 60s. Temperatures across the interior, however, look to remain warm, generally climbing into the mid to upper 80s. Guidance remains in good agreement with a pattern shift starting Monday as a broad upper level trough starts to move into the region. Increased onshore flow will allow for temperatures to cool back down towards seasonal norms, with highs generally expected to be in the 60s along the coast and 70s across the interior. .LONG TERM /TUESDAY THROUGH FRIDAY/...The aforementioned trough will remain over the region through the long term period. This will promote onshore flow, morning stratus, and temperatures close to seasonal norms. Highs generally look to trend in the 60s for the coast and 70s across the interior, with overnight lows trending in the upper 40s to mid 50s. Not much expected in the way of precipitation at this time, though could see a few very spotty showers at times across the mountains late in the week. && .AVIATION...Southwest flow aloft with troughing upstream and ridging downstream with weak low-level onshore flow. VFR/clear skies continue this evening. LIFR/IFR stratus is expected to advance inland along the coast tonight and possibly making it`s way to KSHN- KPWT, and also close to KCLM/KPAE in the Strait of Juan de Fuca. Any lingering stratus should return to the coast by 18z Saturday with widespread VFR for the remainder of the day. Winds expected to remain light out of the north and west at 4 to 8 kt. KSEA...VFR for the remainder of the evening into Saturday. Marine stratus is expected to remain well west of the terminal (might see a few VFR level clouds in the morning). Surface winds north 4 to 8 kt this evening before shifting back to the northwest Saturday afternoon. McMillian/HPR && .MARINE...Surface high pressure offshore with lower pressure inland. Because of this, we`ll see prevailing onshore flow of varying strength. A SCA has been issued this evening into tonight with more headlines likely into next week. Gales are possible Sunday evening into Monday. Seas 3 to 5 ft into Saturday before rising to 4 to 7 ft Saturday night through Monday. McMillian && .FIRE WEATHER...Warm and dry conditions through Sunday. Afternoon minimum RHs again dipping close to critical thresholds Saturday for the lowlands and through Sunday for the Cascades. Instability may increase enough to keep a mention of elevated concern for the Cascades especially Saturday afternoon and evening. Another disturbance may bring a renewed threat for thunderstorms Sunday across some portion of the mountains, with high resolution ensembles including the NBM showing a 10-15% chance at this time. A cooler more humid pattern develops Monday as the pattern switches to be dominated by an upper level trough over the region. && .SEW WATCHES/WARNINGS/ADVISORIES... WA...None. PZ...Small Craft Advisory until 3 AM PDT Saturday for Central U.S. Waters Strait Of Juan De Fuca-East Entrance U.S. Waters Strait Of Juan De Fuca. && $$