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Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA
Issued by NWS Seattle/Tacoma, WA
519 FXUS66 KSEW 160355 AFDSEW Area Forecast Discussion National Weather Service Seattle WA 855 PM PDT Thu Aug 15 2024 .SYNOPSIS...Weak troughing over the region will maintain cooler and partly cloudy conditions through the rest of the week. A low will deepen along the coast through the weekend, increasing the threat of thunderstorms across western Washington on Saturday and Sunday. Unsettled conditions look to continue into next week with cooler temperatures and chances of showers. && .SHORT TERM /TONIGHT THROUGH SUNDAY/...A weak low pressure system will continue to lift northeastward through southern Oregon/eastern Washington into Friday. Radar does show a few showers across portions of the Cascades tonight, however expect showers to continue to wane over the next few hours. The forecast remains largely on track, so no updates are needed this evening. The remainder of the previous discussion follows below, with updates made to the aviation section. 14 The aforementioned low exiting our area will continue to maintain a varying degree of onshore flow on Friday, with temperatures remaining in the low 70s. There is continued chance of thunderstorms and showers, primarily in the North Cascades through the evening as well. Our attention focuses to Saturday as an upper level low drops down from British Columbia and deepens as it spins offshore, with widespread showers expected across W WA. With S/SE flow aloft, latest guidance continues to depict a thunderstorm threat across the lowlands and the Cascades. Elevated instability will increase starting Saturday afternoon, with latest probabilistic guidance indicating a 20-35% of thunderstorms across the Cascades and Puget Sound. Areas along the coast have at least a 10-15% chance, with the primary areas of thunderstorm development throughout the lowlands and the Cascades. Probabilities peak especially Saturday evening into the overnight period, with nocturnal thunder possible at this time. Along with thunderstorms and rain showers, we will continue to monitor the potential for slides, especially in the burn scar areas in the Cascades. See the Hydrology section for more details. Ensemble guidance continues to fluctuate on the exact position of the upper level low offshore, which could lead to differences in rainfall totals, timing, and thunderstorm chances for the area. Confidence continues to remain low on coverage of thunderstorms and showers, but can expect locally heavier precip in any heavier showers or thunderstorms that do develop. The main threat with any storms that do develop will be lightning, gusty and erratic winds, small hail, and heavy rainfall. The threat for thunderstorms continues through Sunday morning but will start to diminish by Sunday afternoon, with still a 10-25% chance of thunderstorms. The highest probabilities will be focused in the Cascades (25%) with lower probabilities in the lowlands. .LONG TERM /MONDAY THROUGH THURSDAY/...Troughing looks to be favored in the Pacific Northwest throughout much of the long term period, maintaining cooler temperatures across the region and general chances of showers. CPC and clusters generally agree with this output, as do most of the ensembles at this time. Mazurkiewicz && .AVIATION...Southerly flow will increase aloft into Friday as upper troughing develops offshore. VFR conditions with mid level SCT/BKN cigs have continued into this evening. MVFR/IFR cigs will develop along the coast this evening and slide inland into southern and central Puget Sound Friday morning. Stratus inland will slowly improve to VFR into the afternoon, with lingering VFR/MVFR cigs along the coast. W/SW winds will continue through this evening before becoming lighter tonight. KSEA...VFR conditions with lingering mid-level SCT/BKN cigs this evening. SCT/BKN cigs likely to continue into this evening. Stratus will move into Puget Sound and near the terminal early Friday morning. MVFR cigs are possible Friday morning, generally 13-18z, with probabilities around 35% of MVFR cigs. Improvement to VFR expected into Friday afternoon. S/SW continuing into Friday. JD/Wolcott && .MARINE...An onshore flow pattern will generally continue through the weekend for area waters. A westerly push through the Strait of Juan de Fuca will result in SCA winds and wind gusts this evening into tonight, with gusts generally ranging 20 to 30 kts. Onshore flow will become a bit lighter later Friday into Saturday for a weaker push through the Strait of Juan de Fuca. Northwest winds will continue for the Coastal Waters through Saturday before becoming southerly Sunday into early next week. In addition, the threat of thunderstorms will increase later Saturday afternoon and continue into Sunday for all area waters, with the threat peaking Saturday evening into Saturday night. Thunderstorms could produce brief gusty winds as well as frequent lightning during this period. Seas will generally remain 3 to 5 feet into the weekend. JD && .HYDROLOGY...Impulses coming out of the low pressure trough aloft will bring very slight thunderstorm chances to the northern Cascades near the crest this evening and Friday afternoon. There is a very low probability of flash flooding for that area if a slow moving thunderstorms sets up over the area Friday. The bigger threat comes over the weekend. The low pressure system that moves over the area Saturday into Sunday will bring plenty of moisture. Rainfall will be widespread, but total QPF has 90% of 0" to 0.25", ranging to a 10% chance of 1" to 3.7". It will be fairly unstable with thunderstorm chances having increased up to 35%. If a thunderstorm sets up over an area, especially a burn scar, rainfall rates and/or amounts could be enough to trigger flash flooding and debris flows. JBB && .FIRE WEATHER...The low pressure system moving over the area this weekend could bring widespread thunderstorms. However, the conditions going into this event - fuel moistures relatively high, showers preceding and with the thunderstorms - fire starts due to lightning will be somewhat limited. But even if there is plenty of lightning and multiple fire starts, the conditions will curtail fire growth. With continued cool, cloudy, and high humidity conditions well into next week, holdover fires may only smolder for quite sometime until we get our next hot, dry, or windy conditions. JBB && $$ .SEW WATCHES/WARNINGS/ADVISORIES... WA...None. PZ...Small Craft Advisory until 5 AM PDT Friday for Central U.S. Waters Strait Of Juan De Fuca-East Entrance U.S. Waters Strait Of Juan De Fuca. && $$