Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA
Issued by NWS Seattle/Tacoma, WA
043 FXUS66 KSEW 180928 AFDSEW Area Forecast Discussion National Weather Service Seattle WA 228 AM PDT Sat May 18 2024 .SYNOPSIS...An upper level trough will produce cool and occasionally showery conditions across Western Washington today into early Sunday. The trough will move east of the region by Sunday afternoon and a weak upper level ridge will give drier and somewhat warmer conditions on Monday. Another trough will arrive on Tuesday for a return to cool and unsettled conditions for much of next week. && .SHORT TERM /TODAY THROUGH MONDAY/...Early morning satellite imagery shows the next in a series of upper level troughs slipping down the British Columbia coast toward the region. Onshore flow will increase through the day as this system digs southward through the area. Shower coverage will increase later this morning...especially across the northern half of the CWA. A convergence zone is likely to form by late morning then gradually drag southward across Puget Sound through the afternoon before dissipating in the Cascades late this evening. High temperatures will be a good 5 to 10 degrees below normal. The upper trough axis will shift east of the Cascades tonight and only a few remnant showers in the Cascades are expected by Sunday morning. Drier northerly flow aloft takes over Sunday afternoon and most lowland locations should see some afternoon sunshine...allowing high temperatures to nudge upward a few degrees. Weak upper ridging will move across the area on Monday and low level onshore flow will weaken. That spells drier and marginally warmer conditions...if only briefly. .LONG TERM /TUESDAY THROUGH FRIDAY/...By early Tuesday, the ridge gives way to yet another upper trough digging southward over British Columbia. We`ll see a return to cool and wetter conditions on Tuesday. The QPF totals with this system look fairly robust by late May standards. Cool and showery conditions look to continue into Wednesday as the upper trough continues to progress southeastward across the Pacific Northwest. The forecast details become more uncertain late next week. In general, ensembles maintain negative height anomalies over the western third of the lower 48 with strong ridging centered over the Gulf of Alaska. But there are considerable differences amongst ensemble members with the placement and depth of the trough over the Western US. Current forecast from the NBM have mostly chance PoPs and temps a little below average late next week. This seems like a reasonable forecast until the overall picture becomes clearer. 27 && .AVIATION...A weak upper level trough will cross western WA today for a chance of showers. The flow aloft is westerly. The low level air mass remains moist with MVFR ceilings likely in the central sound (where convergence zone showers are also expected). Showers will decrease in coverage tonight as the trough exits east. With onshore flow, MVFR cigs will likely reform during the overnight hours tonight. 33 KSEA...MVFR conditions this morning with light showers in the vicinity. Winds shifting to N around 21z then back to S by 06z tonight. 33 && .MARINE...High pressure over the NE Pacific will maintain onshore flow across western WA through the weekend. Strongest winds will be over the Strait of Juan de Fuca with Small Craft Advisory winds extending through late tonight. The next frontal system will reach the region early next week. 33 && .HYDROLOGY...The daily hydrology discussion has ended until the start of the next rainy season; it will only be updated as needed. && .SEW WATCHES/WARNINGS/ADVISORIES... WA...None. PZ...Small Craft Advisory until 5 AM PDT Sunday for Central U.S. Waters Strait Of Juan De Fuca-East Entrance U.S. Waters Strait Of Juan De Fuca. && $$