239 FXUS66 KSEW 150959 AFDSEW Area Forecast Discussion National Weather Service Seattle WA 259 AM PDT Fri May 15 2026 .SYNOPSIS...A very cool upper level trough will move over Western Washington today and remain over the area through Saturday night. Upper level ridge building offshore Sunday with Western Washington on the backside of the ridge through the middle of next week. && .SHORT TERM /TODAY THROUGH SUNDAY/... Satellite imagery shows mostly cloudy skies over Western Washington early this morning. Doppler radar has widely scattered showers mainly over the northern portion of the area. Temperatures at 2 am/09z were in the mid 40s to lower 50s. Main story for the next two days is the very cool upper level trough over Western Washington. The trough will move over the area today and remain into Saturday night. Temperatures aloft very cold for this time of year with 500 mb temperatures dropping below -30C tonight into Saturday. Even with plenty of cloud cover and little daytime heating the very cold air aloft will create unstable conditions especially this afternoon through Saturday. Lifted indexes in the plus 2 to minus 2 range, convective temperatures both this afternoon and Saturday afternoon below the predicted high temperatures, lapse rates with the very cold air aloft in the plus 7 to 8C/km. All this adds up to showers through Saturday with a chance of afternoon and evening thunderstorms. Snow levels, already low near 4000 feet this morning, lowering to 3000 to 3500 feet tonight and Saturday. Could see a couple of inches of snow at the summit of Stevens Pass while higher up Paradise on Mount Rainier looking at 4 to 8 inches of new snow the next two days. Highs in the lower to mid 50s today and mid to upper 50s Saturday. Highs today will only be a couple of degrees warmer than record low maximums. Lows tonight in the upper 30s to mid 40s. Upper level trough weakening Saturday night and moving out of the area Sunday. Convective activity coming to an end Saturday evening. A few breaks in the clouds cover will allow temperatures to drop into the mid 30s in the colder locations Sunday morning. Lows in the upper 30s to mid 40s for the remainder of the area. Upper level ridge building offshore Sunday. Plenty of low level moisture over Western Washington combined with light flow in the lower levels will make it hard for the sun to break through especially in the morning. As the marine layer thins in the afternoon some sunshine will get highs into the upper 50s to mid 60s. Felton && .LONG TERM /SUNDAY NIGHT THROUGH THURSDAY/... Models in good agreement with the upper level ridge remaining in place at least through Tuesday. Differences in the model solutions beginning Wednesday with some solutions moving the ridge inland over Western Washington while other solutions keep the ridge axis offshore. Either scenario keeps the weather dry over the area. The ridge axis moving overhead would result in a little warmer days. For now with the uncertainty will keep highs just a couple of degrees above normal Wednesday and Thursday, in the mid 60s to lower 70s. Highs Monday and Tuesday in the 60s. Lows in the 40s through the period. && .AVIATION... Mostly westerly flow aloft into tonight with perhaps a slight shift to southwesterly in the late morning and early afternoon as an upper level trough begins to dig into the area. Flow will shift more northwesterly by Saturday morning as the trough axis passes east of the Cascades. Surface winds remaining southerly for most terminals although HQM and CLM are reporting more westerly winds. Speeds this morning are varied, as some locations have gone light and variable, however majority of terminals reporting speeds 8-12 kts. Like yesterday, these speeds are expected to pick up in the late morning/early afternoon generally ranging 10-15 kts with gusts up to 25 kts. The only exception here may be CLM, as they may be shielded from the southwesterly winds by the Olympics...so their speeds may remain fairly uniform to what is being observed at the time of this writing. Majority of stations reporting VFR conditions save for PAE where a PSCZ is producing persistent precipitation and thus lowering cigs down into MVFR. SEA and BFI likely will not see the precip, however low clouds have nestled in here too with MVFR conditions also present here. Models suggest that cigs should see some improvement a little later in the morning, getting these sites back up to VFR with the rest of the crowd...yet none of them are handling the current conditions well. May split the difference on the 12Z forecast, allow for some improvement, but keeping these two terminals in a high-end MVFR scenario. As showers become more widespread with the incoming trough, any terminal or location under falling precip will likely see cigs dip into MVFR. Latest progs suggest some recovery after 22Z, which may nudge conditions back up into low-end VFR, except for those locations more prone to lower cigs /HQM, maybe OLM and PWT/. KSEA...MVFR conditions in place. While some improvement may be possible this morning, not sure cigs lift enough to get the terminal out of this category. See above ceiling discussion for more detail. Southwesterly winds throughout the TAF period with speeds 8-12 kts this morning, becoming 10-15 kts with gusts to 25 kts this afternoon and early evening before easing tonight. 18 && .MARINE... Onshore flow will continue over the weekend which will allow for daily pushes through the Strait of Juan De Fuca. However, given the more southwesterly flow of winds...they may not see such a strong push today...that remains on track for Saturday though. These southwesterly winds however do look to trigger advisory level winds for the Puget Sound for much of the day today before easing by this evening. As such, an SCA has gone out with the morning forecast package. Near-shore seas will steadily increase today, likely topping out at 7 to 9 ft. The outer waters however will see more significant increases, reaching 9 to 12 ft late tonight and persisting throughout much of Saturday. As such, have issued an SCA this morning to cover this high seas threat. Seas might be slow to regress, but they are expected to slowly ease back down to 7 to 9 ft Sunday morning. 18 && .HYDROLOGY... The daily hydrology discussion has ended until the start of the next rainy season; it will only be updated during this time as needed. && .SEW WATCHES/WARNINGS/ADVISORIES... WA...None. PZ...None. && $$
NWS SEW Office Area Forecast Discussion
Discussion Script by Ken True at Saratoga Weather - WFO Script by SE Lincoln Weather
