Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA
Issued by NWS Seattle/Tacoma, WA
443 FXUS66 KSEW 292141 AFDSEW Area Forecast Discussion National Weather Service Seattle WA 241 PM PDT Sat Jun 29 2024 .SYNOPSIS...An approaching front will spread increasing clouds and light precipitation across western Washington tonight through early Sunday. The upper trough follows, with stronger onshore flow and a possible convergence zone developing later Sunday. High pressure will then begin to slowly but steadily strengthen onshore early next week, before moving eastward late in the week. This will maintain much warmer and dry conditions much of next week. && .SHORT TERM /TONIGHT THROUGH TUESDAY/...Mild conditions with increasing clouds across the region as the next front is pushing onshore this afternoon. Expect rain to be generally limited (largely less than a tenth of an inch) as the front breaks apart into the interior, but locally higher amounts can be expected in the mountains. With some instability over the Cascades, there remains around a 10% chance of an isolated thunderstorm around Mt Baker, but this remains a lower probability. Expect rather mild temperatures overnight with widespread cloud cover and another passing impulse through the upper trough will provide another focus for showers early Sunday morning. Increasing onshore flow will bring the potential for a Puget Sound Convergence Zone and additional lingering mountain showers. Otherwise, the start of a shift in the pattern will then emerge by Monday as a ridge begins to amplify offshore. Initially, it`s far enough offshore that we maintain light onshore low-level flow. This will keep temperatures on a warming trend, but only gradually so, and maintain widespread morning clouds each day. By Tuesday night, this onshore flow will begin to weaken. .LONG TERM /WEDNESDAY THROUGH SATURDAY/...Confidence continues to be higher with respect to a strengthening ridge that will likely dominate the pattern across the Pacific NW through much of next week. While there are a small subset of ensemble members (15% or so) that would suggest the potential for a trough to sweep through the region by Friday, the majority feature a ridge somewhere in the neighborhood of the region. While the finer details of where the ridge axis set up may determine the details of just how warm it gets (and if we can pivot around to offshore low-level flow at any point), confidence is increasing for a steady ramp-up in temperatures late in the week. Temperatures near or slightly above normal Wednesday will likely climb to 10-15 degrees above normal late in the week. For now, this would correlate to temperatures in the 80s to around 90 and moderate HeatRisk developing Thursday through Saturday. Along with the potential for increasing heat, this warming and drying trend would lead to a potential increase in fire weather concerns around and after the Independence Day holiday. Cullen && .AVIATION...Flow aloft is becoming southwesterly ahead of a shortwave trough approaching from the west today (ridging also located east of the Cascades). A weak front has begun to produce showers along the coast and the southwest interior. Activity is moving in a north-northeast direction. Additional showers may fire this afternoon along the Cascade Crest and in Puget Sound. The threat for thunder is very low, but heavy showers may reduce CIGs/VIS temporarily and create variable winds. Winds remain variably out of the west, but will solidify out of the south this evening as the trough approaches (6 to 10 kt). High VFR ceiling deck has filled in ahead of the system, and CIGs will gradually lower to widespread MVFR late this evening/Sunday morning. There are increasing chances for IFR/LIFR CIGs/VIS in the form of low clouds/fog Sunday morning, mainly for terminals west of I-5. All ceilings/fog will mostly clear late Sunday morning/afternoon (except for potentially the coast). KSEA...Chance of showers this afternoon (between 02-07Z) from the south. If a heavy shower passes over the terminal, may briefly drop CIGs/VIS to instrumental meteorological conditions. West winds 6 to 8 kt will become southerly early this evening ahead of the front. Increasing chance for impactful IFR/LIFR CIGs/VIS Sunday morning before scattering out. HPR && .MARINE...Trough/frontal system is positioned over the coastal waters this afternoon, and will continue to move inland today into tonight. Showers are moving along the coast, with a few heavier showers possible in some of the inner waterways. Widespread low marine clouds/fog is possible Sunday morning, especially off the coast and in the eastern Strait of Juan de Fuca. The visibilities may drop below a quarter of a mile at times. Pushes down the Strait of Juan de Fuca are expected with high pressure building offshore early next week. The push Sunday afternoon is expected to peak at around 20 kt in the central/east sections, with the Monday evening push expected to exceed this. Strong northwesterlies over the coastal waters may also exceed 20 kt Tuesday into Thursday. Seas will reach 4 to 6 feet today, and may increase to 6 to 8 feet in the outer coastal waters Wednesday and Thursday. HPR && .SEW WATCHES/WARNINGS/ADVISORIES... WA...None. PZ...None. && $$