Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA
Issued by NWS Seattle/Tacoma, WA
738 FXUS66 KSEW 020958 AFDSEW Area Forecast Discussion National Weather Service Seattle WA 258 AM PDT Tue Jul 2 2024 .SYNOPSIS...High pressure begins building offshore with northwest flow aloft over western Washington today. This upper ridge will strengthen significantly and move east over the region later this week, ushering in a very hot and dry weather pattern late this week. This pattern may remain in place through early next week, maintaining an extended period of very hot conditions. && .SHORT TERM /TODAY THROUGH THURSDAY/...Upper level high pressure remains centered well offshore this morning, maintaining northwest flow aloft across western Washington. Meanwhile, onshore low-level flow continues to support the expansion of low stratus along the coast. Some lingering clouds remain over portions of the interior near Puget Sound after a few showers developed in an area of weak convergence. With morning clouds in place and the residual onshore component to the wind continuing today, expect rather seasonable temperatures that may run a few degrees cooler than yesterday for most. Not much change to the pattern into Wednesday with temperature again bumping up a few degrees warmer as the offshore ridge strengthens and we see less in the way of cloud cover for the morning that day. A more significant shift in the pattern is in store starting around the Independence Day holiday for the region as the ridge of high pressure offshore begins to shift eastward toward the region. This will bring afternoon temperatures into the upper 70s to mid 80s across much of the interior and signal the start of several days of well above normal temperatures. .LONG TERM /FRIDAY THROUGH MONDAY/...Strong agreement in the ensemble guidance continues to boost the likelihood of very hot temperatures through the long term period with several days reaching the Moderate category of HeatRisk. The high amplitude upper level ridge will likely remain parked over the region into the weekend, with much of the differences in the ensembles only in the precise amplitude and/or placement of the ridge axis, but there`s very strong support for the strong ridge in place in the vicinity of the region. Saturday and Sunday look to the be hottest day for most, with better than a 60% percent chance for the central Puget Sound region eclipsing the 90 degree mark (and higher from Olympia south along I-5 into Lewis County and eastward into the Cascade valleys and foothills). Hedged the forecast a bit closer to the NBM 50th percentile from the deterministic NBM for the interior for the Friday to Sunday time period, which did result in a slightly hotter forecast for many locations. Depending on the overnight low temperatures, it`s even possible for some locations to have a day or two in the Major level of HeatRisk late in the weekend if any significant relief from onshore flow is shut off. It`s notable that there are some fairly large spreads in the potential outcomes by Sunday and Monday, as much will depend on the precise placement of the ridge axis and the degree of offshore flow that develops. Beyond the heat related concerns, expect that daytime relative humidities will steadily move closer to critical thresholds, resulting in the potential for increased fire weather concern (especially in proximity to the Independence Day holiday). Cullen && .AVIATION...Upper level ridge centered offshore will continue to build into the region today with northwest flow aloft. The low level flow remains onshore. MVFR ceilings in stratus remain confined to coastal areas this morning and this is expected to persist through the morning. Areas of stratocumulus between 3500 and 6000 feet will remain across the interior lowlands this morning as the result of remnant convergence. Ceilings along the coast are expected to briefly lift to VFR after 20Z while VFR ceilings across interior areas scatter out this afternoon. Onshore flow will lead to the return of low MVFR in stratus along the coast and locally inland tonight and Wednesday A.M. KSEA...Broken stratocumulus deck between 4000 to 6000 feet expected to linger through the morning before scattering out this afternoon and evening. Presently thinking there`s about 50/50 chance of MVFR ceilings redeveloping again Wednesday A.M. between 12Z and 16Z. Surface winds light S/SW veering once again to W/NW this afternoon. 27 && .MARINE...Summer pattern taking shape this week as broad surface ridge takes up residence over the offshore waters with lower pressure across the interior. This will keep onshore flow in place with westerlies likely reaching small craft advisory levels in the central/east strait each of the next couple afternoons/evenings. The strong surface ridge offshore will also lead to some gusty northwest winds over the outer coastal waters. The fetch of NW winds will generate some seas of 8 to 10 feet tonight into early Wednesday. Thermally induced low pressure expanding northward across the interior Thursday into Saturday will lead to lighter onshore, or occasionally weak offshore, flow. 27 && .SEW WATCHES/WARNINGS/ADVISORIES... WA...None. PZ...Gale Warning until 5 AM PDT early this morning for Central U.S. Waters Strait Of Juan De Fuca-East Entrance U.S. Waters Strait Of Juan De Fuca. Small Craft Advisory from 5 PM this afternoon to 3 AM PDT Wednesday for Central U.S. Waters Strait Of Juan De Fuca- East Entrance U.S. Waters Strait Of Juan De Fuca. Small Craft Advisory until 5 AM PDT early this morning for Admiralty Inlet-Northern Inland Waters Including The San Juan Islands. Small Craft Advisory until 11 AM PDT Wednesday for Coastal Waters From Cape Flattery To James Island 10 To 60 Nm- Coastal Waters From James Island To Point Grenville 10 To 60 Nm-Coastal Waters From Point Grenville To Cape Shoalwater 10 To 60 Nm. && $$