Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA
Issued by NWS Seattle/Tacoma, WA
276 FXUS66 KSEW 150937 AFDSEW Area Forecast Discussion National Weather Service Seattle WA 237 AM 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 will persist into next week with a continuation of cooler temperatures and chances of showers. && .SHORT TERM /TODAY THROUGH SUNDAY/...A weak low pressure system will continue to push inland across Oregon and lift northeastward throughout the day today, maintaining cloudy conditions and below normal temperatures in the mid 60s to mid 70s across western Washington. A wave of lofted smoke will be advected northward into the region, but is expected to stay above the surface, resulting in little more than hazy skies. As the low exits the state, wrap around moisture will generate isolated showers over the Cascades later this afternoon and evening. While confidence is low, a lightning strike or two cannot be ruled out on the western side of the Cascade crest. The weakening low will move northward into British Columbia by Friday, maintaining a chance of showers and isolated thunderstorms over the North Cascades. Elsewhere, weak onshore flow will maintain cloudy skies and cooler temperatures with highs across the lowlands peaking once again in the low to mid 70s. A low pressure system will deepen off the Pacific Northwest coast on Saturday, which will bring the next round of weather to western Washington. Moisture will surge inland Saturday afternoon and evening, generating widespread showers and thunderstorm activity that will linger into early Sunday. Instability will increase as a frontal system lifts northward across the state later on Saturday, with probabilistic guidance showing a 15% to 30% chance of thunderstorm activity into the evening hours. Nocturnal thunder also cannot be ruled out with favorable dynamics in place. Latest ensemble guidance suggests roughly half an inch of rainfall across western Washington over the weekend, though this will be highly dependent on where showers set up and where any heavier showers and thunderstorms develop. While forecast models continue to hone in on thunderstorm potential for the weekend, ensembles still show some uncertainty of the track of the upper level low, which could lead to differences in rainfall totals, timing, and thunderstorm chances. As for now, the threat for storms continues to be monitored as new guidance becomes available. .LONG TERM /MONDAY THROUGH WEDNESDAY/...Forecast models continue to favor troughing into next week, maintaining cooler temperatures and chances for lighter shower activity across the region. Temperatures are on track to stay several degrees below normal through the period, giving us a preview of early fall-like conditions. Lindeman && .AVIATION...A weakening block of high pressure will move east as an upper level low and surface low/cold front push through the region Thursday. Flow aloft will be weak during the transition between high and low. The onshore flow along with the front/low will bring cloud coverage into much of the region early Thursday morning, ranging from MVFR to IFR CIGs via pushes through the Strait of Juan de Fuca, and through the Southwest Interior. These pushes will bring stratus into inland areas (from Shoreline to Alger, and as far east as the West Slopes of the Cascades in the north, and from Chehalis up to Bethel/Kent). Lower LIFR is possible closer to the coastlines. The ceilings may not lift until after 19-22Z, but will improve to VFR during and after this timeframe (except the coastline which will improve to MVFR). Winds will become primarily southwesterly at 6 to 10 kt (higher gusts to 15 kt or higher near the Strait of Juan de Fuca and in the Southwest Interior between Shelton and Aberdeen). KSEA...Conditions remain VFR with light north winds as of early Thursday morning. As the front approaches, winds will calm and turn southwesterly at 6 to 10 kt after 12Z. Expect marine stratus to arrive after 12Z. Confidence remains highest in MVFR CIG coverage from 15Z to 21Z, but there remains a 20% chance of IFR CIGs making it into KSEA from the south (this will be very brief if it lowers to IFR). HPR && .MARINE...A weak cold front will move through the region early this morning. While remaining dry across the region Thursday, areas of low stratus and patchy fog have been observed in the coastal waters, and onshore flow will push some of the stratus through the Strait of Juan de Fuca this morning, covering portions of Puget Sound, Admiralty Inlet, and the Northern Inland Waters. The low stratus/fog may reduce outward visibilities at times through Thursday afternoon. Gusty winds up to 25 kt are expected with this push through the Strait of Juan de Fuca today, which will be hazardous for small craft (thus an advisory was issued Thursday afternoon through Friday morning for Central & East Strait of Juan de Fuca). Winds over the coastal waters will remain light and turn northwesterly Thursday afternoon into the weekend. Winds will also be light during this time out of the south in all inner waterways (with west winds in the Strait of Juan de Fuca). There is a slight chance for thunderstorms Saturday afternoon through Sunday morning for inner waters as a strong upper level low moves in. Threat for gusty sustained winds remains low with this system (except for possible gusts from thunderstorms). Seas will generally remain 3 to 5 feet through next week. HPR && .SEW WATCHES/WARNINGS/ADVISORIES... WA...None. PZ...Small Craft Advisory from 11 AM this morning to 5 AM PDT Friday for Central U.S. Waters Strait Of Juan De Fuca-East Entrance U.S. Waters Strait Of Juan De Fuca. && $$