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Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA
Issued by NWS Seattle/Tacoma, WA
786 FXUS66 KSEW 161633 AFDSEW Area Forecast Discussion National Weather Service Seattle WA 933 AM PDT Fri Aug 16 2024 .UPDATE...A rather tranquil morning with widespread clouds across Puget Sound with some clearing along the coast. Clouds will clear into this afternoon for Puget Sound for additional sunshine. Temperatures will generally be in the 60s along the coast and 70s for the interior. Active weather tomorrow, more info found in the discussion below. No major updates this morning. && .SYNOPSIS...A complex pattern remains in place with a broad trough across the Pacific Northwest today, while several disturbances are lined up offshore over the northeastern Pacific. An upper low will deepen just offshore Saturday, with an increased threat of thunderstorms and widespread rain over the weekend. Unsettled conditions then continue across Western Washington through the first half of next week as cooler conditions and shower chances prevail in the onshore flow. && .SHORT TERM /TODAY THROUGH SUNDAY/...Not much activity across the local area this morning weather-wise with some stratus forming and lowering across portions of the region. Meanwhile, a line of showers across northeastern parts of the state have built a little westward and a few showers in the North Cascades will be possible through daybreak. Otherwise, modest onshore flow will lead to a gradual improvement in the cloud cover through the day for most and high temperatures again reaching the 70s. The focus for any showers or isolated thunderstorms will once again be the North Cascades this afternoon and evening. A much more active pattern then takes shape for the weekend as a deepening upper low slides southward near or just offshore of the coastline. This will bring an increased threat for thunderstorms across much of the region by mid-afternoon Saturday with the threat continuing overnight. Ensemble guidance continues to suggest probabilities in the ballpark of 20-30% across a large portion of the area including the Cascades and the Puget Sound region. Rather steep lapse rates and the favorable placement of a branch of the jet will enhance the potential, and some elevated instability may lead to a more widespread outbreak of lightning. While confidence isn`t high, there does remain some potential of a few storms reaching severe thresholds, particularly wind gusts. Heavy rainfall will also be a primary hazard with these storms and some hail cannot be totally ruled out either. These storms will have abundant moisture to work with with guidance suggesting precipitable water values around 1 inch and the good news is there should be decent steering to support storm motion in the range of 15-20 kt, which should limit very heavy rain for long durations of time in any particular spot. One exception may be if training storms develop off of the terrain in the south/southeast flow aloft - this could be particularly concerning in the context of recent burn scars. Additional details about fire weather and hydrology related hazards can be found in the sections below. With moving into the time horizon of several of the higher resolution convection allowing models, expect to be able to continue to refine the details around the convective threat over the next day or so. While some of the guidance has suggested the bulk of the action may push through in a more organized feature late in the day Saturday and into the overnight hours, expect that a few isolated storms may linger into Sunday. Another round of activity in the North Cascades will return on Sunday. Cullen .LONG TERM /MONDAY THROUGH THURSDAY/...Looking ahead to next week, ensemble guidance continues to suggest a broad trough across the western U.S. through much of the week. This will maintain general onshore flow and support some shower chances through the week. Expect this to be much less impactful than the conditions over the weekend, but there may be a day or two that again features thunderstorm chances mostly confined to the North Cascades. High temperatures generally remain in the 70s next week with a diurnally driven pattern of morning clouds and increasing afternoon sunshine pending the arrival of any disturbances. Cullen && .AVIATION...An upper level low will deepen off the coast Friday into Saturday, which will increase southerly flow (aided by a jet max on the lee/east side of the low). With no flow in between lows currently (one sits in eastern Washington this morning), some cloud coverage will remain this morning. This is expected to remain VFR/MVFR for most areas. A couple pockets of IFR/LIFR clouds are possible in the Cascades, Kitsap Peninsula, and between Forks and Aberdeen. Fog is not expected (but may be patchy in any areas that get clear overnight). All cloud coverage will lift and scatter by 19- 20z , with a scattered to broken deck remaining from Kitsap Peninsula over to Neah Bay (MVFR potentially still at Neah Bay). Winds will be light out of the southwest today at 4 to 8 kt (becoming light and northerly in Puget Sound late this afternoon). Convection for today is expected to remain in the North Cascades. The threat for thunderstorms increases Saturday night for the entire coverage area. A few strong storms will be capable of producing frequent lightning, strong winds with wind shear near the ground, small hail, and heavy downpours. KSEA...CIGs currently VFR. Confidence is 30% in the potential in seeing MVFR CIGs between 15 and 19Z this morning (most likely will see a scattered to potentially broken 1,500 to 2,000 ft layer during this time). Improvement to VFR will take place in the afternoon with mostly clear skies. Light southwest winds 4 to 8 kt will turn north/northeasterly at 4 to 6 kt after 01Z Saturday. HPR/McMillian && .MARINE...Setup remains onshore with a low in eastern Washington today, and a deepening low dropping down from B.C. Canada throughout the day into Saturday. The next round of pushes through the Strait of Juan de Fuca through the weekend are expected to be much weak (not requiring any advisories). Coastal waters will also see light northerly winds through Saturday. As the low slows offshore early next week, the southerly flow aloft will make it down to the seas, flipping winds to the south Sunday through next week over the coastal waters. Seas will remain at 3 to 5 feet Thursday through next week. Additionally, low clouds are possible up and around Neah Bay this morning that may lower visibilities to any mariners out on the waters. This is expected to increase for Saturday along and off the coastal waters as the low deepens. Thunderstorms will be possible Saturday evening with this system, especially over inner waterways. A few strong storms will be capable of producing gusty and erratic winds, small hail, heavy downpours, and frequent lightning. HPR/McMillian && FIRE WEATHER...The low pressure system moving over the area this weekend could bring widespread thunderstorms. However, given the relatively high fuel moisture and widespread rain expected with the thunderstorms, it is still expected that fire starts due to lightning will be somewhat limited. But even if there is plenty of lightning and multiple fire starts, the conditions will curtail fire growth. With continued cool, cloudy, and high humidity conditions well into next week, holdover fires may only smolder for quite sometime until we get our next hot, dry, or windy conditions. JBB && HYDROLOGY...Impulses coming out of the low pressure trough aloft will bring very slight thunderstorm chances to the northern Cascades near the crest this afternoon and evening. There is a very low probability of flash flooding for that area if a slow moving thunderstorms sets up over the area Friday. The bigger threat comes over the weekend. The low pressure system that moves over the area Saturday into Sunday will bring plenty of moisture as stated above. Rainfall will be widespread, but total QPF has 90% of 0 to 0.25in., ranging to a 10% chance of 1in. to 3in.. It will be fairly unstable with thunderstorm chances having increased up to 75%. If a thunderstorm or multiple thunderstorms pass over an area, especially a burn scar, rainfall rates and/or amounts could be enough to trigger flash flooding and debris flows. JBB && .SEW WATCHES/WARNINGS/ADVISORIES... WA...None. PZ...None. && $$