FXUS66 KSEW 171642 AFDSEW Area Forecast Discussion National Weather Service Seattle WA 942 AM PDT Sat Aug 17 2024 .UPDATE...There are a few showers and isolated thunderstorms that developed over south Lewis county early this morning. These storms/showers will continue to drift into portions of the Olympic Peninsula and dissipate later this morning. Active weather is still expected later on this evening, with widespread thunderstorms and showers expected to develop early this evening through tonight. && .SYNOPSIS...A strong upper level low will deepen off the coast of Washington today, and will swing a shortwave trough across the region tonight. A complex of showers and thunderstorms is expected to develop in Western Oregon this afternoon, and move into Western Washington during the evening and overnight hours. A few of the storms may be severe, especially in the Southern Cascades. The unsettled weather pattern will continue into next week, with additional chances for showers and thunderstorms Sunday, and Tuesday through Thursday. && .SHORT TERM /TODAY THROUGH MONDAY/...A strong synoptic weather pattern is taking shape today across the region. Upper level analysis shows a strong upper level low digging off the coast of Washington and Oregon over the Pacific, deepening as it tilts negatively to the northwest. A jet max on the east side of the low will move up from California into Washington today ahead of an embedded shortwave trough that will swing around the low. Dynamics are impressive with strong vorticity advection in the left exit region of the jet max. The cool temperatures and moisture advection aloft will help prime the atmosphere for active weather later today. Daytime heating will help destabilize the atmosphere, as temperatures climb into upper 70s and low 80s today. Moisture will also be present down at the surface, with dew points climbing into the upper 50s to low and mid 60s. The 00Z HREF ensemble has areas of 1000-2000 J/kg of CAPE in the Cascades, Olympics, and around 500 J/kg in the Southwest Interior. Sounding profiles show the classic "inverted v" shape, with capping in place until Saturday afternoon/evening (after daytime heating). The shortwave trough is expected to fire a complex of showers and thunderstorms this afternoon over the Oregon Cascades. Hodographs/SRH favor storm motion to the north, which will bring the storms into Washington by 5 PM along the Washington/Oregon border. They will reach Puget Sound between 7 and 10 PM, and the Canadian Border around midnight. Storm mode is favored to be in clusters/segments, spanning from the Cascades to the Washington coastline. Additional pre-complex isolated storms are possible in the South Cascades/Olympics mid afternoon, but confidence is low. SPC has placed far eastern Lewis County in a slight risk for severe thunderstorms Saturday, and a marginal risk east of a line from Toledo to Puyallup to Gold Bar. WPC has also placed a marginal risk for excessive rainfall for all of Western Washington today. The strongest potentially severe storms may produce damaging winds of up to 60 mph (based on strong DCAPE of 1200 J/kg), hail up to quarter size (particularly in Cascade areas south of Paradise where lapse rates reach 7.7 deg/km), and heavy rainfall (PWAT anomalies up to 1 inch). Remaining storms will likely contain smaller hail and gusty winds. Rainfall rates may approach a quarter of an inch per hour with the strongest storms. If the storms entrain over same areas over any length of time (storm totals expected to be 0.50 to 0.75 inches), it may be enough to trigger flash flooding/debris flows in the Cascades, especially over burn scars (see hydrology section). Persons in Western Washington should stay tuned for possible warnings later today, and have a plan of action if you are under one. Sunday/Monday: Storms will exit into B.C. Canada Sunday morning. The low will begin to track northeast towards B.C. Sunday into Monday. As it slides up, there will remain a chance of isolated thunderstorms Sunday morning into the afternoon (best chances in the Olympics). Flooding/severe weather is not expected with any new storms that develop Sunday. Some sun will make its way back into Western Washington Sunday afternoon, with more clouds on Monday, and a slight chance of showers for the coast/Olympics and North Cascades. High temperatures Sunday and Monday top out in the low to mid 70s (60s on the coast). .LONG TERM /TUESDAY THROUGH FRIDAY/...Ensemble/deterministic models have strong agreement of troughing off the coast moving inland Wednesday into Thursday. This will keep the pattern unsettled through mid week, with the chance of showers Tuesday through Thursday. There is a slight chance of isolated thunderstorms Tuesday afternoon for a majority of the coverage area, and in the North Cascades/Bellingham area Wednesday afternoon. No severe weather is expected at this time with these storms. Friday will be the driest day in the extended, with partly sunny skies. High temperatures will remain in the low to mid 70s through this period (60s along the coast). HPR && .AVIATION...Low pressure remains centered offshore, which maintains generally southwest flow aloft. Generally clear skies over the area with passing high clouds, but low stratus remains just offshore and likely returns to the coastal terminals for a few hours around 12z. Otherwise, attention turns to the next disturbance that will lift north this evening and overnight, providing the trigger for scattered thunderstorms to move from south to north. Strong storms will feature abundant lightning, gusty outflow winds, and low ceilings/visibility in heavy rain. Expect the prime threat starting by 00z in the south and ending early Sunday morning by 12z, with some lingering isolated showers into Sunday. In addition to the direct impacts from thunderstorms with gusty and erratic winds, expect a general increase in gusty southerly winds this evening in association with this disturbance lifting north. KSEA...VFR conditions through the morning. Surface winds north 7 to 10 kt though early afternoon, switching to southerly this evening. Expect scattered thunderstorms to develop after 00z and move north across the terminal around 03-09z tonight. Abundant lightning, heavy downpours bringing IFR ceilings/vis, and gusty and erratic winds are the predominant hazards, though some hail is also possible. Strong thunderstorms taper overnight with some lingering showers into Sunday. Cullen && .MARINE...Low pressure center well offshore with a high over the far northeastern Pacific through the weekend. A passing disturbance will produce southerly winds over most waters and west winds through the Strait of Juan de Fuca through the weekend. Winds generally 15 to 20 kt through early next week, but will need to closely monitor the potential for a push of gusty southerly winds through much of the interior waters starting in Puget Sound and extending through the Strait of Georgia in association with a passing disturbance late Saturday evening and night. This appears like a short duration burst of wind, so will closely monitor the evolution and likely handle this with MWS/SMW hazards rather than a broad SCA given the shorter duration impact. Additionally, scattered thunderstorms will bring gusty winds and frequent lightning, and perhaps some hail as the move from south to north. Seas will hold around 4 to 6 feet over the coastal waters through the early portion of next week and remain dominated by short period waves. Cullen && .FIRE WEATHER...A low pressure system moving over the area will push a fast moving disturbance over western washington today, bringing widespread thunderstorms across the area. 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 rather limited. Even if there is plenty of lightning and multiple fire starts, the conditions will likely 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/Mazurkiewicz/HPR && .HYDROLOGY...The low pressure system that moves over the area Saturday into Sunday will bring significant moisture into the region. Rainfall will be widespread, but total QPF is expected to range from 0.50 to 0.75 inches (locally higher amounts are possible in heavy thunderstorms). 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/HPR && .SEW WATCHES/WARNINGS/ADVISORIES... WA...Flash Flood Watch from 5 PM PDT this afternoon through Sunday morning for West Slopes North Cascades and Passes-West Slopes North Central Cascades and Passes-West Slopes South Central Cascades and Passes. PZ...Small Craft Advisory from 5 PM this afternoon to 11 PM PDT this evening for Central U.S. Waters Strait Of Juan De Fuca-East Entrance U.S. Waters Strait Of Juan De Fuca. && $$