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Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA
Issued by NWS Seattle/Tacoma, WA
930 FXUS66 KSEW 170351 AFDSEW Area Forecast Discussion National Weather Service Seattle WA 851 PM PDT Fri Aug 16 2024 .SYNOPSIS...An upper low will deepen offshore over the northeastern Pacific on Saturday. A shortwave will move northward across the area Saturday night into Sunday bringing an increased threat for thunderstorm and widespread rain. Unsettled conditions then continue into next week with the upper low offshore before it slides over the area midweek. && .SHORT TERM /TONIGHT THROUGH MONDAY/...Troughing will linger across the Pacific Northwest tonight. Radar shows showers along the Cascade crest continuing to diminish this evening. The forecast remains on track, so no updates are needed. The previous discussion follows below. 14 Upper troughing will then deepen offshore of Washington/Oregon on Saturday, acting as the catalyst for an active weather period, more info below: Saturday Afternoon-Saturday Night: A shortwave impulse located within the aforementioned deepening trough will move inland over Oregon Saturday and slide northward across Washington Saturday night. This shortwave will become more negatively-tilted for increasing S/SE flow aloft. Diurnal heating will boost surface instability, mainly over the Cascades and Olympics, Saturday afternoon. Precipitation has also slowed on model guidance over the last 12-24 hours, allowing for additional surface heating on Saturday, and warmer highs into the 80s. Convection is expected to develop over the Oregon Cascades mid Saturday afternoon associated with mid-level height falls. Southerly flow will then allow for convection to move northward into Western Washington by Saturday evening and continuing through Saturday night. Mean MUCAPE values from 12z HREF across Puget Sound will range 700-1200 J/kg ahead of the convection Sat PM. In combination with dCAPE (downward convective available potential energy) values near 1000 J/kg, the primary hazards from thunderstorms will be frequent lightning and gusty outflow as winds aloft mix to the surface. Furthermore, increased sfc based CAPE due to the diurnal heating, along with steep lapse rates, may promote isolated severe storms (gusts> 58 mph, <1" hail), mostly across the southern Cascades, but also perhaps drifting towards the Cascade Foothills in adjacent areas of Lewis & Pierce Counties. This is noted on the latest Storm Prediction Center outlook introducing a Slight Risk for the Cascades of Lewis County (last slight risk in SEW CWA was 5/30/2020). Thunderstorms will likely begin to slowly weaken Saturday night across northern Washington, with HREF calibrated thunder probs ranging from 80-90% from Seattle southward, and near 50-70% over Whatcom County. In regards to timing, particularly given likely outdoor ongoing weekend events, peak concern for thunderstorms is from 5PM-2AM. HREF probs for 0.01" of precip initially begins in Lewis County generally around 5-7PM, with convection moving northwards into central Puget Sound (including Seattle) between 9PM-11PM, and further north into Whatcom County from 12-2AM. With that said, cannot rule out an isolated shower or thunderstorm developing in the afternoon mainly over the southern Cascades or Olympics. Those outdoors during this period will want to monitor the forecast. If you are outdoors during a thunderstorm, make sure to seek shelter in a building or car. In addition to the lightning/severe threat, concerns also exists for heavier rain rates, particularly over burn scar areas in the Cascades. PWATs, expected to range 1.25-1.50", are well above the 90th percentile for August (from climo of RAOBs at KUIL). Instantaneous rain rates (short duration) are likely to exceed 1"/hour (perhaps up to 1.5"/hour) in heavier thunderstorms. With that said, due to relatively fast storm, 1hr accumulation`s are likely to be closer to 0.25"-0.60" during the heaviest period. Despite the fast storm motion, if a thunderstorm or multiple thunderstorms move over an area, especially a burn scar, rainfall rates could be enough to trigger flash flooding or debris flows. Refer to the .Hydrology section below for additional information. Also, for fire weather concerns, refer to .Fire Weather section. All in all, despite some uncertainty existing in the exact locations of thunderstorms, in addition to the timing of storms weakening, this looks to be an impressive event for Western Washington standards for convection, particularly for areas Snohomish County southward. Sunday-Monday: HREF/guidance suggests convection continues to slide northward early Sunday morning across the BC border. With that said, there also remains a chance of thunderstorms on Sunday. However, probabilities are lower around 10-15% on the NBM. Clouds will likely linger into Sunday morning before some improvement in the afternoon. Temperatures will also be cooler with highs in the mid 60s along the coast to mid 70s in the interior. The upper low continues to linger offshore of Vancouver Island on Monday, allowing for continued onshore flow. Although most areas are expected to remain dry Monday, there may be an uptick in precipitation potential along the Olympic Peninsula given the proximity of the upper low. In addition, a few isolated thunderstorms may be possible across the far northern Cascades. Otherwise, similar temperatures in the 60s and 70s are expected on Monday. .LONG TERM /TUESDAY THROUGH FRIDAY/...Ensembles are generally consistent that the troughing offshore begins to slide over Western Washington Tuesday and Wednesday. Precipitation potential increases during this period, along with cooler temperatures. NBM thunderstorm probabilities also increase a bit during this period (mainly 10-20%) though uncertainty exists on the extent of instability. Ensembles are then in agreement in the reinforcement of troughing offshore over Oregon/California Thursday and Friday. Cooler than normal temperatures are likely to continue for this reason with continued onshore flow across the area. JD && .AVIATION...An upper-low remains offshore bringing southwesterly flow aloft. At the surface, mostly VFR conditions are being met area- wide as marine stratus is gradually lifting and scattering. The trend is set to continue tonight but, can`t rule out lingering patches of stratus promoting IFR/LIFR conditions, particularly for coastal terminals (KHQM, KCLM, etc.) and isolated inland locations (KPWT) into early Saturday morning. Thunderstorms today are to remain in the North Cascades however, they`re expected to become more widespread Saturday evening. A few strong storms may be capable of producing frequent lightning, gusty winds, small hail, and heavy downpours. KSEA...OVC skies with VFR conditions being reported. Skies are slated to trend mostly clear tonight with VFR continuing overnight. SW winds 5 to 10 kt turning NNE at 4 to 8 kt after 01z Saturday before increasing out of the NW between 8 to 12 kt Saturday morning. A 30% chance of thunderstorms are in the forecast Saturday evening. McMillian && .MARINE...1035mb surface high is well offshore but maintains onshore flow here across area waters. Westerlies will persist through the Strait of Juan de Fuca but no genuine threat of SCA are currently expected. Seas will remain at 3 to 5 feet through next week. Marine layer activity may limit visibility at times throughout the next several days with the coast seeing the better chance. Also, a potent disturbance may bring thunderstorms Saturday evening, particularly over the inner waters. A few strong storms may be capable of producing frequent lightning, gusty winds, small hail, and heavy downpours. McMillian && .FIRE WEATHER...A low pressure system moving over the area will push a fast moving disturbance over western washington on Saturday, which looks to bring 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 && .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...Flash Flood Watch from Saturday 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...None. && $$