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Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA
Issued by NWS Seattle/Tacoma, WA
999 FXUS66 KSEW 180349 AFDSEW Area Forecast Discussion National Weather Service Seattle WA 849 PM PDT Sat Aug 17 2024 .UPDATE...Showers and thunderstorms are currently tracking northward into Puget Sound and W WA. Current radar trends have the activity gradually weakening but frequent lightning continues to be observed. The previous discussion remains below as the inherited forecast remains on track. An updated marine/aviation section is provided also. && .SYNOPSIS...A strong upper level low will deepen off the coast of Washington this afternoon, and will swing a shortwave trough across the region tonight. A complex of showers and thunderstorms are expected to develop and move through Western Washington during the evening and overnight hours. A few of the storms may have the potential to be severe, particularly in the Southern Cascades. The unsettled weather pattern continues into next week, with additional chances for showers and thunderstorms possible. && .SHORT TERM /TONIGHT THROUGH TUESDAY/...An interesting synoptic weather continues to take shape over the Pacific Northwest this afternoon. 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 with S/SE flow aloft. A jet max on the east side of the low will continue to move up from California into Washington ahead of an embedded shortwave trough that will swing around the low for an interesting evening. THIS AFTERNOON INTO TONIGHT: Diurnal heating will continue to boost surface instability this afternoon, mainly over the Cascades and the Olympics - which will help prime the atmosphere for the expected thunderstorm development. Convection is expected to develop over the Oregon Cascades later this afternoon as mid level heights begin to fall. With southerly flow aloft, convection will start to move northward into western Washington by this evening, continuing into the nighttime hours. The main time frame for thunderstorm development and movement across our area is generally 5PM-2AM, but cannot rule out some stray showers and thunderstorms outside of that time frame. Convection will likely start first in Lewis County (5PM-7PM) and generally move northward as the evening progresses. Latest hi-resolution guidance indicates a cluster of storms moving right through Puget Sound between 7 to 10 PM, reaching Whatcom County/North Cascades by midnight. Again, these time frames demonstrate the highest probability of thunderstorm development, but outside any of these mentioned time frames, there is still potential to have storms in the area. 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. DYNAMICS: Mean MUCAPE values from the latest 12z HREF generally show values across Puget Sound ranging from 500 to 1000 J/kg ahead of the convection, with locally higher values southward in Lewis and Pierce Counties. Along with MUCAPE, DCAPE values (downward convective available potential energy) values ranging from 1000-1200 J/kg, the primary hazard from these thunderstorms will be frequent lightning and gusty erratic outflow as winds aloft mix down to the surface. With increased surface based CAPE due to diurnal heating, along with steep lapse rates (7.0-7.5 deg/km), this may promote an isolated severe threat, with storms potentially having gusts > 58mph, and small hail. This isolated severe threat will mostly exist across the southern Cascades, with the potential to drift towards the Cascade Foothills in Lewis & Pierce Counties. This threat can be noted on the latest Storm Prediction Center Day 1 Outlook, where a slight risk is shown clipping the Cascades of Lewis County. It is also worth noting that areas north/northeast from Toledo are in a marginal risk from the SPC (Seattle, Tacoma, Everett, Gold Bar, and North Bend). In addition to the lightning/severe threat, there are concerns 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 (taken from climo of RAOBs at KUIL). Instantaneous rain rates of short duration may exceed 1"/hour (perhaps up to 1.5"/hour) in locally heavier thunderstorms. With that said, due to relatively fast storm motion, 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 continuously entrain over an area, especially over a burn scar, rainfall rates could be enough to trigger flash flooding or debris flows, which we will continue to monitor. Refer to the Hydrology section below for additional information. Also, for fire weather concerns, refer to Fire Weather section. Again, the primary impacts of ANY thunderstorm that develops is frequent lightning, gusty erratic winds, small hail, and heavy rain. Be weather aware, and have multiple ways to get warnings. Those outdoors during this period will want to continue monitor the forecast. If you are outdoors during a thunderstorm, make sure to seek shelter in a building or car. SUNDAY/MONDAY: The low will begin to track northeast towards B.C. Sunday into Monday. As it slides up, there continues to remain a chance of isolated thunderstorms Sunday morning into the afternoon, with the best chances of thunderstorms over the Olympics and western portions of the CWA (with latest NBM probabilities showing 20-25%). Although most areas are expected to remain dry Monday, latest guidance indicates that there may be an uptick in precipitation potential along the Olympic Peninsula given the proximity of the aforementioned 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 and Tuesday. .LONG TERM /WEDNESDAY THROUGH SATURDAY/...Ensemble and deterministic models have strong agreement of troughing off the coast moving inland Wednesday into Thursday. This will keep the pattern unsettled through the week, with the chance of showers and thunderstorms, primarily in the Cascades through Thursday. Cooler and showery conditions will continue into the weekend. Mazurkiewicz && .AVIATION...Showers and thunderstorms are tracking north through the sound from roughly between now and 07Z, diminishing thereafter to more stratiform rain. Along with the showers there will be a southerly windshift with gusts to 30kt possible. Isolated thunderstorm threat continuities Sunday afternoon. KSEA...TSRA observed at the airfield currently. Gusty, erratic winds possible, as well as a southerly wind shift with gusts to 30kt. Thunderstorm threat eases after 07z but still expecting showers through the night. 33/McMillian && .MARINE...A storm system is tracking north through western WA this evening with a southerly wind shift and a brief period of gusty S winds to 30kt possible. A westerly push is also expected through the Strait of Juan de Fuca. Winds turn more SE on Sunday and Monday with low pressure over the outer Coastal Waters. Diurnal western pushes may bring Small Craft Advisory winds to the strait at times (late afternoon and evening hours) the rest of the week ahead. 33 && .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 across the area. Even if there is plenty of lightning and fire starts, the conditions will likely curtail fire growth as wetting rains are likely with these storms. 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...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 until 8 AM PDT Sunday 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 until 2 AM PDT Sunday for Central U.S. Waters Strait Of Juan De Fuca-East Entrance U.S. Waters Strait Of Juan De Fuca. && $$