Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA
-- Remove Highlighting --
Issued by NWS Seattle/Tacoma, WA
-- Remove Highlighting --
-- Discussion containing changed information from previous version are highlighted. --267 FXUS66 KSEW 150941 AFDSEW Area Forecast Discussion National Weather Service Seattle WA 241 AM PDT Sat Jun 15 2024 .SYNOPSIS...-- Changed Discussion --A low pressure system will track inland today, ushering in widespread shower and thunderstorm activity that will continue through the evening along a convergence zone boundary. Troughing will maintain unsettled conditions through the remainder of the weekend and into next week as cooler temperatures and periods of showers persist across western Washington. * THUNDERSTORMS: Thunderstorm activity will spread across western Washington today, with the highest chance (30% to 40%) east and south of the Puget Sound this afternoon. Storms will be capable of producing small hail, gusty outflow winds to 30 mph, and locally heavy rainfall alongside lightning. * FLASH FLOODING: Localized heavy rainfall is favored this afternoon and evening along a convergence zone east of the Puget Sound, with the potential for flash flooding and debris flows over the Bolt Creek burn scar along US-2 in the Cascade Mountains.-- End Changed Discussion --&& .SHORT TERM /TODAY THROUGH MONDAY/...-- Changed Discussion --A broad upper level low will continue to slowly move inland throughout the weekend, maintaining unsettled and cooler conditions through the short-term. Moisture will continue to stream inland this morning with onshore flow, allowing showers to continue as another front approaches the region. Instability will slowly increase as the sun comes up, with high resolution forecast model soundings showing steep lapse rates and as much as 500 to 600 J/kg of CAPE in the early to mid afternoon over the Puget Sound region. Thunderstorms will be possible across all of western Washington into the afternoon as precipitation continues, with storms capable of producing small hail and graupel, locally heavy downpours, and gusty outflow winds up to 30 mph alongside lightning. While most precipitation will shift eastward by the evening, a convergence zone is favored to set up east of the Puget Sound and persist through early Sunday morning. Forecast models highlight the potential for locally heavy rain with the formation of the convergence zone especially over the Cascades. The Bolt Creek burn scar along US-2 remains susceptible to debris flows, so a Burn Scar Flash Flood Watch remains in effect through the evening. The afternoon instability will also be enhanced by cold temperatures aloft, which will allow snow levels to lower to near 4000-4500 ft. The wet bulb effect may even allow snow levels to drop as low as 3000 ft in heavier showers that develop over the mountains, but confidence is low. The Cascade mountain passes will likely not see any snow accumulation, but the highest peaks will see light snowfall amounts through early Sunday. Cloud cover will stick around for most of the day today, limiting high temperatures across the lowlands to the upper 50s and low 60s. Wet and buoyant conditions will continue on Sunday as a cold front drops southward across the region, spreading another round of showers and embedded thunderstorm activity southward throughout the day. Instablilty will decrease on Sunday as the trough axis shifts further southeastward, with ensembles showing CAPE values peaking at 200 to 250 J/kg south of the Puget Sound. Temperatures Sunday will warm a couple degrees from today, peaking around the 60 degree mark for most of the lowlands. Upper level troughing will finally exit the region on Monday with moist northwest flow filling in behind. A shortwave disturbance passing over the Pacific Northwest will stir up continued light shower activity over higher terrain as conditions start to dry out across the lowlands. Temperatures will warm into the mid 60s with snow levels rising above 6000 ft, diminishing any additional snowfall over the mountains. .LONG TERM /TUESDAY THROUGH FRIDAY/...Ensembles highlight a warming trend through the rest of the week, with temperatures returning to typical mid June conditions near 70 degrees by Tuesday and peaking well above normal near 80 degrees by Thursday. Forecast models have also come into slightly better agreement over weak flow throughout the week, with any chance of light shower activity favored over the Cascades each day. Lindeman-- End Changed Discussion --&& .AVIATION...-- Changed Discussion --Southwest flow aloft early this morning becoming more westerly by this afternoon as an upper level low begins to enter W WA. As the upper low moves south and east, could see NE winds aloft Sunday morning. Generally southwesterly winds at the surface will become zonal with speeds increasing to 10 to 15 kts in the afternoon with occasional gusts up to 25 kt possible. VFR conditions in place over the majority of the area this early morning, although some isolated spots of MVFR have materialized over the San Juans, near HQM and near PAE. Cigs will continue to deteriorate throughout the morning hours with most terminals getting into MVFR by mid-morning...between 14-16Z. Atmosphere starts to destabilize by late morning, with highest instability stretching from HQM eastward through the south Sound, then along the I-5 corridor through SEA and possibly as far north as PAE. Will likely use this area to limit any potential TS mention in 12Z TAFs and eliminate any TS after 03Z. It should also be worth noting that confidence will likely not exceed PROB30 for most terminals, however could see to nudge SEA and BFI up to TEMPOs for the early afternoon period. One plus to the instability will be lifting cigs, returning VFR conditions to the area between 18-20Z. Precip activity expected to start winding down after 03Z tonight. KSEA...VFR conditions in place for much of the morning with any precip well of to the west at the time of this writing. Terminal should squeak through the AM mostly dry, although some showers may pass to the immediate south...so VCSH continues to seem appropriate. As discussed above, should see instability peak starting around 19Z and thus will likely nudge up PROB30 from inherited TAF to TEMPO. Most activity should shut down by 03Z or slightly after. Winds remain generally southerly ranging 10-15 kts, however may nudge upward to 15-17 kts this afternoon with gusts up to 25 possible. Speeds should ease after 03Z down to approx 8-12 kts. 18-- End Changed Discussion --&& .MARINE...-- Changed Discussion --Broad surface troughing will develop across the waters through midday today before ridging gradually rebuilds over the coastal waters this evening into Sunday as lower pressure remains over the interior. This will increase onshore flow with gusty winds exceeding 20-25 kt this afternoon and evening, especially in the central and eastern Strait...where a Small Craft Advisory will go into effect at 2 PM PDT this afternoon through 8 PM PDT tonight. By early/mid next week, broad surface ridging will develop over the coastal/offshore waters, with lower pressure east of the Cascades. This will maintain onshore flow with westerlies reaching small craft advisory strength in the central/east strait daily in the late afternoon and evening. Otherwise, fairly quiet conditions are expected across area waters. 18-- End Changed Discussion --&& .HYDROLOGY...-- Changed Discussion --Widespread showers and thunderstorms will continue to spread across the region today, with convergence zone showers favored to set up this afternoon and evening east of the Puget Sound and continue through early Sunday morning. Locally heavy rainfall is possible with these showers, and forecast models continue to show the potential for 1 to 2 inches of rainfall over the Cascades east of the Sound. The heaviest rainfall is expected to occur between noon and 9 PM before rainfall rates weaken. While this storm system is not particularly moist, the convective nature of these showers could produce rainfall rates that meet or exceed flash flood trigger thresholds. A Debris Flow Flash Flood Watch remains in effect through the evening for the Bolt Creek burn scar, where excessive rainfall may cause debris flows that could impact US-2.-- End Changed Discussion --&& .SEW WATCHES/WARNINGS/ADVISORIES... WA...None. PZ...None. && $$