An upper-level low, or storm system, tracking directly across Western Washington tomorrow will bring chilly air aloft and create an unstable atmosphere capable of producing a few thunderstorms across the region.
While severe weather is not expected, any storm that develops could produce brief downpours, small hail, localized gusty winds, and cloud-to-ground lightning.
As the center of the upper low passes overhead, temperatures a few miles above the ground will drop substantially. When relatively mild late-summer air near the surface interacts with that much colder air aloft, clouds can build rapidly. That enhanced vertical motion can turn an ordinary rain shower into a thunderstorm. For this chance, Thursday is another Pinpoint Alert Day.
The primary focus for thunderstorm activity will be over the higher terrain, but lower elevations south of Puget Sound will need to stay alert as well.
The mountains remain the most favored zone for scattered storm development beginning around midday and continuing into the early evening. Daytime heating combined with terrain-driven lift will help storms develop and intensify.
Farther west, areas across Pierce, Thurston, and Lewis counties, extending toward the Chehalis River Valley, will sit within a corridor of slightly better low-level convergence, where winds come together. A few showers could strengthen enough to produce isolated rumbles of thunder during the afternoon.
Seattle north to Everett and Bellingham will mainly see scattered showers mixed with occasional sun breaks. Still, an isolated thunderstorm cannot be completely ruled out if enough sunshine develops to boost instability.
Activity should taper off fairly quickly Thursday evening as daytime heating fades and the upper low shifts east of the Cascades, setting the stage for a drier pattern heading into the Labor Day weekend.