Holly crap that's some awesome thunder outside! But what's up with it? I mean, in uber-technical, kinda hard-to-understand-but-still-impressive terms? When searching for such information, there's only one weather name to turn to: Cliff Mass.
Why the thunderstorms? We start with fairly unstable air above the surface. In fact, you could see it destablizing during the evening last night with the formation of altocumulus castellanus clouds (castellanus indicating "castles" in the sky!).
A very tight closed low has moved into the region, with the center near Olympia that is rotating moisture northward in eastern Washington, eastward across the northern Cascades and NW WA, and then southward over the Olympics. It is also providing some lift. The key is NOT to have onshore flow through depth...that is death for summer thunderstorms west of the Cascades.
The low will move out tomorrow, but the threat of thunderstorms remain...particularly over the mountains and eastern WA.
You catch all that?
Naturally, Mass has a bunch of technical graphics and awesome lighting pictures to help drive his weather-related points home. You should check them out.
Sadly, Mass also reports that temperatures are expected to cool on Sunday and the awesome weather we've been experiencing will see its awesomeness slightly decreased. The good news is, Mass says the happy times and sunshine will be back by mid-week.