The thunderstorms hit the Puget Sound area around 1:30 PM...here is a nice radar image at 1:57 AM Saturday. You see a line of heavy showers (reds/oranges), with lighter precipitation (green) following behind.
Tonight may see a repeat. Take a look at the upper level forecast map (at 500 hPa, around 18,000 ft) at 5 PM Saturday. You can see an upper level low over Oregon and a lobe of low pressure, just south of the Oregon/Washington border, that is rotating around it. Those lobes are associated with upward motion that can force new thunderstorms.
In fact, the latest visible satellite imagery at around 3 PM shows thunderstorms building over eastern Washington, as does the radar (see images).
One of the most useful forecasting tools over the next 6-12 hours in the NOAA/NWS High Resolution Rapid Refresh (HRRR) modeling system. Here is the forecasting for 11 PM Saturday of a simulated radar image. You will see a band of thunderstorms moving north across Washington, really reminiscent of last night...so be prepared for more thunder!
Update at 5:30 PM Saturday: lighting is widespread over northern Oregon and in a strong cell over the central Washington Cascades.
Lightning network data (for hour ending 9:20 PM) shows many hits.