SEATTLE - We had some cool air move in at high altitudes Thursday night combine with our hot daytime weather. Since warm air rises, and with cold air moving in above, it allowed that warm air to rise very high and result in towering thunderstorms.
Also a factor was the Cascades, which added lift to the air as our east winds ramped up the eastern slopes.
These types of thunderstorms aren't too uncommon -- they just typically blow into Eastern Washington with our normal westerly winds. But since the winds were from the east, they blew them west over the Seattle area.
Why so much lightning? It helps that the air was generally dry. Just like how when you scuff your socks on a carpet and then shock someone or something metal when you touch them, that shock is more prevalent when it's drier outside than when it's wetter.