The first ingredient one needs for thunderstorms is the potential for vertical instability, whereby if an air parcel gets pushed upwards, it will keep on going, rather than fall back to its original elevation. A good measure of the potential instability and the ability of an air parcel to convect upwards when pushed upwards is called CAPE (Convective Available Potential Energy). Knowing this term will not only impress your friends and endear you to meteorologists, but is useful as well.
Around the Northwest during summer, CAPE values are generally low (zero to one hundred) most of the time. Why? Because of the dense, cooler marine air at low levels, modest lapse rate (change of temperature with height aloft), and relatively dry air in lower troposphere (no warm, hot Gulf of Mexico air for us!). In contrast, in the Midwest CAPE values often get to 2000-3000 and sometimes much higher (4000-5000). That is why they get a lot of big thunderstorms and we don't.
But today and tomorrow our CAPE values will surge. Let me show you. Now Midwest meteorologists might laugh at this, but our CAPE values get to around 1200-1400 both afternoons.
All we need is some lift to release the instability. The classic way around here is to have an upper level trough (low pressure) along the coast, with southwesterly flow in the mid to lower atmosphere. The 500 hPa (roughly 18,000 ft) chart for 5 PM today shows we have this feature (although not a particularly strong trough). But good enough to get some action.
Looking at the Cascades at 9:30 AM, there is only some shallow cumulus, but we should expect dramatic changes during the next 6 hours if the models are correct. I am heading out to do some biking with friends in the eastern suburbs of Seattle, perhaps I can do some storm chasing as well.