The question is very broad so I will subdivide it into different parts. However, everything ties into the Pacific Ocean, which essentially acts as a force field against thunderstorms, tropical cyclones, tornadoes, heat waves and polar vortices.

This is a sea surface chart for North America on June 30, 2012, the day after the infamous derecho. It shows that the Gulf of Mexico and Atlantic Ocean are home to very warm waters. Warm water means hot, humid air; and as my poorly-drawn additions show, the Atlantic and the Gulf are a tag-team heat pump for most of the continent. However, the low surface temperatures of the Pacific Ocean, combined with the enormous amounts of energy needed to increase the temperature of water, mean that no matter how much the sun beats down on the ocean, the surface temperatures remain moderate, and as a result you get a cool, westerly flow. It also results in a low amount of moisture, since:

1. Cool water evaporates much more slowly than warm water

2. Cool air has a much smaller capacity to carry water vapour than warm air.

This cool, dry westerly flow is funneled into the PNW by the perfectly-placed jetstream (typical summer position) and high pressure (the East Pacific High); the end result is warm, dry summers with low humidity levels.

This weather pattern is also the reason why thunderstorms are so rare. The westerly flow creates cool air at the surface (since cool air sinks), and the rising warm air from the East Pacific High is trapped in the upper atmosphere. This is the opposite of what you need for convection (warm air at the surface and cool air aloft--a high "lapse rate"). Even then, you also need moisture near the surface, but of course the dominant airflow in the PNW does not have a lot of moisture in it! High pressure kills wind shear, since the jet stream and air mass winds are moving in the same direction (west-to-east). No wind shear means no updraft, and no storms.

There are a few scenarios where this pattern can break down.

• Thunderstorms are most common in the coastal PNW from late April to mid June, when the air can still cold aloft (i.e. backside of a Pacific storm) but daytime heating and/or southerly winds pulling in warmer air from subtropical latitudes resulting in cold air aloft and warm, moist air at the surface.

I have to eat dinner now, but I'll think of some others.

I'm just a layman, but I have a passion for weather and follow Cliff Mass' blog very closely. That's where I get most of my info.