It takes a perfect atmosphere for a tornado to form. Plenty of moisture in the air, instability, strong winds and ultimately, the trigger needs to be pulled to initiate storm formation. There is no better set up in the world than right here in the U.S. More specifically, east of the Rocky Mountains. The moisture roll player is the warm Gulf of Mexico. With mild water temperatures year around, the Gulf of Mexico provides plenty of "fuel" for storm development. All that warm air can travel freely over the land and deep into the Plains due to the lack of "elevated terrain". There is no physical barrier that would wring out moisture or block it from proceeding North or East or any direction until it reached the Rockies.
All that warm air then needs to be lifted. A typical lifting mechanism is simple day-time heating. As the sun breaks through the clouds, it warms the air and causes bubbles of lifting air. Those lifting bubbles then grab the moisture from the Gulf and sends it upwards. A cold front moving through the area will also do the trick. The boundary will help push the warmer, less dense air up and over the colder, more dense air.
So we now have moisture and lift. Next we need to create instability. Those bubbles of air won't continue to rise unless the air surrounding the bubbles is cooler than the bubble itself. That cold air comes compliments of Canada. The still-cool spring air sinks down from the north behind our cold front and interacts with those rising warm bubbles. This creates instability and allows storms to being to form. Those big, towering, white puffy clouds are a result.
Oregon lacks several of the ingredients previously listed. We lack a significant warm body of water to provide moisture. The Pacific Ocean along the U.S. west coast is a cold current body of water. Cool water helps stabilize the air passing over it. Thus, the air coming onshore in Oregon is more stable than the air coming in from the Gulf of Mexico in the South. Coastal and Cascade mountain ranges also pose an issue to the formation of tornadoes. The rugged terrain often disrupts organization of all the ingredients required. Lastly, we just don't have a good source of warm, moist air that can filter in to our area. The most common form of active weather we Pacific Northwesterns' get are summer time thunderstorms. A good southerly push of warm, more moist air from California will typically help spark those storms. I think most of us will take the grey, drippy days instead of a regular threat of severe weather.
The Moore, Oklahoma tornado was just a perfect recipe of weather. The tornado wound up being ranked at the top of the Enhanced Fujita (EF) tornado scale, which is a 5. The EF scale, a re-tooled scale of the original Fujita scale, remains based upon damage caused from the storm. National Weather Service workers will asses storm damage and then use that damage to estimate wind speeds from the twister. A common misconception is that tornadoes are ranked on wind speeds. Hurricanes are classified using their wind speeds. Back in 2007, the NWS implemented the EF rankings, slightly adjusting damage-to-estimated wind speed scale. You'll notice that the new scale does not include winds of 300+ estimated winds but the old Fujita did. It is widely suspected that tornado winds are the fastest winds on earth. The confusion between Fujita and Enhanced Fujita is a bit perplexing but it really shouldn't be. In my mind, they are almost interchangeable but I refer to them using "EF" just to follow along with the NWS.
I had mentioned above that the Moore tornado was officially classified as an EF-5. Since 1950, when the NWS implemented the Fujita ranking scale, 58 tornadoes received the top classification. Moore, Oklahoma made it 59. In fact, of those 59 F/EF-5's, 7 of them have ravaged the state of Oklahoma. That is more than any other state and compromises just over 8% of all F/EF-5 tornadoes in U.S. history. Needless to say, Oklahoma is "Tornado Alley".