Why is severe weather so rare in the Pacific Northwest?
As active as the weather pattern is expected to be Thursday, it still won't hold a candle to what those in the Midwest and East Coast experience during their severe weather. But the fact that we're even talking about potentially severe thunderstorms in the region is a testament to the uniqueness of the current weather pattern.
Let's start with what you need to get severe thunderstorms in the first place - it's a variety of ingredients: Warm, moist air on the ground, a huge difference in temperature (i.e., much colder air) as you gain altitude, and some wind shear helps as well. And then you need a trigger to get the convection (thunderstorm development) process started.
Warm air can hold more moisture than colder air, so when it's warm and muggy - it's plenty of fuel for thunderstorms. Colder air aloft means that warm parcel of air can rise higher in the atmosphere, giving more size to the storms. The greater the temperature difference, the stronger the updrafts. But those updrafts come down at some point, in turn creating strong downdrafts, which can lead to heavy rains and a burst of wind at the surface. A bit of wind shear (as in increasing wind with height) will angle the storm so that the storm's updrafts and downdrafts can function without interfering with each other, allowing the storm to have greater endurance and sustained development.
In the Midwest -- spring is such a severe weather season because you have warm air from the Gulf of Mexico heating the surface clashing with cold air still leftover from winter sliding down from Canada, thus creating massive temperature differences. The incoming fronts will provide the lift to start the convection process.
But around here, the Pacific Ocean moderates everything. For one, it keeps our surface temperatures cool, and storms coming in off the Pacific are moderated from being over 55 degree water than cold, interior Canadian locations. Bottom line: We don't get those dramatic differences in temperatures between the ground and the upper atmosphere, an even when we do (like, a very cold Alaska low wanders this way), our surface temperatures are cooler so the air doesn't hold as much moisture, and there is not as much fuel to feed the storms. We also usually have to rely on the mountains to provide the lift since otherwise, the atmospheric dynamics are too weak to trigger the storms by itself. (That said, the mountains do a pretty good job and that's why the bulk of our region's few thunderstorms hit the Olympic and Cascade foothills.)
So this is why thunderstorms are rare and when we do get them, it's usually due to bare minimum atmospheric requirements and they end up relatively tame -- just not as much fuel; the storms don't go as high and any lightning and hail is fairly weak. To my knowledge, the Seattle area has never had a Severe Thunderstorm Watch and just a few isolated Severe Thunderstorm Warnings over the year.
Yet Thursday's set up is unique: We have the warm, humid air in place, brought in earlier this week via the jet stream that tapped into tropical air near Hawaii (sort of like a dry Pineapple Express) -- there's our fuel on the ground. Then we have an approaching cool trough of low pressure of the Pacific -- not too unlike the zillions of lows we've had this cool, soggy spring -- providing the cold air aloft. And boom: You have a large difference in temperature, plenty of moisture, and a mechanism (the approaching low) to provide the lift to light the fire. It's a very unique situation where a lot of conditions have to be right at the right time (kind of like our snow forecasts) and it appears we've somehow managed to check off all the boxes Thursday.