The Pacific Northwest of North America has been experiencing unprecedented heat these last few days. But “unprecedented” doesn’t even do it justice. Statistically speaking, this is shaping up to be a once in a 1000-year occurrence – and that’s in a normal climate. But our climate is far from normal now due to anthropogenic climate change – these nearly impossible heat waves will not only become possible, but more probable in the coming decades. The PNW alone has warmed by about 3-4 degrees Fahrenheit since pre-industrial times, which is adding intensity to this already strong heat wave.
So, what’s going on? The PNW typically has very mild and cool temperatures. How is it reaching these extreme and record-breaking temperatures? It’s due to something called a “heat dome.” When the summer sun warms air above the ground or ocean, that air can then rush up into the atmosphere to form a mountain – or dome – of slow-moving hot air under higher pressure that blocks new weather systems from moving in. So, essentially, it is a block in the atmosphere (called an “omega block” because it looks like the Greek letter, omega), not allowing the weather to move, while continuing to gather heat.
This dome has been built into a very wavy jet stream, with extreme undulations. When the jet stream — a band of strong wind in the upper levels of the atmosphere — becomes very wavy and elongated, pressure systems can “pinch” off and become stalled or stuck in places they typically would not be. In general, a wavy jet stream slows down the forward pace of weather systems, adding more instability in the atmosphere, resulting in more extreme impacts. Eventually, this heat dome will become too big to remain stable and will release the trapped hot air. But that may not happen for another few days – and, further inland, more areas may experience this extreme heat as well. Prolonged heat is dangerous for people – it will be important to drink water, wear light-colored clothing, sunglasses, a hat – whatever it takes to stay cool. Please stay safe!