The final few days of April played out like weather bingo — deadly tornadoes in Texas, historic rainfall and flooding in the Midwest, a blizzard in the Plains, and extreme heat and humidity closer to home.
Beginning this weekend, the atmosphere will take on an alignment that has been linked to extreme weather events like the devastating heat waves and wildfires in Europe (2003), Russia (2010) and the United States (2011).
We call this setup “wavenumber 6.” The name originates from the six distinct “dips” in the jet stream when you look at it across the entire Northern Hemisphere. These dips are troughs of low pressure, and they are associated with cool and stormy weather.
Taken by itself, a wavy jet stream is not uncommon in spring, when the sun angle is rising. Warm, tropical air interacts with and overtakes what’s left of the cold and dry winter air. The result is giant swings in day-to-day weather. But the pattern emerging this weekend has a troubling tendency.
Pretend you are viewing the image above as you might view a turbulent ocean from the shore. In physics, air is technically a fluid. It moves like water and has waves like water. So not only is it convenient, but it also makes sense to visualize our atmosphere’s behavior as waves in the ocean. In this manner, we can simplify complicated dynamics into the basic facts we know about waves in general.
One such characteristic is the tendency for waves to become standing waves, as on a plucked guitar string, where both ends of the string are vibrating so that the string appears to not be moving at all. Recent research suggests that wavenumber-6 patterns are more likely to develop into standing waves, trapped in a repetitive pattern referred to as planetary wave resonance.
Under this alignment, all forward motion of storms (west to east) becomes stuck in a hemisphere-wide traffic jam, and the probability of extreme weather events increases.
The atmosphere likes traffic jams about as much as you and I do, and the animation below illustrates why.
Notice the stationary nature of the highs (red) and lows (blue) over an eight-day period. Depending on where you live and what type of pressure system sits overhead, you’re stuck with the same type of weather day after day. You can imagine how this might create issues after a few days of persistent rain or unrelenting heat.
The atmosphere wants to get out of the traffic jam. It wants to restore forward motion and balance. That’s the whole point of weather — to balance the energy across Earth.
So it will begin to produce weird conditions to break the traffic jam and return to its normal and preferred west to east flow. This is where our increased vulnerability for extreme weather events comes from. Floods, severe weather, spring snows and even early season tropical storms are all fair game over the next few weeks.
Also troubling is the potential for an increase in this extreme weather pattern as the climate changes. Research published recently in Nature suggests a link between Arctic warming and atmospheric wavenumber-6 patterns. Enhanced warming in the Arctic would lead to a weaker temperature gradient between the tropics and the poles, resulting in more occurrences of a wavy, amplified jet stream capable of producing extreme weather.
Naturally, nothing is guaranteed when it comes to the weather, and this pattern may only result in a damp and dreary early May for both coasts of the United States. But if things start to get crazy, we’ll know why.