Get ready for some terms!
Last Monday, a derecho crossed the upper midwest. A derecho is defined as a line of storms with a bowing effect reaching winds of at least 60 mph (but often 80 to 100 mph) which produces damage swaths at least 60 to 400 miles long.
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Here’s how Exact Track Radar saw the derecho bowing from Milwaulkee (south of Green Bay) across Indiana and Illinois into St. Louis. Look at all those thunderstorm warnings!
The National Weather Service defined the event with this tweet:
And a derecho can cause what’s known as a “meteotsunami” when moving across a body of water, like Lake Michigan!
So what is a meteotsunami?
You’ve probably heard of a tsunami -- a wave generated when an earthquake occurs in the ocean causing displacement of water. That displacement in turn causes a wall of water to move toward the coast. You may recall the devastating tsunami in Thailand in December 2004. But the key here is that the cause occurs in the water, below the surface, and that is the difference from a meteotsunami.
For starters, the term meteo derives from the Greek “metéōros”, meaning “thing high up there” specifically in the air. Think meteor, a thing high up in the air! Or meteorologists, those of us who study what’s happening in the air (well, as long as it’s weather).
The bottom line is that a meteotsunami is created over water when air pressures drop suddenly. And while a strong line of storms, like a derecho, plays a role, these are not just wind-driven waves. The air pressure disturbances are what defines a meteotsunami differently than other phenomenon (like seiches, which are mostly wind-driven). Here is Exact Track Radar showing the intense low pressure right over Lake Michigan on Monday:
Of course, the danger here is the waves, and while some meteotsunamis can be rather tame, this one warned for 6′ waves in Lake Michigan. The highest recorded meteotsunami wave happened in Croatia in June 1978, as waves reached 19.5′ crossing the Adriatic Sea to the coast.
In other news, we may actually have a cold front next week! Hopefully, I’ll be blogging about that on Friday!
Frank