On June 6, Coloradans experienced a rare weather event called a "derecho" that linked up thunderstorms crossing the entire state and caused hurricane-strength wind gusts.
We're all familiar with the word "tornado" — this is a Spanish word for "twisted." "Derecho" is Spanish for "straight" — meaning these storms produce winds that blow in a straight line.
To be classified as a derecho, winds have to blow over 58 mph across a 250-mile-long distance and include some hurricane-force gusts over 74 mph. The National Weather Service received 205 wind reports from the June 6 storm; 51 of them were hurricane strength. Wind gusts in Winter Park clocked in at 110 miles per hour.
The winds are caused by thunderstorm cells that perpetuate one another. As heavy, wet air falls and hits the earth, it spreads out, creating a gust front. The gust front helps create the next thunderstorm cell downwind.
"We call them mother cells and daughter cells," said Dr. Rich Wagner, professor of meteorology at Metropolitan State University (MSU) in Denver.
To get a derecho, three components have to occur at once.
First, you need something meteorologists call "instability." Different layers make up the atmosphere from the ground up. If there is high pressure in these layers, they tend to be solid, like a roof over a house, and hard to move up through. But if there are some big cracks in the ceiling and even more cracks in the roof, suddenly it's possible to move from the ground up. This is what meteorologists mean by instability.
Second, you need flow — to get your flow going, you need a "lifting mechanism" to get up to the ceiling.
"When my kids were little, they loved to climb trees — me too. But you go to the tree and the first branch they can get to is six feet up so you need to have a lifting mechanism to get them up to that six feet, but once they're lifted a little ways, it goes up by itself," said Wagner.
The flow for this type of storm doesn't usually occur in Colorado this time of year. We get flows like this in the spring, when it's dry, but not usually in June, when we have more moisture.
Moisture is the third thing derechos need, and this is the biggest reason derechos are so rare in Colorado and generally in the West. We just don't get enough moisture.
Including the most recent one on June 6, Colorado has seen three derechos in the last 30 years. The others were in 2002 and 1994, but those only touched the northwest corner of the state. This was the first derecho to cross the entire state.
The data on derechos is a little sparse before the early 1990s. They may be infrequent, but Colorado is seeing a change in weather patterns, which may be due to the jet stream.
"The jet stream is controlled by temperature gradient and the temperature gradient has been somewhat relaxed a little bit because due to the change in our global temperature — the southern part is getting warmer, the northern part is not cooling as quickly," said Dr. Sam Ng, another professor of meteorology at MSU. “So we’re getting this weaker temperature gradient, and so the jet stream tends to meander more, a lot more.”
"So it seems like we're getting more extreme weather," Ng said.