As more people are able to get their bearings and have their electricity restored over a week after the derecho that slammed millions from Chicago to Atlantic City, I keep seeing more questions about what exactly happened, how it happened, and why it looked like it did on radar imagery.
Last weekend I explained why it happened (spoiler alert: the heat wave), and today I'd like to answer two of the big questions I've seen lately:
A) how do derechos form, and
B) why did it look like it did on radar.
The structure of the storm
The technical term for what happened is mesoscale convective system, or MCS. An MCS is simply a line of organized thunderstorms that move in sync with each other.
Many MCSes develop into what's called a bow echo, or a line of thunderstorms that has such strong winds within them that it takes on the shape of an archer's bow.
A derecho is a bow echo/MCS that produces extensive wind damage over very long distances. The "official" word from the National Weather Service is that a bow echo/MCS is considered a derecho when it produces extensive damage over a 240 mile swath. Seems like an arbitrary number, but that's what it is.
The difference between an MCS and an individual storm is subtle at first, but becomes a big deal later on in the storm's life. I'm sure everyone is familiar with how a thunderstorm develops...
Unstable air rises and condenses, creating cumulus clouds. This rising of unstable air, called an updraft, continues to build the cumulus cloud as far up into the atmosphere as it can. The taller and bigger it gets, the more likely it is to be a pretty big storm.
Rain, snow (yes, snow), hail, and ice pellets form in the clouds as the updraft continues to gain strength. Eventually, the weight of the precipitation gets to be too much for the updraft to handle, so it starts to fall to the surface:
As the precipitation falls to the surface and gets more intense, it drags cooler air down through the thunderstorm. This eventually creates what's called a downdraft. As the precipitation gets harder, the downdraft strengthens, which in turn increases the amount of precipitation, and so on.
All that cold air the downdraft drags to the surface has to do something. Since cold air is denser and more stable than hot, moist air, it pools up beneath the storm. This bubble of cold air is aptly called a cold pool. The cold pool is more commonly known as an outflow boundary or a gust front.
This cold pool can do one of two things. It can either
1) kill the updraft and kill the storm, or
2) race away from the storm.
This step is key to the formation of an MCS.
As the cold pool races away from the thunderstorm, it acts as a mini cold front by scouring up surrounding warm, unstable air and forcing it to rise. When it rises it creates more thunderstorms and intensifies the ones that are already there.
This process tilts everything, and that's what keeps the line of storms alive. Thanks to the cold pool, the updraft and downdraft are now tilted, and they don't interfere with each other. The thunderstorms start moving in unison with the cold pool and continue to gather energy virtually unimpeded.
As the cold pool moves across the surface, it starts to go much faster than the air around it. The air in the cold pool creates friction with the air above it, causing a horizontal rolling motion:
If you've ever been in a swimming pool and raced your hand through the water, you probably noticed little tiny whirlpools develop. This is the same concept, but on a much larger level.
This horizontal rolling motion creates what's known as a rear inflow jet, which is essentially a jet of higher winds a mile or two above the surface. The faster the cold pool moves, the stronger the horizontal rotation will get, and the stronger the rear inflow jet will be.
This rear inflow jet usually occurs near the center of the MCS, and causes it to bow out (hence the name bow echo). When the line of storms starts to bow out, either end of the storm starts to curl back because of friction. These curls, known as bookend vorticies, cause vertical rotation (going up and down as opposed to parallel with the ground) through the atmosphere.
This concept is shown in this horribly drawn map. Theoretically, this is what it looks like on radar:
As the updrafts are tilted backwards, they're ejecting precipitation tens of miles back behind the main line of storms. This creates an area called the stratoform region. This area of light to moderate rain can extend up to 100 miles behind the main line of storms.
This stratoform region drags more cold air to the surface, which strengthens the cold pool, which strengthens the rear inflow jet, and so on and so forth. As you can probably tell by now, it's all linked together in a delicately balanced system. This is why derechos don't happen too often -- everything needs to be perfect for them to flourish.
Now, as the rear inflow jet races at speeds sometimes exceeding 100 MPH, it's got to go somewhere. That direction is down. The jet of air hits the updrafts at the front of the line of storms and gets shoved down to the ground:
The result is surface winds of 60-130 MPH, as many of you experienced back on June 29th.
Now, what kills a derecho? If the cold pool gets too far out ahead of the line of storms, it can cut off the updraft. If the updraft gets cut off, there's no more unstable air to keep the thunderstorms going, so they die. Without thunderstorm activity, there's no precipitation to sustain the cold pool, so it dies. Eventually, all wind and precipitation will stop, and all that's left is the damage.
Cold pools on radar
Since I'm a weather geek, I have tons of radar images from the June 29th derecho. Radars don't only pick up precipitation. They can pick up bats, birds, smoke, dust, airplanes, buildings, mountains...pretty much anything that gets in the way of the beam can be picked up.
Interference can be bad sometimes, but it's a great thing for spotting things like cold pools.
Here's a radar image from that evening as the derecho was about to enter Virginia:
If you look at the line, you can see a disparity between the northern end and the southern end. The northern end looks much more healthy, whereas the southern end of the line looks emaciated and falling apart.
The storms kept up with the cold pool on the northern end of the storm, and that's what let them keep their strength and slam into the DC area like it did.
The storms on the southern end of the line died because the cold pool raced so far ahead of the storms that it cut off the updrafts, and killed them off. That thin green line out ahead of the storms is actually the cold pool itself showing up on the radar.
Cool, huh?
The Beak
This is the much talked about "beak" that formed on radar around 1AM on Saturday June 30, just as the derecho was heading out to sea:
If you look at it closely, you'll see a thin line extending south from Atlantic City, NJ. That's the cold pool showing up on radar. You can tell by the storms dying out along most of the line:
What's happening on the north side of the line is that the bookend vortex turned into a mesoscale convective vortex, (MCV) or a small center of low pressure within the line of storms. This MCV acts like most large low pressure centers do -- they rotate counterclockwise and have what appear to be fronts attached to it:
The "beak" is thunderstorm activity forming along the edge of the cold pool, which wrapped itself into the vortex and is radiating out from it like a cold front.
Cool, huh?
I hope that answers your questions. If you want more information about this or other derechos, see the Storm Prediction Center's great "About Derechos" page.
For more info on this and other storms, you can follow me on Facebook, and you can also follow me on my newly minted Twitter feed: @weatherdudeDKos.
If you have any other questions, don't be afraid to ask.
