About 80 percent of the summer rain between the Rockies and the Appalachians comes from huge clusters of thunderstorms known as "mesoscale convective complexes" or "MCCs." During the summer of 1993, MCCs went wild with one following on the heels of another without the usual breaks. During July 1993 rain fell on many places 16 to 22 days, compared with the month's normal eight or nine wet days. Many of these MCCs were also wetter than normal, bringing up to 12 inches of rain in 24 hours. The relentless MCCs of 1993 were an extremely rare event. Nothing like them had been seen since records began in the mid 19th century.
During normal years Midwest farmers love MCCs because they bring needed rain during the height of the growing season. A pilot who is also a farmer has to have mixed feelings because MCCs can make flying dangerous, especially for pilots who don't realize how complex and long-lasting these systems can be.
What does the name mean and what are MCCs? Meteorologists use the term "mesoscale" for weather systems that are up to a couple of hundred miles across. "Convective" refers to the up and down movement of air that characterizes thunderstorms. "Complex" is part of the name because MCCs are more than just a bunch of thunderstorms in the same area. Instead, interactions among the individual storms build an MCC into a long-lasting system that affects not only the area it covers, but also the surrounding atmosphere.
An MCC can bring thunderstorms to hundreds of square miles beginning in the afternoon and lasting into early the next morning, with new thunderstorms forming as old ones weaken and die. An MCC usually reaches its peak around midnight. Even after thunderstorms stop forming, the MCC's upper-atmosphere swirl of air can continue to rotate, triggering more thunderstorms that bring the system back to life the next afternoon or evening, usually east of where it began the previous day.
MCCs surely have been forming over the Plains of North America since the end of the last ice age, but meteorologists didn't recognize them as unique phenomena until the 1970s, when weather satellites began sending back images. In pre-satellite days, meteorologists had no way of recognizing MCCs as anything but thunderstorms that happened to spring up over wide areas every few days.
Today, satellite infrared images, which show cloud-top temperatures, are used to identify MCCs. An MCC is a cluster of thunderstorms with an area of cloud-top temperatures colder than -32?C (-25.6?F) covering more than 100,000 square kilometers (38,626.7 square miles), a little smaller than the state of Ohio, for more than six hours. The system also has to be almost circular to qualify as an MCC. In satellite photos the anvil clouds at the tops of the individual thunderstorms merge to create a large blob of clouds that tend to hide the individual storms.
Most pilot weather training materials don't mention MCCs. Pilots can get by without knowing about them as long as they get preflight weather briefings and avoid places where thunderstorms are forecast, says Fred Foss of the National Weather Service's Aviation Weather Center. "If you're a flier, which I am, you should know the best defense against thunderstorms is to stay away from where they are," Foss says. "That's easy to say, but when you're flying a light aircraft it might not be easy to find your away around the convection."
Knowing as much as you can about thunderstorms, including MCCs, helps you avoid flying into an area where dangerous weather is hard to avoid. Knowledge helps you understand what forecasters are talking about. At times you'll see MCCs mentioned in the synopsis part of the area forecasts from the Aviation Weather Center. Forecasters mention MCCs, fronts, and other phenomena to help explain why they're forecasting certain kinds of weather. Pilots who know what the forecasters are talking about will understand what's going on.
Like all thunderstorms, MCCs need a good source of moisture, unstable air, and some sort of triggering mechanism. The moisture comes from the humidity in the air where the thunderstorms are forming and sometimes, especially for MCCs, from winds that bring in more humid air.
When the air is unstable, rising air will continue rising to form the towering cumulus clouds that grow into thunderstorms. As the air rises it cools and the air's humidity condenses into cloud drops, rain, and ice. Condensation releases heat, which encourages the air to continue rising. In other words, the air's humidity is the fuel that powers thunderstorms.
The triggering mechanism, which gives air near the ground the shove needed to make it start rising, can be heating of air near the ground, or a boundary where cool air is pushing warm, humid air upwards, such as a cold front.
When all the conditions are right on a late spring or summer afternoon, thunderstorms can begin forming over the western Plains just east of the Rockies. As the storms grow and rain begins falling, the rain drags cooler air down with it. When the cooler air reaches the ground, it spreads out, moving into the warm, humid air that's already near the ground. As the advancing cool air pushes warm air upward, the rising air forms new clouds that grow into thunderstorms.
"It becomes a self-perpetuating phenomenon," Foss says. "We're talking about a whole lot of thunderstorms that are being generated by the existing atmospheric conditions. Once they get to a certain stage they kind of take on a life of their own. At some point they can influence the environment around them."
Even if the sun's heat was the initial trigger, the sinking cool air from thunderstorms continues providing triggers for new storms even after the sun sets. Foss says that satellites and weather radars, especially the National Weather Service's new Doppler radars, are showing scientists and forecasters just how complicated MCCs are.
A full-blown MCC is a meteorological mix of layers of stable and unstable air and boundaries between cool and warm air that look like miniature cold and warm fronts. This gives MCCs patches of steady rain typical of areas ahead of warm fronts, as well as the heavier but on and off rain of thunderstorms. Frequent lightning often marks MCCs. They can also contain all of the hazards of thunderstorms - icing, hail, dangerous turbulence, and sometimes tornadoes. The worst of this goes on at night with nothing but flashes of lightning to illuminate the sky for any pilot who's flown into an MCC.
MCCs can also create hazards for pilots who are far away from the thunderstorms and clouds. These hazards are often fed by "low-level jets," which are streams of humid air from over the Gulf of Mexico flowing northward into an MCC with winds that can top 60 mph only a couple of thousand feet above the ground. While the jet is roaring overhead, the winds at the surface can be calm. And the sky can be clear with no hint of what's going on aloft.
A pilot who takes off and climbs into a low-level jet could encounter moderate to severe turbulence as the aircraft reaches the boundary between the calm air and the low-level jet. "You shouldn't assume that just because the sky is clear, nothing bad can happen," Foss says. The atmosphere around an MCC, especially to the south, "can be highly perturbed."
Pilots who plan to fly in the south-central U.S. during the late afternoon or night should first look at a satellite photo. A nearly circular area of clouds somewhat to the north means that a low-level jet could be blowing, or will begin blowing.
As an MCC moves eastward during the night with its wind, rain, and lightning, skies might be clear farther east. Clear skies allow the air near the ground to cool. Such cooling makes the air more stable. If the MCC moves into such an area of stable air, its thunderstorms will begin dying.
But, the system isn't necessarily dead. An MCC can create a swirl of air high above the ground that continues moving eastward. Later in the day, as the sun's heating makes the air near the ground less stable, the upper-air vortex created by the MCC can trigger new thunderstorms, and the whole thing begins again.
As scientists have learned more about MCCs, forecasters are doing a better job of predicting them and their effects. But the best forecasts aren't of much use to a pilot who ignores them or fails to recognize the dangers. Some afternoons and nights MCCs are going to make flying dangerous over the central United States. Foss' advice to pilots: "The way to deal with these things is not to be there."
