Weather

Most weather training for pilots focuses on either small-scale weather phenomena, such as thunderstorms and localized fog—or big weather systems, such as fronts that can stretch hundreds of miles and winter storms that bring snow to northern states and rain to places across the South.

When low-level jets die in the early morning, the rain ends and clouds begin to evaporate. The mesoscale vorticity center continues to rotate as it moves east, where it can help a new MCC form the next evening. (Illustration from The AMS Weather Book by Jack Williams. Used with permission of the American Meteorological Society.)

This is like publishing a book titled Airplanes of the World, with chapters on only the small airplanes you’d see at a general aviation airport and jetliners that can carry 300 or more people across oceans. In these cases, a lot of interesting and important weather phenomena—and aircraft—that aren’t small enough or big enough get left out.

Meteorologists use the term “mesoscale” for middle-size weather systems, which range from a few miles wide to maybe 300 miles across. Larger systems, such as extratropical storms and their fronts, are called synoptic-scale events. Smaller weather systems—individual thunderstorms or showers, for example—are microscale.

Mesocale weather varies. While many pilots haven’t heard the term “mesoscale weather,” knowing at least a little about it could help them avoid trouble. Middle-sized weather systems can make life difficult for pilots, especially from May into August from the Rockies to the Appalachians—and in some parts of the world outside the United States.

The American Meteorological Society’s Glossary of Meteorology defines mesoscale weather as “A cloud system that occurs in connection with an ensemble of thunderstorms and produces a contiguous precipitation area on the order of 100 km or more in horizontal scale in at least one direction.” The word “convection” tells you that thunderstorms are involved, and “system” indicates that the thunderstorms are organized, not just a random collection.

Squall lines are only one form of mesoscale weather. They have worried pilots since the early 1920s, when airplanes began regularly flying over the horizon and meteorologists started taking their first, stumbling steps toward aviation forecasts. Today, most pilot weather education includes squall lines (see “Weather: A Line of Trouble,” May 2015 Flight Training).

Other kinds of mesoscale convective systems include polar lows, small but intense storms over polar oceans; lake-effect snow; tropical cyclones, such as hurricanes; and mesoscale convective complexes (MCCs), which are common during warm weather on North America’s Great Plains—but sometimes move east from there.

MCC History. By the 1970s, atmospheric scientists using improved weather radars and satellites were learning in more detail about weather features that had always been around, but weren’t well understood. The discovery of MCCs is a perfect example.

In the 1960s and early 1970s, Robert Maddox was an Air Force weather officer who, along with other forecasters, struggled to predict the large areas of overnight thunderstorms that often form after sunset over the central and northern Plains during warm weather. After he left the Air Force and was working at the NOAA Laboratory in Boulder, Colorado—and studying for a doctorate at Colorado State University in Fort Collins—weather satellite improvements were showing the temperatures of the cloud tops, which are a good measure of cloud heights. The colder the cloud tops, the higher they are. Until Maddox began intensely studying satellite images from over the Plains on rainy nights, most forecasters had thought the storms were random.

“The thing that struck me was the coherent structure of the [cloud top] temperatures,” Maddox says. “It wasn’t a cold blob here, a cold blob there. What really astounded me is how frequently [organized systems of thunderstorms] occurred.”

In November 1980 he published a study in the Bulletin of the American Meteorological Society that described how he had “identified, defined, and contrasted with other types of convective weather systems” a system he called mesoscale convective complexes. Maddox and other meteorologists also used the then-new Doppler weather radars and other data to work out the picture of what’s going on in a MCC.

By definition, a MCC is a nearly circular cluster of thunderstorms with an area of cloud top temperatures colder than minus 26 degrees Fahrenheit that covers an area of about 39,000 square miles—roughly the size of Ohio. They usually begin forming from disorganized clusters of thunderstorms in the late afternoon, and reach their peak in the middle of the night. Low-level jet streams from the south feed warm, very humid air into the complex, supplying the energy that keeps the thunderstorms going.

MCCs normally die down sometime around dawn, when the low-level jet fizzes out. But by then the system has generally developed a swirl of wind, which continues long after individual thunderstorms in the system weaken and die. This swirl, or vortex—called a mesoscale vorticity center—is roughly 10,000 to 15,000 feet above the ground. It continues moving generally toward the east and can trigger a new MCC the following evening.

MCCs are most common on the Plains, often forming just east of the Rockies and moving generally eastward. But they do occur in other parts of the country. As a rule, MCCs don’t bring large tornadoes or exceptionally strong winds, although just about any thunderstorm can spin out a weak tornado or kick up dangerous winds.

One of the major threats they pose to pilots is widespread areas of poor visibility and low ceilings, especially at night, when flying in such conditions is most dangerous.

Today, with the growing understanding of how MCCs form, move, and grow, aviation weather forecasters take them into account. In fact, you will sometimes see MCCs mentioned in forecasts.

As far back as 2006, the National Transportation Safety Board issued a safety alert on thunderstorms, which noted the concern that “terminology often used by meteorologists is unfamiliar to some in the aviation community.” In February 2013 the FAA updated Advisory Circular 00-24C to include such terms as “mesoscale convection complex.” It replaced the thunderstorm advisory circular that had last been updated in 1983. The new AC is available online.

Nevertheless, the 2014 edition of the Pilot’s Handbook of Aeronautical Knowledge does not mention the word “mesoscale” anywhere in its chapter on weather theory.