Beware. The Mesoscale Convective Complex feeds on itself and grows like rapidly-spreading cancer.

Vern Weiss

It is called the Mesoscale Convective Complex and pilots should be keenly aware of the term when it appears in a weather briefing. Of course, all thunderstorms require caution but what makes the MCC so nasty is that it becomes a long-living, slow-moving, self-regenerating system that covers an enormous area of ground.

It wasn’t until 1980 that we even knew about them when meteorologist Robert Maddox identified its characteristics while doing research at the NOAA Environmental Research Laboratory in Boulder, Colorado.¹ Until then Mesoscale Convective Systems were known but primarily in the tropical regions of the world. What made his MCC discovery significant is that it is a product of “the good ol’ USA.”

I am sure we agree that all thunderstorms can be nasty. They all can spawn lots of rain, hail, wind and short-term titillation like wind-shear and tornadoes. With an MCC we cannot even call it a thunderstorm; it is a multiplicity of thunderstorms. If you put a pot of water on a stove top and bring it to a rolling boil, you are watching something analogous to a Mesoscale Convective Complex. As one bubble diminishes, another grows. As that one begins to diminish, another one adjacent to it erupts.

One of the best-known events that was caused by a Mesoscale Convective Complex occurred in 1977 when flash flooding surprised everyone in Johnstown, Pennsylvania and killed 76 people.² In 1985, a Delta Airlines L-1011 got snarled in the grip of wind shear believed to have been associated with an MCC, smashing it into the ground on the approach to DFW and killing 134 people.³

More recently in May 2015, MCCs deluged and clobbered Texas, Oklahoma, Arkansas and Nebraska. Typically 10 to 14 inches of rain fell on concrete bridges that were busted to bits. “It has been one continuous storm after another for the past week to 10 days in several regions of the state,” said Dr. John Nielsen-Gammon, a Texas state climatologist.⁴

Pilots know that all thunderstorms require 3 main ingredients: moisture, unstable air, and a lifting force. It is the lifting force that gets the storm’s engine to start. The four primary means of providing a lifting force are through convection when the sun warms up a parcel of air. Since warm air is lighter than cool air the parcel begins to rise. When it rises high enough the moisture in that parcel begins to condense and there’s your rain. Another means is through frontal activity. The lifting force is provided by the “scooping” action of a front as it is pushed along by the winds rotating around the big “L” in the center of a low-pressure area. By the way, I’ve been flying a long time and seen my share of bad weather but have yet to ever see the “L” at the center of low pressure. One time I was looking up in the sky on a CAVU (Ceiling and Visibility Unlimited) day and I thought I saw the “H” of a high-pressure system but it turned out to be just 3 high altitude aircraft making contrails that crisscrossed.

The third primary type is the nocturnal thunderstorm. A simplification of this one’s description is that it is a variation of the convective type. The sun beats down on the Earth all day, warming up the ground. After sunset, the air cools quickly and then the ground starts releasing its stored- up heat. Warm air rises and the parcel of air adjacent to the ground begins rising. Once it reaches an altitude where its dew point is achieved, the moisture condenses and if the air is unstable the mechanism for a thunderstorm is launched. These typically occur after 10 PM, so don’t ever fly after 10 PM if you want to avoid them.

The fourth mechanism providing a lifting force to unstable and moist air is through orographic means. This is a fancy word that, translated for we who were solid “C” students in school, means hills or mountains.

Now let’s get back to the Mesoscale Convective Complex.

The generation of an MCC is usually detected with satellite infrared imaging. I’m now going to throw a whole bunch of generalities at you. Bear in mind that these are not absolutes; they’re just typical.

Mesoscale Convective Complexes are most often found in the central part of the US but begin with frontal and orographic movement. This is not to say that they don’t occur elsewhere. (remember Johnstown and Delta at DFW?). They generally are strong for 12 hours or more and commonly form in the late afternoon and continue until sunrise the next morning. They typically form when the dewpoint is above 70 degrees Fahrenheit. This last ingredient is particularly savory because a dewpoint above 70 degrees is also considered the trigger for plain, old garden- variety tornadoes. So yes, it should be no surprise that an MCC will be rich in tornado activity.

From a pilot’s standpoint, there are obvious cautions: Wind-shear, heavy rain, high winds, intense lightning, hail and damaging tornadoes; lots of all those things because this is a thunderstorm that covers a wide area and moves slowly, feeding on itself. Even Dr. Maddox (now with the National Severe Storms Forecast Center in Oklahoma) warns pilots that, with an MCC, “the agglomeration and expansion of thunderstorm cells may occur so rapidly that the pilot of a slow-moving light aircraft may find himself literally engulfed by thunderstorms.”⁵

Mesoscale Convective Complexes are huge and minimally will cover an area of nearly 39,000 square miles (or roughly the size of the State of Virginia). Aircraft attempting to skirt the northern side of such a large area will experience extremely strong winds which may be a factor, depending on the direction of travel. Pilots skirting the southern side of an MCC will observe very light winds which may diminish any anticipated “help” from tailwinds. But, c’mon…with such a weather system are we really worried about “on time” arrivals? Of course, if the wonky winds create fuel concerns it becomes a serious matter.

We’ve got some incredible aircraft now. Big…tough…powerful. But even those “heavy iron” monsters are no match for Nature. The more dangerous the weather forecast is, the longer you should study it. Flying in the vicinity of thunderstorms can be dangerous but, carefully executed, is do-able. But when a Mesoscale Convective Complex is sitting on your destination it might be a good time to head to the Motel 6. Because they’ll leave the light on for you.

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References:

1 – Maddox, Robert A. Bulletin – American Meteorological Society, “Focus on Forecasting,” November 1980.

2 – Reynold, Harold, “Mesoscale Convective Complex – An Overview”, 1990

3 – National Transportation Safety Board Aircraft Accident Report, August 15, 1986.

4 – https://weather.com/forecast/regional/news/plains-rain-flood-threat-wettest-may-ranking

5 – Maddox, Robert. A., and J. Michael Fritsch, Weatherwise, “A New Understanding of Thunderstorms-The Mesoscale Convective Complex,” 1984.

^{Featured Image: Keven Menard}