WX BRIEF

Mesoscale convective systems

Recognizing and understanding these midsized but dangerous storm systems.


Flying an MCS

Radar image of a strong bow echo approaching Pleasant Hill MO. Bow echoes form from squall lines and can generate derechos with wind gusts easily exceeding 100 mph.

Because an MCS is a cluster of storms in varying stages of growth and decay—and may encompass a wide area of real estate—they represent a dangerous situation for pilots. These systems can be large enough to force a significant diversion.

They can also move either very slowly, meaning if you choose to wait it out, you could be cooling your heels for several hours, or very quickly—over 50 mph—meaning scant decision time to avoid penetrating it.

An MCS may also be many kilometers deep, with embedded thunderstorms hidden anywhere within. There is no guarantee that, once you make it through the first line of storms, you won’t run directly into an even stronger cell lurking behind them.

Even if you have onboard radar, the attenuation of the instrument will likely underestimate the strength of any storms taking place behind the nearest cells. In addition, it will not show you areas where no rain is yet occurring—even though there may already be a developing cell in that area.

Similarly, a strike finder will not alert you to a developing cell, because lightning is only produced by mature cells. Since MCSs are rather large phenomena, your preflight check of weather maps, prog charts and satellites should alert you to their presence.

Satellite and radar loops may give you additional information about their propagation and movement. Your weather briefer should also give you a lot of this information. If you are already in the air and find yourself faced with an MCS on your route of flight, the first task should be a call to your dispatch or to Flight Watch to determine the extent and movement of the system.

If a diversion is in order, you should plan on diverting away from its direction of movement. New cells are far more likely to form on the eastern and northern sides of the system (eastern and southern sides in the Southern Hemisphere) than on the other sides, which represent the rear quadrant of the MCS.

Unfortunately, every year a number of pilots fly into an MCS. The edges of the system can often be subtle, and a pilot may fly through miles of benign clouds before stumbling on the first embedded cells.

The same rules that apply to inadvertent thunderstorm penetration apply to MCS penetration. Consider the MCS one giant thunderstorm. Reduce your speed to turbulent air penetration speed.

Climb if possible, and don’t fight the up­drafts and downdrafts. It is important that you maintain a straight flightpath. Unless your onboard instruments, Flight Watch or ATC give you a good reason to retrace your steps to extract yourself from the situation, it’s best not to turn around in an MCS.

This is because you really don’t know if you’ve already passed other embedded or developing cells that you’ll run into on your way back. If you are not inside a cell battling to get out, inform ATC of your predicament and they may be able to give you the most direct vector out of the system.

Another useful source of information about the extent of the MCS, and the best route out of or around it, is a satellite weather provider such as XM Weather. In many places, these weather services broad­cast images from the network of surface-based weather radars.

Since these radars are on the ground looking upward, they will have a much better view of the cells within the MCS than you might otherwise have from your other onboard instruments.

However, users of these services should be aware that surface radar images, especially if they are “base” images, will only show precipitation near the ground, and may significantly underestimate the strength of cells further up in the atmosphere or even miss cells in which the precipitation has not yet reached the cloud bases.

Bow echoes and derechos

The MCS that hit southern Illinois started as a small squall line of thunderstorms. However, atmospheric conditions were just right to turn it into something more severe. Small squall lines, or portions of a larger squall line, may begin to bow outward ahead of the axis of the line.

This behavior is known as a bow echo, and such a bow can mature into a very intense and compact storm system in its own right. Bow echoes normally require a specific set of formative criteria.

To form, they usually require strong windshear in the lowest 4000–6000 ft of the atmosphere. This windshear is often enhanced by a midlevel localized jet, known as a rear inflow jet, which is formed by a convergence of air from lower levels to the midlevels of the storm.

The outflow of cool air from the thunderstorm downdrafts creates what is known as a mesohigh cold pool. This is an area of higher pressure where the descending air from the downdrafts hits the ground beneath the MCS and spreads out, and with it the inflow of new humid surface air.

As the rear inflow jet becomes entrained in the thunderstorms, it is forced toward the surface, enhancing the downdrafts and pushing the inflow boundary further forward. The jet’s influence bows out the center of the squall line into the bow echo.

The resultant bow echo outflow can be exceptionally strong and winds can easily reach hurricane speeds. This organized multicell enhanced outflow, covering several kilometers with strong straight-line winds and even strong­er gusts, is known as a derecho.

Derecho is named from the Spanish word meaning “straight,” and was meant to differentiate it from the tornado—derived from the Spanish word meaning “to turn.” To be a derecho, and not just a strong gust front, wind gusts need to exceed 57 mph, although strong derechos may have gusts more than twice that speed.

If there are multiple bow echoes within a long squall line—at least 240 miles in length—the result, called a serial derecho, can cause wind damage over a very large area. These serial derechos are most commonly associated with the squall lines that form ahead of strong cold fronts.

A smaller MCS may produce a single, but very strong, bow echo which generates a progressive derecho. These bow echoes have a propensity to grow dramatically with time and over an hour or two may evolve to more than 200 miles in length and cover hundreds of miles as they move forward.

In the case of progressive derechos, the edges of the squall line do not benefit as much from the forward force of the jet, and lag behind. As a result, strong vortices may form here and are known as bookend vortices.

What appeared as the eye of an “inland hurricane” over southern Illinois was actually the wraparound effect of the counterclockwise bookend vortex of a very strong squall line bow echo. We’ve all learned that thunderstorms are nothing to mess with.

The same holds true for mesoscale convective systems. These organized clusters of thunderstorms can include some of the most dangerous weather conditions a pilot will ever face as embedded storms join forces to generate a veritable IFR minefield for the aircraft that strays too close.

Fortunately, these systems, which include tropical cy­clones and squall lines, are not too difficult to identify on satellite and radar images, and a good preflight weather briefing should be able to keep you in the clear.

Karsten Shein is a climatologist with the National Climatic Data Center in Asheville NC. He formerly served as an assistant professor at Shippens­­burg Uni­versity and was a scientist with NASA’s Global Change Master Directory. Shein holds a commercial license with instrument rating.

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