Weather

On March 29, 2011, the National Transportation Safety Board recommended the FAA work with the National Weather Service to revise AC 00-24B, Thunderstorms, by including explanations of the terms used to describe severe thunderstorms, such as “bow echo,” “derecho,” and “MCS.”

The letter resulted from NTSB investigations of fatal accidents that it blamed in part on pilots’ lack of understanding of the terms, used by meteorologists to describe especially dangerous thunderstorm-related weather.

“Pilots need to be provided guidance that explains these terms because without such guidance, they will not be able to fully understand the weather condition information provided in convective outlooks and mesoscale information products—nor the effects such weather phenomena, such as rapidly changing winds, can have on the flying environment,” the NTSB said.

The revision has yet to be made.

It’s easy to see why students and instructors who rely on FAA weather texts—including the thunderstorms advisory circular, and the weather chapter in the Pilots’ Handbook of Aeronautical Knowledge—wouldn’t recognize those terms, because they apply to a scale of weather events FAA materials do not mention: the mesoscale.

Weather interacts with all scales, and weather-wise pilots should know at least a little about each of them, especially the microscale, storm scale, and mesoscale sizes—which most directly affect flights. Synoptic- and planetary-scale weather sets the stage for the smaller phenomena. For example, an instrument-rated pilot planning a flight north of the surface position of a warm front shown on a synoptic map is most interested in storm-scale details, such as the altitudes of potential icing aloft along a possible route through the area, where warm air is cooling as it rises over cold air at the surface.

FAA weather resources for students and instructors don’t cover all of weather’s scales. These materials describe at least one microscale event: runway turbulence caused by winds blowing nearby around buildings or trees. FAA materials introduce some storm-scale thunderstorm types, and also briefly describe synoptic-scale storms and their fronts, as well as planetary-scale jet streams. FAA materials generally ignore mesoscale events except for squall lines.

Derechos and bow echoes.

Some squall lines produce both derechos and bow echoes. The NWS has been using derecho—deh-RAY-cho—since the late 1980s to describe a squall line’s straight-line winds (as opposed to the rotary winds of tornadoes) of 57 mph or faster, that hit several places along a path that’s at least 240 miles long, over a period of hours.

Squall lines, especially derechos, sometimes form bow echoes—named for the bow shape of the line of thunderstorms seen on weather radar. "Bow echo" should alert pilots to the possibility of dangerous, gusty winds because the squall line winds that do the most damage are often associated with bow echoes. Weather researchers and forecasters have been talking about these since the late 1970s; you will occasionally hear TV meteorologists talk about a bow echo showing up on weather radar.

Mesoscale convective systems.

Potentially the most dangerous mesoscale weather for pilots is found in groups of thunderstorms known as “mesoscale convective systems” (MCS). The word mesoscale tells you the size of these events. “Convective” refers to the up and down air motions found in showers and thunderstorms. “System” tells us the atmosphere has organized the thunderstorms; they don’t just happen to be occurring in the same area.

One reason it’s a mesoscale convective system is that cool downdrafts from mature thunderstorms spread out when they hit the ground to trigger new thunderstorms by pushing up warm, humid air.

Mesoscale systems can form in more-or-less straight lines (squall lines) or as circular areas. In addition to showers and thunderstorms, these areas can include stratiform rain that falls steadily over relatively large areas, creating instrument meteorological conditions. In addition to being large, MCSs last for hours.

Mesoscale convective Complexes.

A mesoscale convective complex (MCC) is one kind of MCS. Meteorologists didn’t recognize MCC as organized systems until the late 1970s and early 1980s. Robert Maddox, a former U.S. Air Force weather forecaster, and other researchers used then-new, nighttime color infrared satellite photos, which indicate cloud-top temperatures, to conclude that what had seemed—from the ground—like unorganized batches of thunderstorms and other rain really were organized systems with very high, and thus cold, clouds.

To be considered an MCC, the system’s cloud-top temperatures of minus 25 degrees Fahrenheit or colder need to cover at least 38,627 square miles, an area roughly the size of Iowa. A center area of cloud-top temperatures of minus 62 degrees Fahrenheit or colder covering at least 19,313 square miles also is needed. To be this cold, clouds have to top out between 40,000 to 50,000 feet above the ground. Thunderstorms reaching this high into the sky are powerful. These high clouds also must be nearly circular in shape, distinguishing the system from a squall line. Finally, the system must hang together for at least six hours.

MCCs grow and persist after the sun goes down, shutting off the solar energy that helps power most thunderstorms, because low-level jet streams bring in warm, humid air from over the Gulf of Mexico. These are fast-moving streams of air 1,000 to 2,500 feet above the ground. The wind begins blowing when the ground south of the thunderstorms cools after sunset. As this happens, an inversion—air aloft that’s warmer than the air at ground level—forms because the air aloft doesn’t cool as fast as air next to the ground.

The inversion shuts down the up-and-down air motions that make flights on sunny days bumpy at low altitudes. Meteorologists say that an inversion decouples the upper-altitude winds from the surface. Without slow-moving air rising from below and some of the fast-moving air aloft sinking, the air aloft is no longer “connected” to slow-moving air near the ground. This reduction in friction allows the wind to blow faster than 60 knots. In the morning, as the sun heats the ground, warm air begins rising, the inversion is wiped out, and the low-level jet dies. Without the jet’s warm, humid air the MCC’s thunderstorms weaken and die. After an MCC’s thunderstorms weaken and die in the morning, however, the heart of the system often stays alive. When the system is at its peak, a mesoscale convective vortex (MCV) forms in the part of the MCC away from the intense thunderstorms. This vortex can stretch from roughly 10,000 feet above the ground up above 20,000 feet, and be 30 to 60 miles in diameter. It can keep going as long as 12 hours as it drifts to the east or southeast in the upper-atmosphere winds.

If a MCV moves into an area where the atmosphere is unstable, it can help generate another round of thunderstorms with strong winds, large hail, and tornadoes the next afternoon.

The dangers to pilots.

A flight instructor with a student or a student flying solo are likely to stay far away from an MCS since they avoid any kind of thunderstorm. On the other hand, pilots moving up in the aviation world, such as by flying charters, could run into trouble if they do not understand the danger.

Even the most experienced pilot can be in danger, as shown by the accident that killed famed test pilot A. Scott Crossfield, whose Cessna 210 crashed in an MCC thunderstorm in Georgia on April 19, 2006.

You can never go wrong by asking, “What in the world is this?” when you hear or read an unfamiliar term during a weather briefing.

The NWS’ Storm Prediction Center publishes Mesoscale Discussions when any mesoscale system is forecast to affect the weather.