Wx Watch: Squall-Line Lowdown

Not your typical convective mayhem

July 1, 2005

It's challenging enough to cope with "garden variety" thunderstorms, what with all the extra preflight weather research and in-flight monitoring that safe flying demands in the convective season. To all that, factor in other brands of convective activity that require as much, or more, vigilance (see " Safety Pilot Landmark Accidents: Midlevel Mayhem," page 66). We're talking about squall lines — a phenomenon that's all too often given short shrift in aviation weather texts. But while this violent weather may be mentioned only in passing, it can ruin your day if it catches you unaware.

From the earliest days of ground school, we've all been thoroughly indoctrinated in thunderstorm dangers and dynamics. You know, that thunderstorms can have frontal, air mass, or orographic origins. They require unstable air, a lifting mechanism, and enough moisture to form and mature. And particularly, that they can bring with them the kind of turbulence, downbursts, microbursts, hail, and lightning that can bring down any airplane — no matter how stout the airframe or skilled the pilot.

The same can be said for squall lines. These narrow lines of convection, heavy rainfall, and intense winds frequently precede a cold front (or a dry line, a "dew point front" that demarcates two air masses of different moisture levels). Squall lines also can radiate away from mesoscale convective complexes (MCCs). MCCs are best known in the Midwest, where they can cover an entire state and persist for days in a kind of self-sustaining circulation. They can move quickly, too — at anywhere from 20 to 50 knots. And while the squall line moves in one direction, the individual storms move in another.

The hows and whys of squall-line formation are pretty well understood. Squall lines show up on some specially tailored, computer-generated weather models, but the models' accuracy and precision aren't ready for public consumption. It's safe to assume squall lines occur under the same sorts of conditions conducive to severe thunderstorms. With this in mind, look for the following squall-line ingredients during your preflight weather briefing and research:

The presence of a well-defined warm sector. This is the zone to the east of a cold front and to the south of a warm front. In the typical frontal complex, the warm sector can see quite a bit of convective activity. The faster moving the cold front, the more likely the chance of squall lines, which are believed to form as ripples in the atmosphere, much like the ripples that radiate away from a stone thrown into a pond.

High dew points in the warm sector, especially in the zones ahead of, and parallel to, the cold front. High dew points (say, above 60 to 70 degrees Fahrenheit) indicate a very moist air mass. This moisture is a direct consequence of the southerly inflow of moisture from the Gulf of Mexico, drawn into the frontal complex and filling the warm sector with juicy, unstable air. Warm, southerly air inflows from the surface to 10,000 feet are best for creating storms, especially if there's...

Cold air aloft. Cold over warm is an unstable situation: Once warm air rises into the colder air above, it gets additional buoyancy. A trough aloft is one sure sign of colder temperatures aloft, as colder, Canadian air is drawn over the United States by the counterclockwise flow moving around the trough's perimeter. Moreover, jet-stream cores with maximum wind speeds of 100 knots or more can create localized areas of strong lifting forces at lower levels of the atmosphere. These really give a boost to the vertical transport of that soggy, tropical air.

Strong southerly wind flows just ahead of a cold front. When you feel a quick rise in heat and humidity at the surface, and the windsock fills with surges out of the south, don't be surprised if a squall line is on the way.

In the warm sector, if the surface winds are strong and southerly, look for strong winds out of the south-southwest aloft. They indicate cold air to the west, which is destabilizing. On the other hand, if the winds aloft are from the west or northwest, this may cut off the flow of moist air and help suppress convection.

Once a squall line forms, thunderstorm cells can build at the southernmost edges of the line, then travel along it — toward the surface low. All the while, the line, or lines (many times there are more than one, all running parallel to each other), travels to the east or southeast, strengthening as it goes.

Will you be able to visually avoid squall lines? Let's hope so, in which case you can do a 180-degree turn and make a landing at a suitable airport — one that won't be experiencing any overtaking squall line's intense winds, turbulence, and heavy rainfall. The good news is that squall-line passage won't take much time. The bad news is that if you try to beat a squall line to an airport, you're gambling with disaster.

There's more bad news about squall lines. They can move so fast that they catch up to the warm front ahead of them and then become drawn into the warm front's cloud mass — which can be extensive. The result? Embedded squall lines within the warm frontal zone. If you're on instruments and unfortunate enough to be in this part of the neighborhood, get ready for a rough ride — or worse. Slow to the appropriate maneuvering speed (VA) if you encounter significant turbulence and fly attitude, not altitude. If altitude excursions occur, ask air traffic control for a block altitude — say, from 5,000 to 10,000 feet. Hopefully, this will give you enough room to ride it out. Now's also the time to call flight watch (122.0 MHz) to learn about viable escape routes and alternate airports.

Preflight precautions

Besides checking the usual sources of preflight weather, be sure to check out the National Oceanic and Atmospheric Administration's Storm Prediction Center (SPC), www.spc.noaa.gov/products, for any current weather watches, mesoscale discussions, and convective outlooks — which go out three days in the future. The SPC's home page ( www.spc.noaa.gov) has a mouse-over feature that shows current radar animation superimposed on the day's watch areas. The National Weather Service's Aviation Weather Center (AWC) also puts out a convective forecast product, called the CCFP (Collaborative Convection Forecast Product), http://aviationweather.gov/products/ccfp, which can give you an idea of the anticipated strength and movement of any thunderstorm activity for the coming two-, four-, and six-hour time frames. Also, Meteorlogix provides good current and forecast convective activity via links through AOPA's Web site (www.aopa.org; then click on the Weather button under the Members Section).

Then, just before takeoff, check your sources again for any new convective activity. Thunderstorms and squall lines can form quickly, so you want the very latest data before launching.

Bow echoes

It's worth emphasizing that individual thunderstorms within squall lines have distinctive radar signatures. So, check your preflight sources for signs of conventional thunderstorms — steep precipitation contouring (i.e., close spacing between precipitation contours), well-defined areas of heavy radar returns, and any hook or scallop shapes in the squall line.

Occasionally an especially strong thunderstorm in the line will produce a downdraft that will induce new cells along the leading edge. This makes the local atmosphere more unstable, and bigger storms develop. This gives squall-line returns a bowed-out, or fan-shape, look. That's why they're called bow echoes.

Bow echoes are sure signs of very strong gust fronts and damaging surface winds (sometimes called straight line winds, as opposed to tornadic winds) and, obviously, severe turbulence. That's not to say that tornadoes can't exist within the densest bow-echo returns; many times, they do. So can hail.

In-flight strategies

Two words: Stay informed! You have a number of valuable aids to stay up to date with convective developments, including squall lines. The old standbys include:

Flight watch (122.0 MHz). Check in frequently for convective updates, METARs, and TAFs. Make sure you find out where the best weather is, in case you have to turn and run.

HIWAS (hazardous in-flight weather advisory service). These weather broadcasts are transmitted via the voice channels of certain VOR stations. Airmets, sigmets, convective sigmets, urgent pireps, and center weather advisories are updated at 15 minutes after the hour, then broadcast continuously. Coverage areas are within a 150-nautical mile radius of the broadcasting station.

ASOS and AWOS broadcasts from select airports. These automated surface observations are continuously broadcast and updated. As you fly, it can be useful to check them for surface weather indicating possible convection — such as wind direction, intensity, and gusts; high dew points and lowering ceilings and visibilities; rainfall; and nearby lightning reports.

Air traffic control. Controllers can be very helpful in passing along information about storm-cell movement and intensity, as well as good escape routes from dangerous weather.

Radar, lightning detection, and datalink weather

Use of these options for in-flight avoidance is a separate topic altogether, but suffice it to say that airborne weather radar probably won't work too well for detecting squall lines or other strong storm returns. That's because the radar on most general aviation airplanes has a small-diameter antenna, which cast diffuse beams that do a poor job of correctly defining storm-cell boundaries and contours. Attenuation of these weak beams means that much of the radar energy sent out to the precipitation doesn't make it back to the cockpit display screen. This paints a dangerously misleading picture of precipitation patterns. What's more, thunderstorm research flights have shown that the worst turbulence can occur in the clear air near storm cells.

Lightning-detection equipment is better than nothing, but using it properly means steering very, very far from any lightning plots. It's definitely not a close-in tool for weaving around the densest storm cells.

Datalink weather imagery offers the promise of attenuation-free, reasonably up-to-date views of storm and squall-line boundaries and intensities. But datalink has its own drawbacks and is subject to delays in image delivery. And when you're flying in or around convective activity, a few minutes can be a long time indeed; a whole lot can happen in a very short time. Some have even called into question some of the methodology behind today's current datalink imagery processing.

The upshot: As always, the best way of avoiding squall lines or any other convective storm is to maintain at least a 20-mile visual separation from any angry-looking clouds or rain shafts. If that's not possible, then make plans for watching the weather from the safety of the nearest airport.

E-mail the author at tom.horne@aopa.org.

Links to additional information about thunderstorms may be found on AOPA Online ( www.aopa.org/pilot/links.shtml).