Wx Watch: Steering Clear of Convection

It's summer, it's prime time for flying, and everyone is eager to make those trips planned so enthusiastically during winter's gloom. Unfortunately, we all know that this time of year is also prime time for thunderstorms, one of aviation's most serious weather hazards. Right off the bat, pilots are faced with a dilemma: how to formulate a strategy that will both maximize their flying time, and minimize — or, more preferably, eliminate — their exposure to convective weather.

Reconciling these two objectives can be a real challenge and a high-stakes proposition if there's no well-thought-out plan for avoiding thunderstorms.

Reliable sources

The best plan begins not with a thorough preflight weather briefing, but with building a knowledge of recent weather trends. Keeping track of the previous two or three days of national weather goes a long way toward familiarizing yourself with the conditions you'll most likely encounter. The Weather Channel's WSI Nowrad imagery is especially helpful, thanks to frequent updates of regional radar returns. A.M. Weather, another user of WSI color graphics, is also excellent source of a pilot's daily fix of weather information. Other televised weather broadcasts can also be helpful in filling out the big picture.

Once armed with this background information, the preflight weather briefing shouldn't contain any major surprises. Naturally, you'll note surface observations, terminal forecasts, and winds aloft as part of the process. However, there are a few other weather reports that bear special scrutiny in thunderstorm season.

One is the area forecast (FA), which will identify any areas of hazardous weather on a state-by-state basis. The information in an FA should be familiar because all or part of its text is usually included in every standard, abbreviated, or outlook briefing. So are bits of pertinent convective data from satellite imagery, radar summary charts, sigmets, convective sigmets, and airmets.

Less familiar might be two other National Weather Service products that are of great significance in thunderstorm season. These are radar reports (rareps) and severe weather outlooks (ACs).

Of these two, rareps are the most useful once convection has begun. These are reports from the nation's network of weather radars, and when storms begin to pop, they can provide precise, near-real-time updates of thunderstorm location, structure, intensity, and movement. However, this information is of little use without a map showing NWS radar sites. Without one, the azimuths and ranges of points defining echo boundaries can't be easily visualized, let alone the locations of maximum tops or any widths or diameters of thunderstorm lines or cells. For the best views of radar imagery, try one of the commercial weather information vendors, which can provide facsimile or real-time imagery of regional radar returns.

ACs are definitely worth a close look. Four types of thunderstorm risk areas are outlined. One is for "general" thunderstorms, meaning any storm categorized as less than severe. The other three show areas expected to have a slight (2- to 5-percent coverage), moderate (6- to 10-percent coverage), or high (more than 10-percent coverage) risk of severe thunderstorms. Severe thunderstorms, by the way, are defined as those having any combination of the following fun features: 50-knot or greater surface winds; three-quarter-inch-diameter hail; or tornadoes. The AC comes out once a day, at 12Z. This makes it current in the morning hours, and after years of consulting them, I've found ACs quite accurate in predicting the zones that will have the worst storms later in the day.

As a matter of fact, in the morning hours, the AC may be the only reliable hint of things to come. Most thunderstorms — except those of the nocturnal variety — build in mid-afternoon. Morning editions of rareps and radar summary charts often give no clue of future thunderstorms, for the simple reason that none have formed yet.

That's the graphic form of a convective outlook. Another type of convective outlook is provided in text form. It's available either as an addendum to a convective sigmet, or as a stand-alone statement early in the morning on days that forecasters believe have the potential for convective activity. This convective outlook gives the forecasting community's best estimate of conditions that may meet convective sigmet criteria (i.e., areas with four-tenths coverage of level-four — or greater — radar returns or isolated severe storm cells) from two to six hours in the future. These outlooks are sometimes overlooked by briefers, who tend to concentrate on the body of a convective sigmet.

The moral: Be sure to ask for the convective outlook, even if convective sigmets haven't been issued yet. It can save you from being trapped in the middle of a watch or warning box while enroute.

Avoidance strategies

Okay, let's say you've decided to depart. Of course, your common sense prevents you from launching into squall lines, fast-moving cold fronts, huge mesoscale convective complexes (MCCs), or other known thunderstorm areas, but we'll assume that the convective outlook and other weather information sources indicate the possibility that thunderstorms may form along your route of flight. In other words, you've received the standard summertime briefing.

There are several very good ways to stay out of trouble:

• Depart early in the morning. The earlier the better, because thunderstorms build and worsen in the heat of the day. The only exceptions here are nocturnal thunderstorms of the kind that accompany MCCs. MCCs are often associated with stationary frontal activity and can renew themselves daily, persisting for days over areas covering an entire state. On an average, say, three-hour trip, taking off at 6 a.m. puts you at the destination in mid-morning — well before the ground has enough time to start or renew the convective process. If there's to be another leg or two on your flight, the mid-morning stop will be a perfect time to thoroughly reacquaint yourself with any new weather developments and make any go/no-go decisions. Better then, in the calm of a briefing room, than in flight, facing worsening weather.
• File IFR. Or obtain flight following, if possible. Air traffic control can provide you with information on the weather ahead, and you can often hear firsthand of any escape routes from a convective situation. Many's the time that I've heard a briefing warning of dire, impenetrable weather, only to receive generous help from controllers in circumnavigating storms. Filing IFR isn't a panacea, but it often helps, especially when things go sour and you need some special handling to find an alternate airport. For those without instrument ratings, these and other reasons make compelling arguments for becoming instrument qualified.
• Stay visual. This rule holds regardless of your pilot qualifications. VFR or IFR, stay away from clouds. Keeping good visual separation from clouds — and having adequate visibility in haze — is the only way to guarantee that you won't inadvertently stumble into a thunderstorm.
• Don't let yourself be trapped on top. A common scenario goes something like this: A pilot climbs above a scattered layer of clouds to avoid cruising in the bumpier air below. As time passes, the cloud tops rise and the scattered layer turns broken, then undercast. Fighting to stay on top, the pilot climbs higher and higher, but soon the airplane is engulfed in building cumulus, soon to become one or more full-blown thunderstorms. Now the only way out is to descend into certain chaos. Terrain permitting, the best strategy here is to descend below the cloud deck before it closes up, then fly as long as practicable — to the destination, I hope. Or to the nearest storm-free airport, if thunderstorms threaten.
• Don't let yourself be forced too low. Flying VFR below a cloud deck is fine, unless a lowering ceiling forces you to dangerously low altitudes. This rule is of special concern to those who've followed the previous strategy. Mountain flying in thunderstorm season calls for extra caution; you obviously don't want to descend below a safe altitude, even if this means staying in the clouds while executing a 180-degree turn. The same thing applies to lowering visibility.
• Use Flight Watch. Keeping abreast of fast-changing weather situations is the whole idea behind Flight Watch, so stay in touch. Also, remember that sigmets and other weather reports are broadcast over certain VORs, that you can listen in on ATIS information as you fly, and that you can still call flight service on 122.1 (while listening over certain VORs for a response), 122.2, 122.4, 122.6, and other designated frequencies for full briefings.
• If in doubt, turn around or land. The last thing a VFR-only pilot needs is to fly into instrument conditions when a thunderstorm is near. And an instrument rating isn't a license to fly into convective clouds, no matter how fat the pilot's logbook.

Escape hardware

The subjects of radar and lightning detection device usage could fill an entire book. The complexities and nuances of interpreting each and every model of this equipment make generalities difficult, but there are a few rough guidelines.

Radar is nice to have, but there are limitations. Unless antenna size is relatively large — greater than the 12-inch-diameter ones used in many light airplanes — the quality of the radar returns is apt to be compromised. Small antennas cause smearing of radar returns, making small areas of precipitation seem larger than they really are. This "beam smearing" can make a series of small cells (ones with plenty of room to allow safe passage) appear like one huge cell, causing you to make an unnecessarily wide circumnavigation.

Another problem is attenuation, or weakening, of the radar signal returns. This occurs chiefly because of inadequate radar signal strength. Remember that radar works by transmitting a radio pulse, then bouncing that energy off a target — in this case, precipitation — and back to the airplane's antenna. The stronger the transmitted signal, the farther the radar can "see" into and, more importantly, through precipitation echoes. Weak signals are notoriously bad if flying in precipitation. That's because the transmitted signal is so badly attenuated that it doesn't travel much more than 10 miles or so from the airplane before being reflected back. The result? A permanent echo immediately ahead of the airplane and no information about what lies farther down the road. In some cases, a deceptive radar "shadow" of echo-free air can appear behind an attenuated echo. That doesn't mean there's no weather there — in fact, there may be the worst imaginable — just that no radar energy has been able to make the trip out and back.

By now, it should be obvious that without a larger antenna (at least 12 inches in diameter and preferably larger) and a powerful transmitter, you will be at a disadvantage trying to use radar to circumnavigate storms or detect embedded thunderstorms.

That said, the rough guidelines are to avoid all storms by at least 10 nautical miles (20 nm if the tops are above 30,000 feet) and never to fly into a radar shadow. This applies to all radars, by the way — even the most powerful.

Lightning detection equipment has other limitations. The principal one is a false depiction of range. Stronger strokes of lightning can appear closer than they really are. Also, because this equipment "sees" only electrical discharges, there is no way to absolutely, positively determine the size and shape of a storm.

Still, there are many who swear by lightning detection, saying that it accurately defines the worst areas of convective activity. The best method of using lightning detection seems to be as a means of gross avoidance. Steer well clear of the dots, it's said, and you'll always evade the worst storm cells.

Although storm avoidance equipment can be useful and reassuring, having this gear is no substitute for good judgment. Leaving early, filing IFR, staying visual, and knowing when to throw in the towel are skills of far greater importance to survival than having a panel full of the latest bells and whistles. This holds true for pilots of everything from Cessna 150s to the most modern corporate jets.