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Flying SeasonsFlying Seasons

Storm SavvyStorm Savvy

Gearing up for thunderstorm season Let there be no doubt: Thunderstorms inspire fear in all pilots, high time or neophyte. And for good reason.

Gearing up for thunderstorm season

Let there be no doubt: Thunderstorms inspire fear in all pilots, high time or neophyte. And for good reason. Thunderstorms have huge amounts of destructive power and, according to the AOPA Air Safety Foundation, caused 25 percent of all fatal weather-related accidents in 2004. It's a thunderstorm's turbulence that causes most of the trouble — although convective storms also can contain deadly icing conditions, hail, and lightning. This turbulence can set off a monumental roller-coaster ride, causing dangerous swings in airspeed and/or radical upsets as the airplane rolls, pitches, and yaws out of control. Pilot disorientation usually follows, incorrect control inputs follow that, and ultimately the airplane can experience structural failure. How many times have thunderstorm accident reports mentioned witnesses saying words to the effect that airplane "parts fell out of the bottom of the clouds"? Quite a few.

Knowledge, skill, and judgment

Aviation safety depends on a triumvirate of attributes: knowledge, skill, and judgment. Lack any one of those attributes, and your flying isn't as safe as it should be. Assuming we all know that thunderstorms can be deadly, that leaves skill and judgment as our weapons in dealing with them.

What's the safest way of avoiding a thunderstorm encounter? Staying on the ground whenever convective activity is mentioned, of course! For any pilot — especially newly minted, non-instrument-rated pilots — that may be the best strategy. It all depends on your comfort and experience level in dealing with convective situations. But you won't develop the skills and judgment you need for wisely coping with convection by canceling flights. And you certainly won't get the most out of your airplane's utility.

Experienced instrument-rated and current pilots will find that gradual immersion is the best way to build the skills and judgment necessary for thunderstorm avoidance. The skills include developing a savvy for interpreting preflight weather information, knowing how to safely maneuver your airplane, and interpreting the in-flight weather situation. Good judgment means knowing, yes, when to stay on the ground, plus mastering the ability to formulate alternate plans of action and carry them out. The emphasis, as always, is on thunderstorm avoidance.

Know before you go

The preflight weather briefing is the first step toward safe flying in thunderstorm season. You'll want to be sure to obtain any convective sigmets, along with all the other usual products that briefers should issue.

Surface analysis charts can be quite helpful, although they're sometimes overlooked. It's important to remember that most thunderstorms occur in the "warm sector" of a frontal complex. This sector is east of any cold fronts and south of warm fronts. It's in this region where high temperatures and dew points conspire to set off thunderstorms as the day progresses. When checking METARs from airports in the warm sector, look for dew points of 65 degrees F or higher. This indicates a high water-vapor content. As air parcels rise, this high moisture content provides the fuel for further lifting aloft. Dew points above 70 degrees should serve as warnings, so don't be surprised if convection occurs when a region of high dew points appears.

The warm sector is also a place where southerly winds prevail. Except for the western states, these winds transport lots of moisture from the Gulf of Mexico, along with plenty of warm air from more southerly latitudes. You'll notice that those southerly winds are strongest just in advance of any oncoming cold fronts. This is one reason why lines of thunderstorms and huge mesoscale convective complexes (MCCs) can crop up ahead of cold fronts.

Internet tools

Telephone briefings are fine in most cases, but these days we have many additional sources of convective information via the Internet. Most of these briefing products use graphics, so interpreting them is usually stone simple. A picture is worth a thousand words, so please be sure to check out the graphics on the following links on the Aviation Weather Center's Aviation Digital Data Service (ADDS) Web site's convection tab:

  • Convective sigmets. These are graphical representations of any convective sigmets currently in effect. Cloud top, and direction and speed of movement, of lines and clusters of active storms also are shown.
  • National Convective Weather Forecast (NCWF). This ingenious forecast uses a Java tool to depict individual storm cells, and you can use your cursor to zoom in on a localized grouping of cells. The direction and speed of cell movement can be selected, and METARs also can be superimposed. Best of all, there are blue polygons that give an "extrapolation" of the cells' forecast locations in the coming hour. Previous locations of the cells also can be selected. The NCWF is a supplemental source of weather information; it has not yet been conferred official status by the FAA.
  • Collaborative Convective Forecast Product (CCFP). Also supplemental is the CCFP, which defines the areas where the worst storms may affect air traffic routings. This tool, which makes predictions of storm area coverage two, four, and six hours into the future, is intended for use in managing flow control of airline traffic, but as a nice-to-know snapshot of where definitely not to go, the CCFP is worth a look.
  • Convective outlooks. These can help you in advance planning of any flights you might take two to three days in the future. The ADDS site gives you the "day one" outlook (in other words, today's), and outlines areas where the risk of severe (surface winds greater than 50 knots, three-quarter-inch hail, or tornadoes) thunderstorms is expected, as well as "general" (those with less than severe attributes) thunderstorms. For day two and day three outlooks, you have to click on a link at the left of the page labeled All SPC Forecasts. SPC stands for Storm Prediction Center, and ADDS merely links its site to the SPC's for convective outlooks.
  • Radar. ADDS' radar links give you either a national view of the precipitation situation in a national mosaic or site-specific information. The site-specific information is based on the nation's network of Doppler weather radars, so their coverage areas are circular in shape — reflecting the site's area of radar coverage. Boxes delineating thunderstorm and tornado warnings are posted, and you can select between base-reflectivity or composite-reflectivity views. Base reflectivity shows only the echoes received by the lowest scan angles of the radar antenna; composite reflectivity takes in higher scan angles, and shows all the precipitation signatures in nearly the entire vertical extent of any cells. Lesson: Base reflectivity shows precipitation at low altitudes; composite reflectivity shows all of a storm's moisture, and is a better gauge of its extent and growth potential (all of the precipitation aloft may not yet have reached lower altitudes — but soon may!).

Yes, there are many other excellent Internet sources for great weather depictions. But ADDS, to me, is a wonderful, free site for one-stop shopping. And it includes some of the latest products from our best centers of meteorological research.

Flying smart

To keep the weather flying excitement to a minimum, try using these rules of thumb:

  • Limit your flying to early in the day. Take off at the crack of dawn, when solar heating is at a minimum and air-mass thunderstorms have yet to form. This rule won't work all the time for thunderstorms associated with frontal activity, or for storms that are part of mesoscale convective complexes. MCCs are creatures of the Midwest, where warm, moist low-altitude jet-stream winds can sustain storm complexes overnight, and make them last for as long as three days. In any event, aim for your flight to end well before noon.
  • Stay visual. Whether you're flying under VFR or IFR, steer clear of clouds. If you enter clouds, you obviously can't see any buildups. By keeping your distance from any building cumulus clouds (a 20-nm distance is often recommended, although distances are difficult to judge) you're guaranteed not to stumble into trouble. Deviate as necessary, but be careful about flying between closely spaced buildups. You may become trapped as the gap closes up. For the same reason, avoid large-scale areas of storm coverage. Don't try to pick your way through a line of storms that's 100 nm wide, for example.
  • Don't try to top building cumulus. Unless you're flying an F-16, you probably don't have the performance to outclimb fast-growing tops. Better to go around, or perform a 180-degree turn.
  • Got datalink, lightning detectors, or radar? Good for you. Datalink weather can give you near-real-time information on the extents and shapes of the precipitation returns in thunderstorms. Because datalink uses ground-based Nexrad radar imagery, you see an accurate representation of the storm situation. Lightning detectors provide real-time lightning information, but can be challenging to interpret. Most general aviation airborne weather radars, although nice to have, don't have enough power to see through storm cells. Their signals are blocked by heavy precipitation, and the view on your radar screen can show a storm or lines of cells that are much smaller and/or narrower than they really are. This phenomenon, called attenuation, has lured even pilots of the best-equipped airliners to their doom. So play it conservatively with weather radar and keep your distance from radar returns — preferably by flying in visual conditions. Onboard weather gear is great to have, but it doesn't give you carte blanche to weave through clusters of cells that cover thousands of square miles.
  • Use air traffic control. ATC can be helpful when you're trying to circumnavigate thunderstorms. Just be aware that its radars cannot see light precipitation, so they may believe you're flying in the clear — even though you're on the gauges in rain. You should ask ATC for help in deviating, but remember that as pilot in command you can refuse a clearance or a vector, or deviate at will if it will keep you out of storms — and if you inform controllers. ATC also can share information about the experience of pilots who have previously flown along your route, but again, you must take the initiative and ask. Flying on an IFR flight plan is also a good idea if thunderstorms will be factors. Your handling will be better, and if the weather deteriorates, you'll be prepared for an instrument approach to an alternate, or a deviation in instrument meteorological conditions.


Let's say the worst has happened: You've flown into a thunderstorm. Now what? The top priority is slowing the airplane to maneuvering speed and preparing it for a bout with turbulence. In retractables, extending the landing gear can help stabilize the airplane. Go to the power and pitch settings that you (hopefully) have previously determined will yield maneuvering airspeed. Above all, do not extend the flaps. This will reduce the size of your maneuvering flight envelope, and much more easily expose you to structural overloads.

Next comes a call to ATC to alert it of your predicament. Controllers may be able to help direct you away from the worst of the cell, but first they need to identify you on radar. This is where being on an IFR flight plan can really help speed your exit from danger.

Any turns should be made with very gentle banks. This keeps wing loadings and G-loadings down. By all means, do not try to maintain an altitude. This will only cause dangerous airspeed excursions as you chase the altimeter's and vertical velocity indicator's needle swings. Instead, ask ATC for a block altitude — say, from 5,000 feet to 10,000 feet, or as is appropriate for your level of turbulence — and fly attitude. Keep the wings as level as you can, keep the attitude indicator's "nose" on the artificial horizon, and accept any altitude deviations.

Icing conditions are common in thunderstorms, so be sure your pitot heat is turned on, and that you use any other anti-ice equipment at your disposal.

It's not a good place to be, that's for sure, and it's certainly not the sort of weather-flying experience we are all looking for. But if you follow the rules we've just discussed, you shouldn't ever have a front-row seat in a thunderstorm encounter.

E-mail the author at [email protected].

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