Wx Watch: Frontal Scope-Out

Let a preflight briefing mention frontal activity, and most general aviation pilots won't even think of flying. That's too bad, because all fronts are not created equal. Sure, some can create thunderstorms, instrument meteorological conditions, and other adverse weather, and to cope with them pilots need to be both instrument-rated and current. Weather avoidance equipment (lightning detection, weather radar, datalinked weather information) is also necessary for making good in-flight weather decisions around aggressive fronts. And yes, sometimes even the best combination of pilot, airplane, and equipment has to yield to severe weather and wait it out.

The next time a front's in your weather picture, start analyzing beyond the immediate facts. Of course you'll check the METARs and TAFs for current and forecast ceilings and visibilities, check the winds aloft, and ask for any airmets or sigmets. That's a given. But here are some other items that bear scrutiny:

Speed of movement. A cold front that's traveling across the ground at 50 mph is a completely different creature than one with a 15-mph groundspeed. Fast-moving cold fronts generate the worst thunderstorms and can create violent squall lines ahead of them. Slow-moving cold fronts tend to be more benign. Either way, you can do the math and find out if the cold front will affect your route of flight. Most of the cloudiness and precipitation associated with a cold front occur ahead of the front.

Warm fronts move much more slowly than cold fronts. Generally speaking, their cloud shields and precipitation fields are farther-reaching than those of cold fronts. This means that a warm front 300 miles away can still produce clouds and precipitation during your flight. Ceilings should be higher, though, and barring any embedded thunderstorms, flight under VFR is often possible when flying well ahead of a warm front.

Tops. High tops mean trouble. They signal strong convection, and any cloud tops above 10,000 feet or so require a look at radar imagery and a check of convective sigmets to see if thunderstorms are present. For most nonturbocharged airplanes, high tops also mean that pilots on IFR flight plans have to cruise in the soup. That's a problem because you can't see cloud buildups and can easily stumble into a convective cell or a full-blown thunderstorm.

History. A scan of the METARs for the past few hours can fill you in on a front's character. Are ceilings and visibilities going down, coming up, or staying the same? Are surface winds strong or gusty? If so, from what direction are the winds blowing? Answering these questions tells you a lot about the severity of a front, plus confirms its location. If surface winds show a switch from the southerly to the westerly or northwesterly points of the compass, then a cold front has most likely passed through. Winds slowly switching from the east or southeast to the south or southwest? That's a sign of a warm front's lazy passage.

Wind patterns aloft. Low pressure circulations aloft strongly affect the weather below them. If a front or its associated low pressure center is near your route of flight, look at a 500-millibar constant pressure chart. This pressure level roughly corresponds to 18,000 feet msl, and any wind barbs or height contours showing a tight, counterclockwise wind rotation are a dead giveaway of a low aloft. The weather below it will be especially cloudy, precipitation-laden, and convective. This kind of information also can tip you off to weather problems later in the day, when surface heating can trip the convection that leads to afternoon thunderstorms.

The three-digit code. On some surface analysis charts you'll see a three-digit code somewhere along the front's plotted location. It sums up a lot about the front's type and character (see " Decoding a Front"). After decoding you can see that there's a whale of a difference between a front labeled "411" and a front with a "493" designation.

So let's say you're planning a summertime VFR flight and a front is in the picture. Cloud bases are high, cloud tops are low, there are no lows aloft, and a surface analysis chart calls the front a 422. This is a weak cold front at the surface that shows little sign of change. There are no troublesome airmets and no sigmets, so you decide to launch. What's the best strategy for a safe VFR passage? There are several key rules, and they apply to VFR flight through all types of fronts.

Depart early. Unless morning fog is a factor, take off well before 9 a.m. This lets you complete your flight before daytime temperatures rise and create the worst turbulence and cloud buildups.

Cruise on top. This assumes that you can make a VFR climb to, and descent from, on-top conditions. Being on top lets you see the early signs of any cloud buildups and allows you to keep good visual separation from threatening clouds — a good idea whether you fly VFR or IFR. It's less turbulent up there, too.

Steer toward the light. It can be difficult to spot menacing cloud buildups if you're cruising beneath a cloud layer. Rain shafts, however, can signal the presence of storm cells. Steer away from the dark zones that rain shafts create, and you have a better chance of avoiding convection, downbursts, and storm-related turbulence.

Work your radios. Keep track of the weather en route by tuning in ATIS, AWOS, and ASOS broadcasts at airports along your flight path, and by checking in with flight watch (122.0 MHz). Don't be bashful about asking for detailed, nontechnical descriptions of the weather situation. The best time to call is about 15 minutes past the hour. That's when all the latest surface observations should have been posted.

Know when to throw in the towel. Feel uncomfortable about the weather? That's your good judgment talking. Listen to it by landing — or turning around — before things reach the high-stress level.

Percentagewise, it's surprising how often the weather exceeds VFR minimums. It's worth remembering that bad weather is heavily advertised, and thus should come as no surprise to the well informed.

So, VFR-only pilots, I say broaden your weather horizons. Don't let every front cold-cock your flying plans. You can often get through or around frontal zones with a minimum of fuss. Of course, staying on the ground is always a safe option — but it's also a way of cheating yourself out of valuable weather flying experience. Still nervous? Then take a more experienced, or instrument-rated, pilot along for the ride.

E-mail the author at [email protected].

Decoding a Front

Many surface analysis charts use a three-digit code to describe a front's personality. Here's how to decode this bit of arcana.

Translating the code is simple. Let's say a front is given a 492 label. Going from left to right, this decodes as a cold front at the surface (4) that's strong and increasing in strength (9), and that shows little sign of change (2).