Wx Watch: Preflighting for Storms

Advice for the neophyte and the old pro

June 1, 2001

It's that time again — time for thunderstorm season, and the angst that can come with it. Is it OK to fly today? What are the signs that a stormy flight is in the offing? Will forecasts prove true, or will they be a bust? Questions like these are common in the warmer months, and here are some guidelines (hard-and-fast answers simply don't exist in the world of weather-related decision making) that will help you make the wisest preflight decisions.

The goal of the preflight process is to make a good, safe go/no-go decision. And sure, the weather is a big, big part of the decision. But the pilot and the aircraft are extremely important variables in the process that are often overlooked.

Pilot. If the weather's iffy (it doesn't need to be convective weather, either) then you need to be candid with yourself. Do you have the experience, the skill level, the currency, and the inclination to take on adverse weather? You may have an instrument rating and plenty of hours, but in thunderstorm season it won't mean a thing if you haven't been exposed to some real-world weather flying. This is where tagging along with a more experienced pilot can pay off. Fly around convective weather with an old hand and you'll learn volumes more than any textbook can impart to you. That's not to say you can't gain experience on your own. By playing it very conservatively and scrupulously avoiding any cloud buildups, you can slowly teach yourself how to steer clear of trouble. So there's preflight rule one: Don't fly in or near convective weather if you haven't done it before.

Aircraft. Next, think about the aircraft you'll be flying. Does it have enough power to let you climb high above lower cloud layers and let you see buildups as they grow? Is it fast and long-legged enough to let you make a wide circumnavigation of a line or cluster of thunderstorms? Does it have pressurization or oxygen to help you out should you have to spend long periods of time at high altitude as you keep visual separation from buildups? If the answers to those questions are no, then think long and hard before you attempt a long cross-country through areas with a strong probability of thunderstorms.

Then there's the issue of storm-sensing equipment. Is your airplane equipped with lightning-detection equipment? Weather radar? With regard to those last two issues, the pilot element figures in here, too. If you aren't completely proficient in operating and interpreting radar and lightning-detection imagery, then the most powerful radar in the world won't help you. If you haven't taken a course in radar operation and interpretation, then consider yourself unprepared to use airborne weather radar. What you don't know can kill you. This gives us another preflight rule: For longer flights, have an airplane with ample performance margins, and, if the airplane is equipped with them, have the know-how to operate and interpret radar and lightning-detection equipment inside out.

The weather briefing. Of course, you need a thorough preflight weather briefing any time you fly. You'll want a standard briefing at first — complete with METARs, TAFs, area forecasts, airmets, and convective sigmets (if storms are already active) — followed by abbreviated briefings if you need to check for changes or updates in the interval before your projected takeoff. For thunderstorm season there are some elements that you should make an extra effort to check out for yourself. These are:

  • The convective outlook. Produced by the National Weather Service's Storm Prediction Center (SPC), the convective outlook shows the areas forecast to experience severe thunderstorms (those with surface winds at or above 50 knots, and/or hail at least three-quarters of an inch in diameter, and/or tornadoes) or "general" thunderstorms. The SPC issues two outlooks — one for the current day and one for the next day, or "day 2." Visit the Web site ( www.spc.noaa.gov/products/outlook/day1otlk.html) to see an example. A text description of any convective goings-on follows the storm, hail, and damaging-wind charts. In my experience the convective outlooks are seldom wrong.
  • Convective sigmets. These outline areas where thunderstorms have a confirmed presence. Tagged with an identifier and described in text format, these are the prime warning signs. When first issued, ATC will broadcast convective sigmets over frequencies used in instrument flight rules operations. VORs with HIWAS (hazardous in-flight weather advisory service) capability will broadcast convective sigmets. And, of course, flight service will tell you of any convective sigmets via flight watch (122.0 MHz) or one of its designated flight service frequencies (such as 122.2, 122.3, 122.4, 122.5, or 122.6 MHz). They are good to know when you need an in-flight update of changing weather.

Outlooks are fine for predictions, but the radar summary charts can let you know what's happening in near-real time. Examine these for lines or clusters of storms, as well as cloud-top altitudes and directions of movement. Just be aware that radar summary charts can be as much as three hours old, so look at the time stamp to learn just how old it may be. Also, as they say in the investment world, "past performance is no guarantee of future behavior." In other words, just because a storm is behaving one way or moving in one direction on a radar summary chart is no guarantee that it will continue on what appears to be a trend. Thunderstorms can suddenly intensify, dissipate, or change their direction or speed.

Similarly, satellite imagery can help you observe storm and cloud behavior. The infrared imagery generated by the GOES-8 and -10 satellites shows the highest, and therefore coldest, cloud tops in bright white. Lower-lying clouds are progressively grayer in color. The darkest shades of gray are the lowest cloud or fog layers. Most Internet weather providers let you "loop" (animate and replay several hours' worth of imagery) satellite shots and radar plots, and this can be a great visual aid in watching storm motions and apparent trends.

You'll learn about dew points from any METARs a briefer gives you, but don't let them get lost in the rest of the report. Relatively high (above 60 degrees Fahrenheit/15 degrees Celsius) dew points are a sign of trouble. When air masses become this saturated with moisture, the stage is set for convective troubles as the heat of the day bears down. Dew points above 70 degrees F/ 21 degrees C are convective dynamite, and any mention of dew points this high should set off alarms.

Constant pressure charts show pressure patterns aloft by means of height contours — lines connecting points of equal pressure. Pronounced southward-dipping troughs on the 500-millibar (about 18,000 feet msl) chart can be a tip-off of severe thunderstorms later. The southeast section of a trough's U-shape is a place where lifting actions aloft can cause strong convection at the surface. If the trough's shape is similar at higher pressure levels (as shown on the 300-millibar/30,000 feet msl, for example) then this is evidence of additional lifting motions contributed by a jet stream. The southeast corner of the U won't be a nice place to fly — unless you happen to be researching severe thunderstorms.

RAREPS/RAOBS — short for radar reports and radar observations — are text descriptions of individual storm cell, -line, or -cluster dimensions, intensities, and directions of travel. They use the connect-the-dots method of defining the extent of any precipitation returns — the same method used in convective sigmets and airmets, for example. RAREPS and RAOBS can paint a finer picture of echo activity you have already observed on radar summary charts, satellite imagery, or ground-based Doppler weather radar imagery. Moreover, they'll point out any especially dangerous-looking radar signatures such as hooks or bow echoes — which are associated with tornadic activity.

High negative numbers (minus 3 to minus 6) in a lifted index are a sign of very unstable air and are therefore another indicator of thunderstorm potential. This number is found on the four-panel composite moisture stability chart. Other useful charts in this product show the freezing level and average relative humidity.

By now you should have noticed that some of the weather information listed above is predictive in nature (convective outlook, dew points, constant pressure charts, lifted indices) and some is indicative and descriptive of actual thunderstorms (convective sigmets, radar summary charts and ground-based radar information, satellite imagery, and RAREPS/RAOBS). The predictive information is good for guesstimates and advisory purposes; the descriptive information is the real deal. Which brings us to some final preflight rules: Don't take off in convective conditions and don't knowingly fly into an area marked as a convective sigmet. That's easy to say from an armchair perspective, but what if your briefing made no mention of thunderstorms, you took off, and now you're flying along and find yourself inadvertently stumbling into clouds that ultimately turn ugly, or find yourself trapped and flanked by thunderstorms? How do you avoid those situations?

That's another story. And we'll address that in next month's installment of "Wx Watch."


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