May 1, 2004
By Thomas A. Horne
We've all been there. You're at the airport or standing in your yard and you notice the dark, low-lying clouds rushing by on strong southerly winds. An oppressive humidity lends palpable moisture to the air. The temperature has sharply increased over the past few hours, too. There's a definite sense of imminence about this weather.
What's imminent is usually one or more thunderstorms, probably of the severe variety. As you recall from ground school, any thunderstorm is bad news, but severe thunderstorms are the worst of the worst. By definition, a severe thunderstorm has one or more of the following: surface winds of 50 knots or greater, hail three-quarter inch or more in diameter, or tornadoes. The scenario just painted also implies that the impending thunderstorms are associated with passage of a cold front. Remember that as a cold front's advancing air plows beneath the warmer air ahead of it, tremendous lifting forces go to work on the humid, unstable warmer air mass being displaced. It's a perfect recipe for thunderstorms.
And why are the winds out of southerly directions in the air ahead of the cold front, and why are the winds picking up? Winds flow counterclockwise around low-pressure systems, and fronts are just extensions of low pressure. So ahead of a cold front, winds are out of the southerly points of the compass. Behind it, winds blow from westerly and northwesterly directions. This explains why a wind shift defines a front's passage. Look at a surface analysis chart, study the isobars, and you can visualize how winds change direction when a front moves through.
Using all these cues, judging thunderstorm likelihood is easy when we're watching from a ground-bound perspective (which isn't a bad way at all to experience the onset of convective weather). But we also need a way to know when severe storms are in the offing when we do our preflight briefings. Sure, a telephone or other official briefing should convey any pertinent convective sigmets or thunderstorm watches and warnings, but you can also make use of some of the same forecast rules of thumb that expert analysts rely upon. Here's where Web sites publishing surface and upper-air charts come in handy. You can look at these charts (surface analysis and constant-pressure charts are available, free, on AOPA Online, but there are no height contours on the constant-pressure charts, so you have to look at the wind barbs and envision the flows aloft) and develop your own insights, compare them with what actually happens, and be on guard while flying near suspect regions.
Forecasters at the Storm Prediction Center in Norman, Oklahoma, have used a "crib sheet" to help them quickly focus on trouble spots. The idea is to identify certain signature alignments of surface and upper-air features that have a history of producing severe thunderstorms.
The illustrations on the preceding page show some of the severe-storm setups, and you can see for yourself where the likely convective zones are located. The setups have a few things in common.
One is the presence of strong winds aloft, as shown on constant-pressure charts. These winds can work at low levels — as low as 3,000 to 5,000 feet — as well as altitudes at the jet-stream level. Jet streams, high-speed winds blowing at speeds from 70 knots to as high as 200 or more knots, are usually found at altitudes between 18,000 and 30,000 feet. They flow around troughs aloft — U-shape southward extensions of low pressure — and can be identified on constant-pressure charts. The 500-millibar constant-pressure chart shows the height contours of the pressure surface at about 18,000 feet msl. The 300-millibar chart shows the pressure patterns at approximately 30,000 feet.
Strong winds aloft are big factors in storm development because they cause the air to diverge, or spread out. This, in turn, causes surface wind flows to converge in the lower levels of the atmosphere and, with nowhere else to go, rise and feed the lifting forces in an already-unstable warm air mass.
The other common factor is a conducive surface low-pressure complex — conducive to the low-level convergence we just talked about. These setups can be seen on surface analysis charts, and often consist of young lows with warm and cold fronts extending from them.
The areas of greatest convergence — and therefore the areas with the greatest potential for severe storms — are in the triangular region formed around the low's center and bounded by the two fronts. Called Larko's Triangle after the meteorologist who stressed its importance as a forecast tool, it is a prime trouble spot because of all the unstable air being stuffed into it by the converging winds near the center of low pressure. The triangle extends one to two isobars deep into the warm sector of a low-pressure complex.
Next time you're surfing for preflight weather information in the thunderstorm season, compare the surface- and constant-pressure charts, and look for the signposts we show here. They could spare you a nasty surprise.
E-mail the author at firstname.lastname@example.org.
AOPA Pilot Editor at Large Tom Horne has worked at AOPA since the early 1980s. He began flying in 1975 and has an airline transport pilot and flight instructor certificates. He’s flown everything from ultralights to Gulfstreams and ferried numerous piston airplanes across the Atlantic.
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