Thunderstorms predictable at last

The anvil-shaped crown looms high in the air; the wind-wrapped clouds bristle with spikes of lightning and resound with thunder, evoking images of ancient sky gods -- Zeus wielding lightning bolts in Greek legend, Thor flinging his thundering hammer in Nordic myth. Throughout human history, thunderstorms have been a source of awe and mystery. The ancients invoked deities to account for the apparently capricious effects of nature. For centuries now man has turned to science for explanations. But only in the last year has science shed enough light on the process of thunderstorm formation that meteorologists can predict precisely when they will occur.

``In the past, meteorologists have assumed that thunderstorm formation was largely a random process. But we have determined that thunderstorms don't appear for no reason,'' explains James Wilson, a researcher at the National Center for Atmospheric Research (NCAR).

In the last year, Dr. Wilson and his colleagues have developed the ability to pinpoint spots where thunderstorms will develop 20 to 30 minutes before the first blot of cloud appears. The key, he explains, is in the pattern of surface winds.

Using a sophisticated instrument called Doppler radar, which can detect not only the position of clouds and rain (as conventional weather radar does), but also their motion, NCAR researchers have connected thunderstorm formation with surface-wind boundaries called ``convergent zones.'' These are lines, typically 20 to 30 miles long, where winds collide.

``Characteristically, the wind might be blowing 10 miles per hour east on one side and 5 to 10 m.p.h. west on the other,'' Wilson explains, breezes so light that most people wouldn't notice. They are strong enough, however, to trigger thunderstorm formation. When even such gentle winds converge, there is no place for the air to go but upward. And the resulting updraft triggers the formation of a storm 20 to 30 minutes later.

To locate areas where thunderstorms will appear requires identifying and tracking these lines of wind convergence. This is not as simple as it might seem, however. Radar, even the Doppler variety, does not ``see'' clear air. But it does pick up airborne objects, like raindrops or insects. And insects, it turns out, are swept up by convergent winds and clearly outline these boundary zones on the brightly hued Doppler radar screen. ``It appears as a long, thin line,'' Wilson says.

Thunderstorms always form on convergent lines, but every convergent line does not produce a thunderstorm. The most predictable condition, the scientist explains, is where two convergent lines collide. ``Over 90 percent of the time this produces a thunderstorm,'' he reports. NCAR researchers used this technique to predict a dozen thunderstorms that occurred last summer during a joint experiment with National Oceanic and Atmospheric Administration scientists and the Federal Aviation Administration at Denver's Stapleton Airport. Predicting the size of the storm that will develop is much more ``iffy,'' Wilson says, but if the converging winds are extremely strong, in the range of 30 to 40 m.p.h., then a large storm, or ``supercell,'' is frequently formed.

Thunderstorms appear to multiply naturally. They frequently unleash strong downdrafts that create new lines of convergence with prevailing winds. Scientists want to mount a major field study of this phenomenon.

Don't expect to see such predictions on the TV weather anytime soon, however. It will be some time before operational forecasters will have this capability.

First, Dr. Wilson and his colleagues must test it in other parts of the country. ``We suspect it happens everywhere,'' he comments. But they can't be sure until it has been tried out.

Second, the technique requires Doppler radar, currently used only in research. There is a federal program called NEXRAD to provide the National Weather Service with these sophisticated radars. The Office of Management and Budget has repeatedly cut it from the federal budget, while Congress has repeatedly reinstated it. Contractors are developing prototype systems, but the timetable for deployment is uncertain. Photos:1. Doppler screen shows a 20- to 30-mile-long line of mild easterly surface winds meeting a similar line of westerly breezes (pinpointed in diagram above). 2. Fifteen minutes later, thunderstorms begin to form -- circles on diagram. 3. Thirty minutes after original radar sighting, the converging winds have turned in to a strong line of storms -- large circles.

Now, for the first time, meteorologists can accurately predict thunderstorms half an hour before they form. The key is a sophisticated new tool, Doppler radar. Boundaries where surface winds converge are the clue; Doppler radar makes these visible.