Article: Storm Tracking

Storm Tracking 

In tracking the development of storm systems, a meteorologist relies on two essential observational tools: satellite imagery and weather radar. Satellite images reveal the formation of storm fronts over broad areas: the whole state, several states, entire nations. From above, the satellite can detect rising columns of warm air, which often heraldagain on a broad scalethe development of a severe storm. Satellites are particularly useful for tracking tropical storms and hurricanes, which cover thousands of square miles and typically form far out over the ocean, beyond the range of other observing methods. 

For tracking smaller, fast-moving storms over the mainland, however, radar is the tool of choice. In use since the 1950s, radar enables the forecaster to monitor storms in more detail and to track their motions on a minute-by-minute basis. At its simplest, weather radar works by detecting the precipitation in incoming storms. (It can also pick up other small objects, from dust motes to migrating butterflies and birds.) The radar unit emits a radio wave at very high frequency, then measures how long the signal takes to return after bouncing off of raindrops, hail, snow or whatever else the coming storm has to offer. Since the radio wave travels at a fixed rate (the speed of light, more than 350,000 kilometers per second), the length of the echo delay reveals how far away the storm is. Most radar units send out about 1,000 radio pulses per second and aim them at several altitudes simultaneously; the result is a rich, real-time view of the storm.

Though a marvelous advance at the time, the first-generation weather radar pales in comparison to the technology in use today. In the late 1980s, the federal government began building an advanced, nationwide radar network called the National Weather Service Next Generation Weather Radars, or NEXRAD. The new radars make extensive use of computers; among other advantages, this means they can be programmed to sound an alarm when weather patterns are beginning to appear dangerous, and they can be operated by automation. (In the old days, a forecaster had to watch the screen continuously; taking readings of high-altitude activity involved pointing the radar upward with a hand crank.) More important, NEXRAD radars gather critical data using so-called Doppler technology. Imagine a distant car: as it advances, its sound registers at a slightly higher frequency than it would if it remained stationary. Likewise, precipitation that is blowing toward a radar antenna shifts toward a slightly higher frequency, while precipitation that is blowing away from the antenna shifts to a slightly lower frequency. From these Doppler shifts, the radar's computer calculates the direction and speeds of the winds surrounding the droplets. The result is a detailed map that reveals the location, intensity and direction both of wind bursts and precipitation across dozens of square miles.

Doppler radar has drastically improved the ability of forecasters to track the development of severe storms like tornadoes and derechoes. When a tornado takes shape, its winds blow raindrops in a circular pattern; on a radar screen, that pattern is as distinctive as the whorls of a fingerprint. A derecho also leaves a unique radar fingerprint. When a broad, fast-moving line of thunderstorms begins to take on the characteristics of a derecho, a wide band of high winds, all blowing directly forward, develops at the front edge of the storm. This band of wind produces a spray of raindrops that, when it appears as a radar echo, typically forms the shape of a plow or a bow; this distinctive pattern is called a "bow echo." With these patterns in mind, and with a deep knowledge of the kinds of storm systems and radar patterns that often precede the formation of tornadoes and derechos, today's meteorologists can provide the public with remarkably accurate storm warnings. Of the two storm types, tornadoes are easier to monitor: though they pack strong winds, tornadoes themselves move relatively slowly (30 to 50 kilometers per hour), so their paths can be predicted with reasonable confidence. Derechoes are trickier: they are both faster and more amorphous than tornadoes, and since only a dozen or so occur annually in the U.S., their vicissitudes are less familiar to atmospheric scientists and forecasters.

Radar does have its limitations. Due to the curvature of the Earth, an individual radar station can't "see" effectively beyond about 200 kilometers. For that reason, to track storms accurately over long distances, meteorologists rely on a network of widely spaced radar stations. In addition, although an individual radar unit can measure precipitation and winds that are moving toward or away from it, it can't detect winds that are moving directly perpendicular to it. With tornadoes, that's not a problem: their winds move circularly, so there is always some wind moving toward the radar and other wind moving away from it. Once it develops, a tornado generates a distinctive echo pattern that remains visible to the radar regardless of which way the tornado itself moves.

Derechoes, however, can present a unique challenge to the meteorologist. Their winds blow largely in one direction: forward. If the derecho is moving toward a radar unit, even at an angle, its distinctive wind-pattern--a bow-shaped front--will become visible, enabling the forecaster to follow the storm as it moves and develops. But there are times, right when the derecho passes the radar station (often at a distance of several miles), when the storm and its accompanying winds are moving exactly perpendicular to the radar. At this point the winds produce no radar signature at all, and the storm front dwindles on the radar screen. This can be a distressing moment for the forecaster, who suddenly (and for several minutes) suffers an acute lack of information.

That's what happened on July 4, 1999, shortly before a derecho struck the Boundary Waters Canoe Wilderness Area in northern Minnesota. Forecasters in nearby Duluth, Minnesota, had been monitoring the advance of a large, severe storm from North Dakota. Just as the storm passed to their north, barreling toward the Boundary Waters region, the wind signature diminished on the radar screen. Meteorologists could still view the bow-echo reflection of raindrops well above ground, so they still had a rough idea of where the gust front was located at the surface. But they could no longer determine actual wind speeds at ground level. As it turned out, this was precisely the moment when the storm evolved into a full-blown derecho. Typically, a forecaster could compensate for the loss of wind data by checking data from another radar station. As fate would have it, however, Duluth was the last radar station along the storm's path to northern Minnesota--and the last radar to see the storm before it turned into a derecho.