A World of Weather: Chapter 13 Introduction

CHAPTER 13

RECENT GREAT FLOODS AND THE DROUGHT OF 1988
Flash floods strike with dangerous swiftness. They can happen when a slow-moving thunderstorm concentrates heavy rain over a limited area. They can happen when thunderstorms erupt downwind of a given locality and, like cars in a freight train, are steered over a single track, bringing repeated downpours. The adjective "flash", like the "flash" bulb on a camera, sheds light on the nature of these floods: water in creeks and small rivers rises rapidly (generally in less than six hours) as run- off from heavy, localized rains overwhelms a drainage system. Flash floods can also be incited by dam failures and sudden releases of ice jams.

When a flash flood is possible but not certain, the National Weather Service may broadcast a flash flood watch. Once flash flooding is in progress, a flash flood warning is issued. The National Weather Service makes a slight distinction between flash floods and small stream floods because flooding of small streams can occur so quickly that monitoring the stages of streams is unrealistic. Thus, watches and warnings are issued when small stream floods are a threat or are forecasted to occur. Urban floods should not be confused with flash or small-stream floods. Flooding of streets, underpasses or other low-lying urban areas caused by agents other than rivers and streams (clogged storm drains, for example) are in a class by themselves, though they, too, occur over relatively short periods of time.

In contrast to floods that happen over short time scales, flooding along major rivers such as the Ohio, Mississippi and Missouri typically builds in a slower swell of water. If flash floods can be likened to a flash bulb on a camera, then large- scale floods are like time-delayed photography, with rivers taking longer to rise, crest and eventually recede. Indeed, it takes time for rainwater entering a drainage basin through streams and creeks to make its way to the main river. Thus, crests on major waterways can be delayed for days, sometimes weeks, especially when recurrent, heavy rains continue to feed run-off into the river basin for an extended period of time.

Flooding along major rivers can occur at any time of the year. The clockwork floods of spring, for example, usually result from combinations of melting snow, ice jams and teeming seasonal rains. These floods are costly and, in some cases, quite extreme. A case in point þ after a bevy of blizzards and heavy snow during the winter of 1996-97, the northward-flowing Red River inundated parts of North Dakota, Minnesota, and the Canadian province of Manitoba as snow rapidly melted and ice jams occasionally dammed the river. The city of Grand Forks, ND became a ghost town after record flooding forced massive evacutations.

Despite the extreme nature of some spring floods, the main focus of this chapter will be on floods generated solely from recurrent heavy rains. In particular, we will delve into the causes of the Great Flood that ravaged the Middle West during the summer of 1993, the extreme flooding that occurred in California during January 1995, and the record flooding in Georgia spawned by the sluggish remnants of Tropical Storm Alberto in July 1994.

In many cases, the catalyst for major flooding is a stagnant weather pattern, one that can be likened to a stereo needle stuck in a scratch of an old vinyl album. Each scratching sound can be thought of as a new cluster of thunderstorms or an area of heavy rain being generated by a stalled or slow-moving weather system.

Stagnant weather systems can also breed drought. In summer, for example, an expansive high pressure system holding steady in the upper troposphere can suppress showers and thunderstorms, paving the way for drought. With abundant sunshine and high temperatures also accelerating evaporation rates from the ground, soil rapidly dries and crops wilt. Later in this chapter, we will probe the devastating drought that withered the Great Plains during the long, hot summer of 1988, as well as the drought in the northeastern United States during the summer of 1995.

Color Plate 52 shows the major and record flooding along the main river systems of the Middle West during the summer of 1993. Nine states were catastrophically impacted by this flooding, and more than 75 towns were inundated by the flood waters. The recurrent bouts of widespread heavy rains required to inflict such flooding can be traced to the jet stream, which, like the stereo needle caught in a scratch on a vinyl album, got stuck in an atmospheric groove over North America. As a result, rhythmic waves of heavy thunderstorms were generated over the course of three to four months. How can the mid-latitude jet stream, whose duty is to keep mid-latitude weather systems moving along, get bogged down in a pattern of stagnancy? One way to stop the determined jet stream is to block its path.