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Flash Floods
Flash floods are significantly different from normal floods, according to Oracle Thinkquest. Normal floods occur when water rises and overflows its normal path. A flash flood, however, appears quickly and moves swiftly across land with little warning. The water in a flash flood moves at such high speeds that it can move large objects, such as cars and boulders, uproot trees and even cause bridges and small buildings to collapse.
Occurrence of a Flash Flood
Rainfall intensity and duration are the most important factors associated with flash-flood occurrence, as explained by the NOAA. Topography, land-use and soil conditions also play a key role. Generally, flash floods occur following a period of short, intense rainfall, or after a period of sustained rainfall in which the soil has become saturated, encouraging surface runoff. Therefore, the types of weather that cause, and come with, flash floods are slow-moving thunderstorms, thunderstorms repeatedly moving over the same area or tropical storms.
Thunderstorms
Thunderstorms are the type of weather common to all flash floods (except those caused by human infrastructure failures in periods of calm weather, such as dam failures, which can occur under any weather conditions). Thunderstorms bring heavy rainfall, sometimes hail, and are synonymous with lightning strikes. In addition, high wind speeds can occur during the storm and as it passes through a particular area. Lightning and high winds pose a serious risk to lives and property.
Tropical Storms
Flash floods can occur with extreme weather events such as hurricanes. Tropical revolving storms such as hurricanes develop over the oceans at tropical latitudes and tend to move inland. Extreme weather such as very high winds, thunder and lightning and torrential rain is associated with hurricanes. Weather such as this poses a severe threat to people and property.
Tornadoes
Tornadoes usually occur during severe thunderstorms, which means they can often occur with flash floods. A tornado is a violently rotating column of air, extending from the thunderstorm to the ground. According to NOAA, wind speeds within a tornado can reach 250 miles per hour, and they can travel for up to 50 miles across the ground, at speeds reaching 70 miles per hour. Like many of the other weather events that can occur with flash floods, tornadoes pose a serious risk to human life.
Snow Melt
Warmer spring temperatures can induce melting of snow and ice, with excess runoff causing rivers to rise rapidly. For example, the Utah Center for Climate and Weather documents how, in 1983, rapid warming during the last week of May and early June caused rivers to rise dramatically throughout Utah. In addition, widespread rains during the melting period can warm up snowpacks, causing them to melt earlier than they would normally, potentially leading to flash floods, as indicated by the Flood Safety website of the NOAA.