Extreme weather is a common occurrence in Colorado. Thunderstorms are common east of the Continental divide in the spring and summer, and Colorado is one of leading states in deaths due to lightning. Hail is a common sight in the mountains east of the divide and in the northwest part of the state. While not as common as some of the states to the east, much of the Eastern Plains are prone to tornadoes, and there have been some damaging tornadoes there. An example is the 1990 Limon F3 tornado which devastated the city. Floods are also a factor in the plains, not just from the thunderstorms, but also due to heavy snow in the mountains followed by a warm, dry period which swells rivers with melted snow.

In the United States, flash floods are responsible for more deaths than any other thunderstorm phenomenon. Year to year in Colorado, only lightning is more deadly. Flash flooding usually is the result of very heavy rain in a short period of time over small area, which causes small streams to rage out of control, out of their banks. 

It is hard to imagine the destructive force of moving water, but the result was clear to see after the Big Thompson Canyon Flood of July 31, 1976. That flood killed 145 people, destroyed 418 homes and 52 businesses, and resulted in $35 million in damage. Survivors spoke of a wall of churning water racing down the canyon, an incredibly rapid rise in water level, and a loud roar. There was no way to outrace the water down the canyon, even with a vehicle. The only safety was found by climbing the steep canyon walls.

Flooding in Colorado can also result in the Spring when the mountain snowpack melts. Flooding of rivers and streams usually only occurs, though, when the snow run-off coincides with widespread heavy rainfall, but swift and high (non-flooding) water can be a definite safety hazard to rafters and other water enthusiasts.

If a dam fails, flash flooding will likely result. The destruction from flash flooding in July 1982 following the Long Lake Dam failure is preserved for all to see in Rocky Mountain National Park. 

Floods are the most common and widespread of all natural hazards. Some floods develop slowly, but flash floods can happen in just minutes.

Floodprone areas have been identified in 267 cities and towns and in all of the 64 counties in Colorado.

Over 250,000 people are living in Colorado’s floodplains. There are estimated to be 65,000 homes and 15,000 commercial, industrial, and business structures in identified floodplains. There are likely many more structures located within unmapped flood hazard areas. The value of the property, structures, and contents located in the identified floodplains is estimated to be over 11 billion dollars (1996).

Average annual flood losses in Colorado are estimated to be $17,600,000 in property damage based on data from 1896 to 1984.

Cumulative flood losses for the most damaging floods in Colorado between the turn of the century and 1993 include 331 people killed and $3.3 billion (1995 dollars) worth of property damage.