Severe Weather Awareness Week: Flooding and Flash Flooding

Severe Weather Awareness Week: Flooding and Flash Flooding
Flash floods happen quickly and are a serious threat to life and property. (National Weather Service Birmingham)

Alabama is susceptible to flooding year-round because of its proximity to the Gulf of Mexico and the nearly unlimited supply of moisture it provides. In our case, this can occur from slow-moving summertime thunderstorms, a decaying tropical system or large storm systems.

Sometimes floods develop slowly and forecasters can anticipate where a flood will happen days before it occurs. Often flash floods can occur within minutes and, in a mountainous area, sometimes without any sign of rain.

Flooding is the most damaging, costly and deadly severe weather-related phenomenon, costing the United States more than $5 billion in property damage yearly. On average, flooding is responsible for more deaths each year than lightning or tornadoes.

Different types of floods occur across Alabama.

Flash flood

A flash flood is flooding that normally occurs within six hours of heavy or intense rainfall. These floods develop very quickly and are a significant threat to life and property.

Areal flood

An areal flood develops more gradually, usually from prolonged and persistent moderate to heavy rainfall. These floods usually result in a gradual ponding or buildup of water in low-lying, flood-prone areas as well as small creeks and streams.

Ponding in a field shows the effect of areal flooding, during which water builds gradually. (National Weather Service Birmingham)

River flood

A river flood is flooding caused by a river or stream overflowing its banks because of recent heavy rainfall or prolonged rainfall over the drainage basin. This, in turn, inundates areas that are normally dry, such as low-lying areas near the river bank.

A river floods the nearby land in Guerneville, California. (NOAA Habitat Conservation)

Here are a few helpful flood safety rules to follow:

  • Know what to listen for. A Flash Flood Watch or Flood Watch means conditions are favorable for sudden short-term flooding (less than six hours), or long-duration flooding (longer than six hours), respectively. A Flash Flood Warning or Flood Warning means flooding conditions are imminent or occurring and you should take action immediately. A River Flood Warning is issued when river, stream or lake levels are expected to rise above bankful.
  • Move to higher ground away from low-lying areas, storm drains and stream beds. Do not return to flooded areas. Flood waters carry debris that could cause serious injury or death. Water could be moving very quickly just below the surface. Only 6 inches of fast-moving water can knock an adult over. Children should not be allowed to play or walk near flowing water.
  • Never drive across flooded roadways or around barricades. Flood waters can rise very quickly, covering your vehicle or sweeping it downstream. Just 2 feet of water can move most vehicles, including trucks and large SUVs. Road surfaces could be washed away or large debris might be below the surface. If your vehicle is caught in rising water, abandon it immediately and seek higher ground.
  • Be especially cautious at night. Flood dangers are harder to recognize at night.

On Wednesday, we take a look at tornadoes. Read more about severe weather awareness here and about severe thunderstorms here.

For more weather news and information, visit AlabamaWx.

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