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Severe Weather Awareness Week: What's The Difference Between A Flood And Flash Flood?

Flash Flood Safety

Floods are no stranger to America and no stranger to South Carolina. The historic floods of October 2015 resulted in nearly 1.5 billion in losses and killed 19 people in the state. Our state's proximity to the Atlantic Ocean and Gulf of Mexico make it easy for copious moisture to reach us. Add in the low-lying terrain of the Lowcountry, the hillier and even mountainous terrain in parts of the Upstate, and the many rivers over the state -- and one has the ingredients for flooding.

You may often hear the terms "flood" and "flash flood", and wonder whether there is a difference between the two. In meteorological terms, a flood is an overflow of water on normally dry land that can last for days or weeks. An example of a flood is a river flood; some of the recent major flooding along the Waccamaw River would be characterized as a flood. The National Weather Service issues Flood Warnings for these events that may last for weeks or even months. You may see a "Flood Warning" show up on your favorite news web site or mobile application on a perfectly sunny day for your county or location. That is a good indication there is a river in flood nearby.

A "flash flood" occurs much faster -- in a flash. Typically, heavy rain in a 6-hour period or less that causes flooding is considered a flash flood. The National Weather Service will issue a Flash Flood Warning when conditions are severe enough to significantly disrupt travel, or when the flash flood poses a threat to life or property. The most serious flash floods -- those that National Weather Service Meteorologists believe will have considerable or catastrophic effects -- could also trigger an alert on your cell phone. These alerts are part of the Wireless Emergency Alerts program (WEA, for short) and these alerts are available if you have the feature enabled on most modern mobile phones.

Flood Watches and Flash Flood Watches may precede the issuance of warnings for both hazards, but not always. The watches are issued many hours to a couple of days in advance, and are designed to give everyone early notice about the possibility of flooding or flash flooding. A "watch" simply means the ingredients for flooding could be in place; it does not necessarily mean a flood or flash flood will occur. There are many instances, especially in the summer, when a slow-moving thunderstorm sits over an area and produces flash flooding. In these situations, watches may not be in effect because of the isolated nature of the flooding. Instead, warnings would be issued.

Flash flooding is the most deadly hazard from thunderstorms in the United States and many of these fatalities occur in automobiles. There are things you can do to protect yourself: never drive through flooded roadways. It is often difficult to judge the depth of the water, particularly at night. Your car may stall or could easily be carried away by several inches of fast-moving water. If you're at a home or business, seek higher ground until help arrives. If you're in a flood-prone area, having a plan in place to take quick, protective action is a must. It is possible you will only have minutes to react to a warning from flash flooding.