Relentless rain unleashed catastrophic flooding across West Virginia late last week. Officials say it was the state’s worst flood disaster in more than a century.

According to the NWS, the Mountain State received about 25% of its average annual rainfall in just a few hours. In Greenbrier County, more than 10 inches of rain fell between Thursday and Friday. This massive amount of precipitation in such a short period of time overwhelmed rivers and streams throughout the area. In Kanawha County, which includes the state capital of Charleston, the Elkview River crested at 33.37 feet – its highest crest in more than 125 years of record keeping.

The raging torrents of floodwater damaged or destroyed thousands of homes and businesses, as well as infrastructure across the state. To date, twenty-three weather-related deaths have been reported and more than 10,000 customers are still without power.

This type of rainfall is considered a one in one thousand year event in West Virginia. That does not mean it can only happen once every thousand years. It is the recurrence interval, a statistical calculation that means an event has a one in one thousand chance (0.1%) of happening in any given year in a given location.

The cause of this devastating flooding was twofold and involved a combination of weather and topography. First, “training” thunderstorms developed along a boundary between cooler air to the northeast and warm, moist air to the southeast. This is a meteorological phenomenon where strong storms flow continuously over the same area for a relatively short period of time – like train cars traveling along a track – dumping excessive amounts of rain.

The second major player in this deadly deluge was the state’s mountainous topography. When substantial rain falls in hilly terrain, it runs downslope very quickly and causes flash flooding in valleys, where most people tend to live. Moving with tremendous force, this type of fast flowing water can pick up and destroy almost anything in its path.

The Governor of West Virginia, Earl Ray Tomblin, has declared a state of emergency in 44 of the state’s 55 counties as a result of the flooding. Additionally, President Obama declared a major disaster in three of the hardest hit counties – Kanawha, Greenbrier, and Nicholas – which allows federal funds to supplement state and local emergency efforts.

Flooding in Richwood, Nicholas County, West Virginia. Credit: J. Rose/ABC11