Thursday, July 25, 2013

Columbia's Extreme Rain Event

A complex of heavy thunderstorms formed over the Columbia, SC area last Sunday, July 21, 2012.  The heavy rains resulted in what is known as an extreme rain event.  That means that rainfall for the day had to equal or exceed 2.52 inches to be in the top 1% (99th percentile) based on the current climate statistics.  The official total at the National Weather Service Forecast Office was 2.55 inches, thus an extreme rainfall day.

The slow moving thunderstorms unleashed a torrent of rain producing two to five inches of rain in a couple of hours.  The highest rainfall totals stretched from Lexington to Forest Acres on the east side of Columbia.  Reports of 4.89 inches came in from west of Lexington, 4.56 inches from West Columbia, and 3.48 inches from Forest Acres.

As one can imagine with all of the rain this summer the heavy rains led to flooding.  All of the usual spots flooded, but there were areas not prone to seeing so much water.  Flooding was reported in parts of Forest Acres.  Rain was so heavy in West Columbia that motorists on I-26 had to slow down and pull over in some cases.

A graph of the water levels for Rocky Branch Creek for July 17-24, 2013.  Note that a previous flood occurred on the 17th with bigger flood on the 24th.  Click on the image for a larger view.  Image Credit: USGS.
Water rushed into Rocky Branch Creek that flows through the Five Points Area.  By the time it reached Whaley Street it was the third highest flood on record.  The water level reached 11.46 feet, whereas flood stage is at 7.2 feet.  Roads were closed and cars were flooded.

Click on the image for a larger view. Image Credit: Climate Central.

According to the National Climate Assessment, released in draft form back in January, the most extreme precipitation events (those in the 99th percentile of intensity) have increased in every region of the contiguous states since 1950. As the map above shows, the rise in intensity has been greatest in the Northeast and least in the Northwest — and in all cases, climate scientists believe, the reason is simple: in a world warmed by heat-trapping greenhouse gases, there’s more evaporation, and the atmosphere can hold on to more water. And when that water vapor condenses as rain or snow, there’s more of it.

If you go all the way back to 1901 and focus just on the Southeast, below, the picture is very much the same. It is important to note that while the trend in intensity has been upward, it has not been steady: there are ups and downs from one decade to the next. This shows something else climate scientists often point out: human-caused climate change hasn’t replaced natural variability: it comes on top of it.

Click on the image for a larger view. Image Credit: Climate Central.

Local studies have shown that even as the climate is getting drier the number of heavy downpours and extreme rain events is increasing.  This was discussed last year in a previous post.  As the climate warms more moisture enters the atmosphere.  This leads to heavier downpours when it does rain.

Forecasting such events is quite challenging.  Usually they cannot be forecast before the event.  Radar observations are often the first indication that something unusual is occurring.  The observations led to a flash flood warning for Columbia in the case of last Sunday.

Whenever faced with heavy rains and flooded streets it is prudent to turn around.  The National Weather Service slogan is “turn around, don’t drown”.  This is good advice.