Tuesday, July 23, 2013

From One Extreme To Another

June was a month of extremes across the U.S. Record heat dominated the West, where all time record June temperatures were being set in multiple states. Meanwhile, the same weather pattern that supported the prolonged, intense heat across the West locked in an extremely wet set up across the East. Not only were numerous daily rainfall records set, but both Philadelphia, PA and Macon, GA, ended up with their wettest June ever.

Some of the extreme weather across the U.S. in June 2013.  Click on image for a larger view. Image Credit: Climate Central.

You can also add Augusta, GA, to the wettest June on record.  Bush Field in Augusta received 10.83 inches of rain which was 6.11 inches above normal. An observer near Martinez, northwest of August, received 16.02 inches for June.  This is remarkable since that area of Georgia and South Carolina had been suffering from nearly 3 years of drought until recently.

Just over a year ago the short-term drought indicator had the central Savannah River Valley in an exceptional drought.  The same indicator now has the region in the wettest category.  The area has gone from drought to flood in a year’s time, from one extreme to another.

Objective short-term drought indicator blend from April 7, 2012 and July 13, 2013.  Click on the image for a larger view.  Image Credit: NOAA/CPC.

A number of rivers have flooded across South Carolina due to the heavy rains this summer.  June was a wet month for the entire state, but July has proved to be even wetter.  The maps below show the observed rainfall for the month of June and for the first 15 days of July.  June rainfall was 6.17 in. at the Columbia Metropolitan Airport, but it was 6.43 in. for July 15.

Observed June 2013 rainfall for South Carolina.  Click on the image for a larger view.  Image Credit: NOAA/AHPS.

Observed rainfall in South Carolina for July 1-14, 2013.  Click on the image for a larger view.  Image Credit: NOAA/AHPS.

This has given rise to the thoughts that this may be the wettest summer ever in Columbia.  Not quite yet.  The wettest summer (June, July, & August) on record was in 1971 when the rainfall total was 29.28 in (June = 7.46, July = 11.13, & August = 10.69).  We are tracking close to the wettest, but much will depend on August.

Why is it so wet?

The weather pattern this has been one of a ridge of high pressure across the West with a trough of low pressure in the East.  There have been daily changes, but this has been the norm this summer.  The trough axis has been west of the Appalachians putting much of the East coast under a very moist tropical flow.  The rain has been persistent and locally heavy which has contributed to flooding.

In addition, studies have shown that the warmer climate has led to higher moisture levels in the atmosphere.  This tends to get concentrated in the moist flows from tropical regions.  Thus, the heavy rains may be heavier.  Flash floods are now more likely due to the saturation of the ground.

A torrential downpour hit Columbia this past Sunday producing an extreme rainfall event.  This resulted in the third highest flood of Rocky Branch Creek.  More on this in my next post.