Sunday, July 7, 2013

From Wet To Steamy

Drought has been the rule since the fall of 2009 in parts of South Carolina.  Only 6 out of the next 36 months saw above normal rainfall in Columbia, South Carolina until 2013.  Then the pattern changed and 2013 has been wetter than normal.  In fact, 4 out of the last 6 months have been above normal and July is likely to add to that total.

The first week of July has been extremely wet. The airport has seen 2.19 inches of rain while the city has received 4.35 inches of rain in just six days.  July is normally the wettest month of the year, but this year has been wetter than normal.  In fact, the last two weeks have been quite wet.

The map below shows the observed rainfall across South Carolina for the past two weeks ending on July 6.  Note that the area east of a line from Lancaster to Columbia to Barnwell has seen the greatest rain with 10 to 15 inches of rain in eastern Orangeburg and southeastern Clarendon counties.  The rain has been less west of the line until you get into the upstate of South Carolina.

The 14-day observed rainfall ending at 8 a.m. July 6, 2013.  Click on image for a larger view.  Image Credit: NOAA/AHPS.

Heavy rainfall has resulted in significant rises in many of the rivers with minor flooding of the Congaree River in the Columbia area.  The Congaree Swamp has seen considerable flooding over the past month closing some of the trails.  This is likely to continue as much of the flood waters from the Upstate have yet to pass through the Midlands.

So, why is this happening?  It turns out that our weather is linked to a pattern which has caused record heat in the Southwest and cooler than normal conditions in the Midwest.

The spring weather pattern was one of a persistent trough of low pressure over the eastern U.S. and a ridge of high pressure over the western U.S. in the upper atmosphere.  The unusually cool spring and northwesterly flow over the central U.S. contributed to a late tornado season.  This pattern began to change in mid-May and tornado activity surged toward the end of the spring season.

Normally the jet stream, a river of higher velocity winds in the upper atmosphere, retreats to the north.  This year it retreated north of the U.S./Canadian border as expected, but it left a ridge-trough-ridge pattern in its wake and persisted for much of late June into early July.

The upper atmospheric pattern for 00z July 4, 2013.  The dashed black lines are the axis of the troughs.  Click on image for a larger view.  Image Credit: WLTX-TV.

The high pressure system aloft in the West contributed to a brutal heat wave in the Southwest.  Death Valley, CA, reported the highest temperature ever recorded in the U.S. for the month of June when it reached 129 degrees F on the 30th.  This tied the record with 129 observed at Volcano, CA, a former town near the Salton Sea, on June 23, 1902.  Las Vegas reached 117 degrees F at McCarran International Airport to tie the all-time record high temperature.

Meanwhile the trough in the central U.S. brought relief to two years of scorching heat to the Plains.  Temperatures were as much as 30 degrees cooler than readings a year earlier in parts of the Midwest.  Recent rains had relieved much of the drought in that area.

East of the trough a persistent southerly flow brought abundant moisture to the East coast.  This has been the pattern for much of the last two weeks.

The surface map for the evening of July 3, 2013.  Note the extensive cloud cover for the eastern third of the U.S.  The arrow indicate the prevailing wind at the surface.  Click on image for a larger view.  Image Credit: WLTX-TV.

The trough has combined with the flow around the Bermuda high to bring copious amounts of moisture to the Southeast.  The result has been heavy rainfall with flooding from Alabama to the East coast.

The Bermuda high has moved slightly west over the past few days.  By moving westward the axis of heavy rain has been from the Florida Panhandle through Alabama and Georgia northward.  Some areas of southwestern North Carolina have seen 15 to 20 inches of rain in the past two weeks.

This shift will allow more sunshine to return to the midlands of South Carolina.  There will still be a chance for rain, but now it will be hotter.  High temperatures will be in the low 90s which is normal for this time of year.  However, the abundant rainfall will result in high humidity.  This will produce heat indices above 100 degrees F even with highs in the low 90s.  Thus, the pattern will shift from wet to steamy.  Drink plenty of water.

Side Note: The past month has been a busy one for me and there have not been any posts to the blog.  I have been on vacation and at the AMS Broadcast Conference.  If you follow me on Twitter (@JimGandyWLTX), you already know this.  I was fortunate to be in a climate change short course before the conference which had presentations from some of the leading climate scientists.  There were some fascinating discussions which I will share in the coming months.

Needless-to-say, there is a great deal of research in progress on the earth’s climate.  New research is bringing to light why the recent pattern we have experienced may not be so unusual.

Further reading:

Climate, Ice, and Weather Whiplash