A Tale Of Two Halves

This summer has been one of persistence across the U.S. in the general weather pattern.  The West has been too dry and the East too wet.  Severe to exceptional drought covers much of the western half of the country, while flooding and downpours have drenched the Southeast and Mid-Atlantic.  Some areas have seen relief from the devastating drought of last year and it has gotten worse for others.

The U.S. Drought Monitor for August 6, 2013.  This shows the areal extent of drought across the U.S. at this point in time.  Click on the image for a larger view.  Image Credit: NOAA/USDA.

The worst of the drought continues to be in the High Plains, but it is getting worse over much of the Southwest.  This has already led to an above normal wildfire season in the West.

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% (99^th 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.

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.

2013 Hurricane Season: Very Active?

It has been a little over a month since the last of the 2013 hurricane season forecasts were made.  All of the forecasts call for an active season with the possibility of a very active season.  The third named storm of the season (Chantal) formed late on July 6, which is just over a month earlier than when the average “C” storm forms.  Is this a sign of a very active season?  Given conditions in the tropics I would say, yes.

Tropical Storm Chantal moves into the eastern Caribbean Sea.  Click on image for a larger view.  Credit: NOAA.

Two storms have already formed in the deep tropics (Barry, Chantal) and one is a Cape Verde storm which usually doesn’t get going until mid-August.  As Jeff Masters points out on his blog:

"Most years do not have named storm formations in June and July in the tropical Atlantic (south of 23.5°N); however, if tropical formations do occur, it indicates that a very active hurricane season is likely. For example, the seven years with the most named storm days in the deep tropics in June and July (since 1949) are 1966, 1969, 1995, 1996, 1998, 2005, and 2008. All seven of these seasons were very active. When storms form in the deep tropics in the early part of the hurricane season, it indicates that conditions are already very favorable for TC development. In general, the start of the hurricane season is restricted by thermodynamics (warm SSTs, unstable lapse rates), and therefore deep tropical activity early in the hurricane season implies that the thermodynamics are already quite favorable for tropical cyclone (TC) development."

This quote is attributed to Colorado State University researchers Phil Klotzbach and Bill Gray.

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.

Is Andrea About To Form?

Early Start

Hurricane season began June 1^st and by all accounts it is expected to be an active season.  Is the season about to get underway?  It seems the computers think so.  The vast majority of the computers have been indicating that something could form in the Gulf of Mexico this week.

There was an area of disturbed weather extending from the Yucatan Peninsula northeast into the Gulf of Mexico Monday evening.  Moisture was streaming north from the western Caribbean Sea.  No indications of development have been noted over the past 24 hours.

However, computer models suggest that thunderstorm activity will be on the increase today into Wednesday.  The latest Regional Precision Model (RPM) has a circulation forming in the Gulf north of Yucatan on Wednesday.  The system would then move north curving northeast into the Apalachee Bay on Thursday.

The 72-forecast valid Thursday evening at 00z June 7, 2013.  Click on the image for a larger view.  Image Credit: WSI.

The above model has the system coming onshore late Thursday afternoon and moving northeast toward coastal South Carolina.  That is where the heaviest rainfall would occur.  It could be breezy for the coastal sections, but the system is not expected to be very strong.

Just Another Number?

Numbers have meaning only in context.  A temperature of 98.6 degrees F is normal body temperature, but only in context of the mean.  This is not normal for every person.  The S&P 500 crossing 1600 is meaningless unless it is in context of what the market is doing.  Is it going up or down?  How far has it gone from previous levels?  Only then does the number have meaning.

A milestone was reached recently, Thursday, May 9^th, at the Mauna Loa Observatory in Hawaii.  Levels of carbon dioxide reached 400 parts per million (ppm) for the first time in human existence.  Daily measurements are made at the observatory of the gases that lead to atmospheric change.  One of the main gases monitored is carbon dioxide (CO2).  Levels of 400.03 and 400.08 ppm were measured independently by the National Oceanic & Atmospheric Administration (NOAA) and Scripps Oceanographic Institute respectively.

Mauna Loa Observatory in Hawaii.  Click on the image for a larger version.  Image Credit: NOAA/ESRL.

This was a daily mean that was measured for the first time.  Hourly measurements exceeded this number quite a few times since mid-April.  The monthly mean of CO2 will likely peak this month, at a level of about 399 ppm, and then fall until October.  It is likely to pass the 400 ppm in the spring of next year.  The yearly mean will not likely cross this level until 2015 or 2016.

A Drought Worth Having

Droughts can be quite nasty with it comes to rainfall.  However, droughts can be a good thing when it comes to severe weather.  The U.S. is currently experiencing such a drought for tornadoes.

The monthly number of tornadoes for 2013 compared to the 1981-2010 average. Click on the image for a larger version.  Image Credit: Climate Central.

The year started off looking above normal, but that quickly turned in February.  Significantly fewer tornadoes were observed in March and April.  May is normally the peak of tornado season for much of the country.  The drought has continued and will likely continue into the middle of May.  It is hard to say if it will continue beyond that, but it is likely.

A Cool Start To May 2013

Just as we were getting used to hotter springs, Mother Nature pulls a fast one.  It has been unusual to say the least for South Carolina weather since April 26^th.  Cloudy, cool conditions have been the rule with plenty of rain.  The griping has begun as many are tied of conditions we normally attribute to Seattle.

My favorite tweet during the past week has been “Breaking news: This just in – Cleveland wants its weather back.”  I’m sure those in Cleveland might think this an improvement over what they normally get in early May.

So what has been happening?  Why has it been so cloudy and cool for so long?  When will this go away so that we can get on with summer?

The first half of spring was influenced by a weather pattern associated with a negative phase of the Arctic Oscillation.  This was described in the previous post.  However, that has not driving the weather pattern recently.  Instead the jet stream has retreated to the north leaving upper-level low pressure systems isolated from the main flow.  This has resulted in a blocking pattern where weather systems slow to a crawl and can persist for days.

The weather pattern at 500 mb (~18,000 ft) on Saturday, May 4, 2013 at 12z UT.  The jet stream had retreated farther to the north leaving isolated pockets of low pressure over the mid-latitudes.  Click on the image for a larger version.  Image Credit: WLTX-TV.

A Dramatic Change This Spring

It is amazing how fast the weather pattern can change.  Temperatures were much below normal at the end of March.  There was a brief warm up at the beginning of April, but a brief cool snap followed.  Finally the cold air retreated leaving much of South Carolina with above normal temperatures.  In a matter of five days high temperatures in Columbia went from 55 degrees F on April 4^th to 87 degrees F on April 9^th & 10^th.

A previous post talked about the delay of spring this year.  High pressure centered near Greenland continued to pump cold Arctic air south into North America.  This was part of the negative phase of the Arctic Oscillation which became extreme in March.  This led to colder temperatures in March than in January.

This image shows the sea level pressure anomaly calculated from NCEP climate reanalysis data from February 25 through March 25, 2013. The area of anomalous high pressure over Greenland illustrates the negative phase of the Arctic Oscillation, by which persistent high pressure blocks Arctic air from moving eastward, causing it to drop down into the middle latitudes and is a contributing factor in keeping temperatures unseasonably cool in Canada and the US.  Click on the image for a larger view.  Image Credit: NOAA.