Friday, May 3, 2013

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 4th to 87 degrees F on April 9th & 10th.

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.


The chart below shows the temperature anomalies for the period of March 14-20.  The blue color is for colder than normal temperatures.  Notice the extent of the cold air across North America and Eurasia.

Click on the image for a larger view.  Image Credit: NASA Earth Observatory.

March turned out to be colder than normal for much of the central and eastern U.S.  In Columbia, SC, the temperature averaged 4.6 degrees below normal for the 10th coldest March on record.  It was the coldest March since 1996.  However, for the earth it turned out to be the 10th warmest March on record.

Recently released analysis by NOAA scientists at the National Climatic Data Center reveals that the March 2013 global average temperature of 55.94°F (13.28°C) was tied for 10th warmest year on record (dating back to 1880). This image, using data from NCDC’s Global Historical Climatology Network shows areas of warmer (red) and cooler (blue) than normal temperatures.  Click on the image for a larger view.  Image Credit: NOAA.

The pattern in the Northern Hemisphere was typical of the negative phase of the Arctic Oscillation.  The arctic region was warm while the mid-latitudes were colder than normal.  Meteorologist Deke Arndt of the National Climatic Data Center in Asheville, NC, explains:

A reversal of the phase to a strong positive phase in the Arctic Oscillation led to a dramatic warm-up in South Carolina in mid-April.  The change in temperatures from the end of March to mid-April was more reminiscent of weather in March.  Thus April weather became more like March with big swings in temperatures and severe weather.  In the end the average temperature in April was 65 degrees F or 1.6 degrees above normal.

The Arctic Oscillation will have less of an effect in May as it exerts its greatest influence in the winter.  However, weather systems become naturally sluggish in May and slow down as blocking patterns develop.  That is occurring now and can lead to little change in the weather as it becomes lock in place.  Areas lock in a rainy pattern can often experience floods due to the abundant rainfall.

This will likely break toward the end of May as the summer weather pattern establishes itself.  Enjoy the cooler weather while it lasts.  The outlook is for a hotter and more humid summer than normal.