EarthRisk Technologies, the San Diego based weather analytics company, has confirmed that the Midwest/East U.S. experienced the most extreme and persistent heat on record since 1948 in June and July 2012.
HeatRisk, the company's web-based platform that utilizes historical data to analyze the risk for extreme warm weather events up to 40 days in advance, verified several key patterns conspired to drive the record-breaking temperatures.
"The cumulative HeatRisk index for the Midwest/East United States was higher than any other June-July period on record since 1948," says David Margolin, EarthRisk's Director of Meteorology. "That's nearly 10% warmer than the next highest HeatRisk value which was recorded in 1988."
The geographic region in EarthRisk's study stretches from Chicago to Boston then southward to Charlotte and westward to Little Rock. Residents may remember last summer's blistering heat in early July, but 2011 pales in comparison to 2012.
"According to our HeatRisk index, 2012 was 36% more extreme than 2011," adds Margolin.
EarthRisk also quantifies a measure of extreme cold, called ColdRisk.
"As you might expect, the ColdRisk value for June/July was very low," says Margolin. EarthRisk estimates that ColdRisk in 2012 was the fourth lowest on record since 1948. Only 1949, 2010 and 2011 featured a smaller measure of extreme cold temperatures during the first part of the summer.
EarthRisk also analyzes the overall atmospheric patterns that correspond to extreme heat and extreme cold throughout the year. EarthRisk cites a strongly northward shifted jet stream across the eastern Pacific and North America as one of the primary factors leading to heat, especially in June.
"We call this a 'pattern dipole' focused over the eastern Pacific and it was one of the strongest such jet stream patterns observed since 1948," says Stephen Bennett, EarthRisk's Chief Science and Products Officer. "This particular pattern was so strong, nearly three-and-a-half standard deviations, on June 16th, 2012 - which is very rare."
Strong low pressure in Alaska in late June and early July was also a culprit. "It's also very rare to see this particular pattern so strong," adds Bennett. "On June 17th, we observed a value stronger than 2.5 standard deviations."
These synoptic scale atmospheric conditions are related to a broader pattern that has been stubbornly persistent throughout June and July known as the "Global Wind Oscillation" or GWO. EarthRisk scientists Dr. Klaus Weickmann and Ed Berry have been researching the GWO for the past decade and their research indicates that when the GWO is persistently low, as has been observed most of the summer so far, the weather pattern across the U.S. tends to be repetitive.
"These patterns tend to feature repetitive heat from the Rockies eastward across Canada and the Northern Tier of the U.S.," Bennett says.