Now take a look at June 2011 (last year was the same but there were no good shots.
The dark area in the middle latitudes was built by a strong jet stream. The subsequent mega-ridge was a persistent feature for more than a year, disrupting weather patterns worldwide by locking extremes in place.
While the weather is never based only on 1 factor. It is great to take a feature off the table that literally fried us, 3 out of the past 4 summers.