Many folks across northern Illinois and northwestern Indiana have noticed that summer seems to be missing in action during the first few weeks of July. At a time of year where heat waves and high humidity aren't at all uncommon, afternoon highs through July 15 at Chicago O'Hare hadn't risen above 85 degrees in July, with two days where the mercury only managed to make it to an October-like 65 degrees. Looking back, how does this summer stack up against history? Below are a few statistics through July 15. Please note that these statistics were calculated starting in 1942, the year that the official observing site was moved away from Lake Michigan.
July 1-July 15
Average Temperatures: 2009 is the 3rd coldest, 68.9 degrees
Coldest on record: 1967, 67.9 degrees
While the atmosphere is a chaotic system, there are certain medium to long-range atmospheric phenomena, often referred to as teleconnection indices, that provide at least some explanation as to why this summer has been so cool. These teleconnection indices are a way of measuring semi-cyclic variability in certain atmospheric fields, in this case, atmospheric pressure at certain locations. Below are time series plots and a short discussion for the North Atlantic Oscillation (NAO) and Pacific North America (PNA) pattern. These indices generally show more variability, and their effects are more strongly felt, during the winter season. That being said, there is likely still some effect being felt across the area during the summer.
Since mid-May the North Atlantic Oscillation (NAO) has been in a neutral to negative phase. A negative phase of the NAO favors troughing over the eastern United States (and associated lower temperatures). Normally, the effects of the NAO are felt most during the winter months, however, this extended neutral to negative phase could be a contributing factor to our cooler than normal summer. Unfortunately, the NAO tends to have little in predictive skill.
For much of the spring and early summer, the Pacific North America (PNA) pattern has been in a neutral to slightly positive phase. Since late May, the PNA has trended slightly positive. A positive phase of the PNA often favors ridging over the western United States and some troughing over the eastern United States. This, combined with a negative NAO seems to explain the rather highly amplified pattern that has been seen over much of the summer so far. As with the NAO, the effects are usually more noticeable during the winter, and more so over the western United States . This index also has little in the way of predictive skill.
Finally, it should be noted that NOAA recently confirmed the existance of an El Nino event this year (more information here). El Nino generally has little effect on the sensible weather across this part of the country during the summer, but stronger El Nino episodes tend to result in warmer than normal winters across the upper Midwest.