In Illinois, an autumn-like July historically gives way to a cool August, but state climatologist Jim Angel says it's too early to pack away fans and flip-flops. In an interview with News Bureau editor Jan Dennis, Angel discusses what Mother Nature and a developing El Nino may have in store for central Illinois. Angel is affiliated with the campus-based Illinois State Water Survey and Institute of Natural Resource Sustainability.
What's behind the un-Illinois-like weather this July?
Yes, it has felt more like September than July. In a typical July, we most often experience very warm, moist air from the Gulf of Mexico with occasional relief as a cold front passes. For this July, we have seen a lot of cooler, drier air out of Canada. In the upper atmosphere, the persistent weather pattern has been higher pressure in the West and lower pressure in the Great Lakes region. This has produced northwest winds over Illinois, allowing for air to move right out of Canada into the U.S. While the pattern in itself is not unusual, its persistence through the entire month has been.
Historically, just how unusual has this summer been so far?
We are well on the way to being one of the coolest Julys on record for Illinois. Currently we are running 6 degrees below normal for the month. This situation is not unique to Illinois because much of the U.S. from the Dakotas, Nebraska and Kansas eastward has experienced a much cooler-than-normal July. Meanwhile, much of the West and Southwest have been warmer-than-normal. Here in Champaign-Urbana, our average high has been 79 degrees, compared to a normal of 85 degrees, while our average low has been 61 degrees, compared to a normal of 65 degrees.
Besides the overall cooler conditions, Illinois has seen few days in the 90-degree range. The northern two-thirds of the state have seen none, while only one to five have been reported in sites around the southern third of the state. In a typical July, we would expect to see about five days in the 90s in northern Illinois, 10 in central Illinois, and 15 in southern Illinois. Here in Champaign-Urbana, with records extending back to 1889, it is rare to go through July without seeing 90 degrees - only three times in all (2000, 2007, and 2008).
Does a cool June and July typically mean anything later on, such as a blistering August, a longer summer or milder winter?
Historically, a cool July is most often followed by a cool August. The specific odds on August temperatures after a cool July are 45 percent cooler-than-normal, 34 percent near-normal, and 21 percent above-normal. But sometimes Illinois weather can surprise you. In 2007, after having no days in the 90s in July in Champaign-Urbana, we had 16 in August, nine in September, and 4 in October!
The National Weather Service outlook for the next two weeks indicates a continuation of the cooler weather. Their monthly outlook for August and seasonal outlook for August-October both show an increased chance of cooler-than-normal weather in Illinois. So both the historical evidence and the forecast suggest that cooler conditions will more likely prevail in coming weeks.
How could an El Niño developing in the Pacific Ocean influence Illinois weather patterns - and when?
The National Weather Service says that El Niño conditions will continue to develop and are expected to last through the winter. This is the typical life-cycle of these events - developing in the summer and fall, reaching a peak in winter, before declining in spring. As a result, the strongest impacts are in winter and are most clearly seen with the stronger events. It is not clear at this point how strong the event will be, but we know from past research that during the strong events Illinois has seen milder winters with warmer temperatures, fewer days below zero, and much less snowfall. Its impact on weather patterns in the fall is less certain, but there is a tendency to be slightly cooler.