A Publication of WTVP

There is evidence that some extreme events have become more likely, especially as they relate to temperature and precipitation.

In our profession, we are frequently asked, “Is our weather becoming more extreme?” There are several answers to that question, depending on how you measure extreme weather. It is human nature to remember the latest event or series of events better than those of the past. That is why we rely so much on historical records to put current conditions into perspective and detect any changes. In fact, Peoria has the longest continuous weather records in Illinois, started by Dr. Frederick Brendel in 1856.

Looking at those records, there’s evidence that some extreme events have become more likely, especially as they relate to temperature and precipitation. We’ll start by discussing this past year (2015), and then look at broader changes and trends that have become more prevalent—and arguably important—to those who live and work in Illinois and surrounding states.

When Wet Gets Expensive
In many respects, 2015 was a remarkable year, with the wettest June on record (9.4 inches—5.2 inches above average) and the second wettest December on record (6.7 inches—four inches above average). As a result, it was the sixth wettest year on record, with 48.5 inches of precipitation, according to the National Oceanic and Atmospheric Administration (NOAA). Wisconsin, Iowa, Missouri, Kentucky and Indiana also had above-average precipitation; nationwide, it was the third wettest year on record.

Historical data for Illinois indicates that annual precipitation has increased by about 10 percent over the last century. In fact, four of the 10 wettest years on record have occurred since 2008. Several studies suggest the trend toward wetter conditions is the result of an increase in heavier rainfall events—those producing two or more inches of rain in one day. Furthermore, many of these events are producing these amounts over wider areas. For example, the December 2015 storm produced two or more inches of precipitation across the entire state—with areas from Peoria southward receiving five to eight inches.

These heavy rain events often cause widespread damage, including impairments to agriculture (flood- damaged crops, delayed planting and harvesting, soil erosion, loss of nutrients); transportation (flooded roads and railways, rivers too high for navigation); and urban areas (flooded streets and basements, closed schools and businesses, as well as other infrastructure damages). The IDNR, aided by the Illinois State Water Survey, recently produced a report for the Urban Flooding Awareness Act that outlines the challenges many cities in Illinois face on this issue, with damages of $2.319 billion documented between 2007 and 2014.

Other Trends
Despite the trend toward wetter conditions, droughts have continued to occur in Illinois, with recent droughts in 1988, 2005 and 2012. However, they are not as frequent, nor as intense, as those experienced in the 1930s, or from 1954 to 1956. Even so, it is still important to include drought planning in water-related activities due to the impact on crops, water supply, river navigation, ecosystems, and health and energy sectors.

In terms of temperature, 2015 was Illinois’ 24th warmest out of 121 recorded years. At the same time, the nation was the second warmest it has ever been, and the earth far warmer than ever recorded. Over the last century, temperatures in Illinois have warmed by about one degree. This is much less than the warming seen in some of the western and northern states. Most of the warming trend in Illinois can be found in winter and spring, while summer and fall have shown little or no change.

Oddly enough, minimum daily temperatures have shown the greatest rise in recent years. Like precipitation, there are large variations in annual and seasonal temperatures as the trend more broadly shows increases. While 2014 was the sixth coldest year on record for Illinois, just two years earlier, 2012 was the warmest year on record. Some studies have shown variability has increased; thus, there have been large swings in both temperature and precipitation.

Deciphering Illinois’ Mixed Tendencies
So what can we say about these trends? Several studies point to a general increase in precipitation across the Midwest, but more importantly, the way rain falls has changed. We have experienced an increasing number of heavy rainfall episodes over the last 50 years, so when it does rain, it trends toward larger amounts at once. Temperatures are also increasing across Illinois, and interestingly, the warmer the air, the more moisture it holds. When the moisture is wrung out—say, from a thunderstorm—more rain falls in a shorter period of time.

But what about heat waves? Most of the really hot weather in Illinois has occurred during droughts, with many high-temperature records set in the 1930s (especially 1936), the mid-1950s, and most recently in 2012. However, if you take into account both temperature and humidity using the heat index, there is some evidence to suggest recent heat waves are driven more by humidity than temperature.

What about tornadoes? The tornado climatology for Illinois shows no real trend when you look at the stronger events. Instead, what we have seen is a spike in the number of tornadoes in the weakest category because of better detection by radar, spotter networks or alert citizens with cameras.

Planning for the Worst Case
In the short term, the NOAA outlooks for this spring show an increased chance of above-average temperatures and below-average precipitation, with a better chance for above-average temperatures this summer. Rainfall is a tougher question. Predictions are not clear one way or the other. Some have pointed out that on years following a strong El Niño (like the one that is weakening now), there is a tendency for drier summers. This is hardly conclusive, however, and the official outlook does not provide any evidence of wetter or drier outlooks. Often in the summer, it depends more on spring rainfall and a host of other considerations that can only be forecast in the short term (a week or two out).

Even longer term—maybe 20, 50 or 100 years into the future—many scenarios paint a much warmer climate here, with a potentially ever-increasing frequency of heavy rainfall events. Drought also may be an occasional visitor, with repercussions similar to or even worse than 2012, if it is sustained longer than a year.

Regardless of this mixed bag, we can still move forward in planning. Whether floods, droughts, heat waves or tornadoes, experience has shown that careful planning can help alleviate the negative impacts of any extreme weather. Both the Illinois State Water Survey and NOAA have considerable data and knowledge to help the process of drought and wet weather planning at the state and municipality levels. These plans, if updated regularly, can be a useful tool to minimize negative impacts. iBi

Doug Kluck is the Central Region Climate Services Director at NOAA’s National Centers for Environmental Information. Dr. Jim Angel is the State Climatologist for Illinois at the Illinois State Water Survey, Prairie Research Institute, University of Illinois. Learn more at and