Planning for Precipitation in Every Season

At the risk of speaking too soon, it seems like spring has finally arrived. This month on the blog, we’re taking a look at spring’s proverbial raininess. First we’ll examine the last 10 years of precipitation data for spring; then we’ll compare spring precipitation data with fall precipitation data to see if, in our study period, the Champaign-Urbana area has seen more or fewer showers in April than in other months.

A few notes: as we’ve stated in previous weather-related posts, climate is complex, and this analysis is basic. Any conclusions that can be drawn from the data below are limited to the strictly descriptive (e.g., “in 2016, there was more precipitation in May than in March or April,” and “in both seasons, 2011 had more precipitation than 2010”) as opposed to analytical (e.g., “2010 data suggests that fall is rainier than spring”). We’re using meteorological seasons, meaning that spring includes March, April, and May, and fall includes September, October, and November[1].

This data, like the climate data table on the Regional Dashboard and discussed in previous weather-related posts, is sourced from the Water and Atmospheric Resources Monitoring Program’s Illinois Climate Network at the Illinois State Water Survey, and is collected at the Champaign station. That means it reflects conditions at the station’s site, and that conditions in other areas, even other areas nearby, will vary. The data source does not distinguish between rainfall and snowfall, so neither will our analysis: some of the precipitation in the tables may be an early or late snow. Finally, total precipitation data was missing for May 2014; thus, we are omitting 2014 from our analysis.

Source: Water and Atmospheric Resources Monitoring Program. Illinois Climate Network. (2019). Illinois State Water Survey, 2204 Griffith Drive, Champaign, IL 61820-7495. <http://dx.doi.org/10.13012/J8MW2F2Q.> (Retrieved 27 March 2019).

Of the 10 analyzed years, every meteorological spring saw more than 10 inches of precipitation except for 2010 and 2012; 2012 had the least spring precipitation in the study period. March was the driest month in six of the 10 springs, and was never the spring month with the most precipitation. April was the driest spring month three times, and May only once; May had the most spring precipitation six times, while April had the most spring precipitation four times.

Source: Water and Atmospheric Resources Monitoring Program. Illinois Climate Network. (2019). Illinois State Water Survey, 2204 Griffith Drive, Champaign, IL 61820-7495. <http://dx.doi.org/10.13012/J8MW2F2Q.> (Retrieved 27 March 2019).

Similarly, only two meteorological falls during the study period saw fewer than 10 inches of precipitation (2010 and 2013). September saw the least fall precipitation three times and the most fall precipitation five times; October saw the least fall precipitation five times and the most three times. November was the driest fall month twice, and the wettest fall month twice.

In five of the 10 analyzed years (2009, 2011, 2013, 2016, and 2017), meteorological spring saw more precipitation than meteorological fall. The opposite was true in the other five years: 2008, 2010, 2012, 2015, and 2018. This data suggests that, while precipitation can certainly be expected in the spring in most years, it is far from absent in the fall most years as well.

So why does planning for precipitation matter?

Precipitation, depending on how much, how quickly, when, and where it accumulates, can lead to flooding, a problem that can range in magnitude from minor to catastrophic. Predicting the amount of precipitation likely to occur in a region – again, using far more complex analyses than the one we did above, with additional data as inputs to climate models – can provide some guidance on the region’s stormwater management needs. These needs can be addressed by “gray” stormwater infrastructure (e.g., storm sewers, pipes, culverts), by green stormwater infrastructure (e.g., permeable pavement, rain gardens, bioswales), and by a combination of the two. Engineers and planners need an understanding of regional precipitation, and how these patterns are changing and are anticipated to continue to change, in order to create stormwater elements, plans, and policies that promote safety, sustainability, and resilience.

[1] “Meteorological Versus Astronomical Seasons.” National Oceanic and Atmospheric Administration. National Centers for Environmental Information. (No date). https://www.ncdc.noaa.gov/news/meteorological-versus-astronomical-seasons.

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