A New Index to Characterize Variations in Extreme Weather and its Application in Wisconsin

Many types of extreme weather are becoming more frequent, intense, dangerous, and expensive as the climate changes. But quantifying these trends and adequately characterizing the variability in extreme weather is surprisingly challenging in the absence of a common measuring stick. Many definitions of extreme weather exist, and they are often highly subjective and not directly comparable to one other for quantifying the aggregate behavior of extremes. For example, the widely used HadEX database from the Hadley Centre contains more than two dozen measures of short-term weather variables, while NOAA’s Climate Extremes Index synthesizes several temperature and precipitation metrics on a range of timescales. These and other tools have proven useful in documenting trends and variations in extreme weather, but they are subject to inherent limitations. Such challenges include: obtaining complete daily data records, combining disparate temperature and precipitation measurements, and assigning arbitrary thresholds to define an extreme (e.g., highest or lowest 10^th percentile).

To help remedy these problems, the Wisconsin State Climatology Office is testing a simple, non-parametric index of extreme weather based on the percentile rankings of temperature and precipitation (primarily at monthly or longer timescales). The Extreme Weather Index (EWI) quantifies the magnitude of a temperature or precipitation extreme as its ranked deviation from the median (50^th percentile) of the entire data set, thus allowing a range of values that express the magnitude of extreme conditions without imposing arbitrary thresholds for defining an extreme. The percentile-based nature of the index also allows direct comparison of individual temperature and precipitation extremes and applies equal weighting of the two variables to create a combined index value. The EWI can be used for both a single location and a region, such as state climate divisions and statewide areal averages.

Application of the EWI to data from NOAA’s National Centers for Environmental Information (NCEI) starting in 1895 reveals that Wisconsin has experienced unusually pronounced extreme weather in recent years and that the 2010s were the most extreme decade statewide for annually averaged conditions in both temperature and precipitation. The long-term trends differ substantially by location, however, and the EWI reveals that southern Wisconsin has experienced the largest increase in extreme weather of any region of the state.