NWC REU 2020
May 26 - July 31



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A 180-year Climatology of Severe Weather Environments in the United States

Hope Skypek, Harold Brooks, and Kimberly Hoogewind


What is already known:

  • Prior studies have suggested an increase of severe weather days in the Southeast U.S. and a decrease in the Midwest U.S.
  • Annual cycles have changed and shifted towards earlier in the year (March, April, May).
  • Evidence has also signified increased variability in tornadoes produced by severe weather suggesting a decrease in the number of tornadoes in single events, even though the number of tornado days have remained relatively stable.
  • Tornado and storm reports over time have often been inconsistent, making the creation of reasonable storm climatologies with sufficient datasets a challenge.

What this study adds:

  • 20th Century Reanalysis version 3 allows analysis of severe storm environments from 1836-2015, a longer time range than many past climatologies.
  • Spatial patterns supported prior research with increases in the number of severe weather days (NDSEV) in the eastern U.S. and decreases in the central U.S. and Northern Plains.
  • The beginning and peak of severe weather season has shifted towards earlier in the year, but the end has shifted towards later in the year. This suggests a range increase of severe weather season.
  • Evaluating severe storm magnitudes with 20CRv3 may require different thresholds at different time periods.


This study evaluates a 180-year climatology for the period 1836–2015 of severe weather environments in the United States using 20th Century Reanalysis version 3. Various composite thresholds are utilized to establish severe weather environments and the number of days of these severe weather environments (NDSEV) calculated for each year. Regional comparisons demonstrate trends of frequency of severe storm environments for various geographic regions of the U.S. Annual cycles are analyzed to explore seasonality of severe weather. Compared to the mid 19th century, NDSEV in the Southeast has increased by 15.7 days and decreased in the Northern Plains by 15.5 days. The beginning of severe weather season has moved over a month earlier in the year. Additionally, potential inconsistencies are in the reanalysis are subjectively identified, specifically in evaluated magnitudes of severe storm parameters.

Full Paper [PDF]