Preliminary Assessment of Climate Factors Contributing to the Extreme 2011 Tornadoes

Draft Assessment - Updated: 08-July-2011

Disclaimer: This draft is an evolving research assessment and not a final report. Comments are welcome. For further information, contact Dr. Martin Hoerling (
Overview | El Niño/La Niña | Climate Change | Q & A | References


Severe storms produced tornadoes across the Southern United States during the Spring of 2011, killing hundreds of people. The month of April alone produced the largest number of tornadoes for one month (875) on record. There were several large outbreaks during that month, culminating in the record setting outbreak of April 25-28 across the South Eastern United States which killed 321 people.1 Of pressing importance is to explain how this happened, and determine what factors may have been its cause.

This site provides an assessment of the climatic factors that may influence extreme tornado outbreaks such as the ones that occurred in 2011.

  • The El Niño/La Niña page discusses the impact of ocean temperatures on tornado events
    • Summary: The April 2011 period of very active severe weather occurred during the waning phases of a La Niña. Climate simulation and forecast data indicate the tropical SSTs were nonetheless an effective forcing of U.S. weather patterns during early spring. The co-variability of 20 severe spring (March-May) tornado outbreaks over the contiguous US and phases of the El Niño/Southern Oscillation (ENSO) during the past 100 years presents a complicated picture of the historical relationships.
  • The Climate Change page discusses the impact of anthroprogenic climate change on tornado events
    • Summary: We conducted a rapid, preliminary investigation to determine if climate change contributed to the unusually large number of tornadoes experienced in the United States during April 2011. A change in the mean climate properties that are particularly relevant to major destructive tornado events could not be detected for April during the last 30 years.
  • The Q & A page addresses questions that this analysis of tornado-climate links is raising.
Given the importance of these questions, more rigorous research must now be conducted to verify or alter our conclusions.

Click link for animation: GIF (7.5M)   QuickTime ( 14M)

Synoptic Conditions for April 26, 2011

What the Animation Shows

The daily evolving 500mb heights (contours) and 850mb wind (arrows) from April 1-30, 2011 (00Z analysis) depict the migratory upper air troughs and low level jets. These features help determine the daily environmental conditions in which tornado risks evolve. The risks are displayed by the color shading depicting the daily maximum value of the Brooks et al. 8 index: category 4 (orange) indicative of enhanced severe storm risk and category 5 (red) indicative of enhanced tornado risk. Tornado reports for the 24 hours centered on the analysis time (12Z-12Z) are plotted with blue diamonds based on the preliminary data from the Storm Prediction Center, while other fields are derived from the CFS Reanalysis. As the animation progresses, one can see the alignment of the high risk indices with the tornado reports. In particular, the major outbreaks of April 4-5 (Southeast US), 9-10 (Midwestern US), 14-16 (Central and Southern US), April 19-22 (Southern US, including the St. Louis Tornado) and 25-28 (Southern US) are well aligned with the regions of high risk.

Overview | El Niño/La Niña | Climate Change | Q & A | References