How active is the Wisconsin tornado season historically in La Niña years?

What is ENSO (El Nino/Southern Oscillation) and what are La Niña and El Niño?

Here is a description of these terms courtesy of the Climate Prediction Center (CPC):

"ENSO stands for El Niño/ Southern Oscillation. The ENSO cycle refers to the coherent and sometimes very strong year-to-year variations in sea- surface temperatures, convective rainfall, surface air pressure, and atmospheric circulation that occur across the equatorial Pacific Ocean. El Niño and La Niña represent opposite extremes in the ENSO cycle."

"El Niño refers to the above-average sea-surface temperatures that periodically develop across the east-central equatorial Pacific. It represents the warm phase of the ENSO cycle, and is sometimes referred to as a Pacific warm episode."

"La Niña refers to the periodic cooling of sea-surface temperatures across the east-central equatorial Pacific. It represents the cold phase of the ENSO cycle, and is sometimes referred to as a Pacific cold episode." For more information on this phenomenon, click here.

Current ENSO Conditions

We are currently in a La Niña, which corresponds to cooler-than-average sea-surface temperatures in the equatorial Pacific. For a graphic of the current sea surface temperatures, and anomalies, click here. The La Niña event was much stronger this past winter.

The current CPC forecast calls for a moderation in sea-surface temperature anomalies and a trend towards ENSO-neutral conditions through the summer and towards the latter half of the year.

How Active is the Wisconsin tornado season historically during La Niña events? 

With many famous tornado outbreaks in the United States having occurred during La Niña years, it is easy to expect more tornadoes during a La Niña event. The famous "Super Outbreak of 1974" occurred during a La Nina year. Even this year, there was a significant outbreak of tornadoes on February 5th and 6th across the South, while the same storm system brought parts of Southern Wisconsin over 20 inches of snow! But, do La Niña conditions typically correspond to an increased number of tornadoes in Wisconsin? Consider the following facts, based on tornado and ENSO data from 1970 to 2007:

  • Only once when there was a La Niña during the preceding winter was there an above average number of tornadoes in the state of Wisconsin (based on the 1971-2000 average of 21). This was out of 10 such cases.
  • 10 of the 12 years with substantially cooler than average sea-surface temperatures (anomalies of -0.5C or less in the Niño 3.4 region) had below average numbers of tornadoes in Wisconsin. Of the two outliers, one had 21 tornadoes, or the average amount.
  • Of the 15 years with above average numbers of tornadoes in Wisconsin, 14 of them had El Niño or ENSO-neutral conditions the preceding winter.
  • 9 of the top 10 years, in terms of numbers of tornadoes in Wisconsin, had either El Niño or ENSO-neutral conditions the preceding winter.
  • Of the 19 years with 4-or-more strong to violent tornadoes in Wisconsin, 15 had either El Niño or ENSO-neutral conditions the preceding winter.

Also, keeping in mind that we have seen a "warming trend" in the sea-surface temperature anomalies recently, consider the following fact:

  • 11 of the 15 years that showed warming greater than 0.3C from February to June had below average tornado counts in the state of Wisconsin. Such warming would be typically associated with a weakening La Niña or a developing El Niño.

In summary, based on the statistics from 1970 to 2007, if there are sea-surface temperature anomalies at or below -0.5C in the Niño 3.4 region, historically it corresponds to an 83% chance of a below average number of tornadoes in the state, or 5 to 1 odds of a below average year.

Don’t Let Your Guard Down

Just because the statistics above indicate a historical trend for La Niña years to produce below average numbers of tornadoes in Wisconsin does not mean that you should let your guard down now. In fact, in La Niña years, Wisconsin has experienced some of its most significant tornadoes!

Take for instance, 1996. The winter prior to the severe weather season had La Niña conditions in the equatorial Pacific. That year also only had 21 tornadoes in Wisconsin, the average in the state. However, this was also the year that saw an F5 tornado sweep through the town of Oakfield in Fond du Lac County (more information). Two other strong tornadoes occurred that year in Wisconsin, both rated an F2 on the Fujita Scale.

This year we saw a significant tornado in January. This tornado swept through far Southeastern Walworth County and Western Kenosha County, and was rated an EF-3 on the Enhanced Fujita Scale (more information). A long-tracked tornado occurred this April in Southern Wisconsin, moving northeast through Columbia and Green Lake Counties for 27 miles (more information).

There are many more examples of significant tornadoes in Wisconsin during La Niña years: the Siren, Wisconsin F3 tornado in 2001 (more information courtesy NWS Duluth), an F4 tornado in Fond du Lac and Winnebago Counties in 1974, and 7 strong tornadoes statewide in 1985 just to name a few.

We are entering the climatologically most favorable time of the year for tornadoes in Wisconsin. Now is the time to review your severe weather safety plan, or create one if you don’t already have one. Know what to do in the event a tornado strikes your location.

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