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Is El Niño to Blame For These Winter Tornadoes? | The Weather Channel
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Is El Niño to Blame For These Winter Tornadoes?

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We've seen more tornadoes than average in the U.S. so far this winter. You may be tempted to blame El Niño for this. But does even one of the strongest El Niños on record actually have anything to do with the more tornadic winter?

Several Notable Severe Events Since November

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Maps of preliminary tornado reports in November 2015, December 2015, and 2016 year-to-date through February 22.
(NOAA/Storm Prediction Center)
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Preliminary U.S. monthly tornado count from Nov. 1, 2015 through early morning on Feb. 24, 2016, compared to the latest 20-year monthly average. (Note the February average is month-to-date.)
(Dr. Greg Forbes/The Weather Channel, NOAA/Storm Prediction Center)

First, there's been a number of notable severe weather events since the beginning of November, including:

According to Dr. Greg Forbes,  severe weather expert for The Weather Channel, a preliminary total of 235 tornadoes occurred in the U.S. from November 2015 through early morning, Feb. 24, 2016.

This count is almost 57 percent higher than the 20-year average count through that same time period of 150 tornadoes, according to Forbes.

Breaking the totals down by month, only January was less tornadic than average in the U.S.

All this has happened during what has been a record-tying strong El Niño, so surely El Niño is the cause, right? 

Well, in the words of the late Leslie Nielsen playing in the classic 1980 comedy, Airplane, "Don't call me Shirley."

El Niño-Tornado Connection: Hazy, At Best

El Niño does not, by itself, spawn tornadoes or tornado outbreaks.

El Niño is the periodic warming of the central and eastern equatorial Pacific water that exhibits a forcing on the overall atmospheric pattern, particularly in the winter months.

Typical location of subtropical jet stream from November through February and greatest vector wind anomalies during five previous strong El Niños. (NOAA/ESRL Physical Sciences Division)
Typical position of the subtropical jet stream from November through February during five previous strong El Niño seasons (denoted by the green jet stream). Strongest jet-stream level wind anomalies during those previous strong El Niño seasons, relative to average, are shown by the light blue (higher) to purple (highest) contours from the eastern Pacific Ocean to north of the Bahamas.
(NOAA/ESRL Physical Sciences Division)

One thing common to strong El Niños is a more bullish southern-branch, or subtropical jet stream. 

While this subtropical jet stream typically camps, at times, over parts of the southern U.S. each winter, in a strong El Niño, it is most anomalously strong farther south, from the eastern Pacific Ocean through Mexico, into the Gulf of Mexico, then into the Florida Peninsula.

This tends to pull the track of winter frontal systems closer to the Gulf Coast and Florida. Coupled with a stronger southern-branch jet stream, severe thunderstorms can be more numerous in parts of the South. 

Numerous studies since the 1990s have examined the potential link between El Niño and tornadoes, not simply in the winter, but also in the more active spring and summer months.

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"Year-to-year variations in tornado frequency and strength are not well correlated with the ENSO phase (El Niño, La Niña, or a "neutral" phase lacking either)," says Dr. Forbes.

(MORE: Weakening El Niño -> More Dangerous Hurricane Season)

Forbes says year-to-year tornado statistics can be skewed by either the occurrence or lack of one or more large outbreaks. "These are due to strong transient weather systems (such as strong jet-stream streaks and their associated strong frontal systems), relatively independent of such large-scale influences (as El Niño)."

Essentially, tornadoes, and tornado outbreaks occur at a much smaller scale than the huge scale of an El Niño. Tornadoes are notoriously fickle, forming on some days with a favorable overall environment for severe weather, but not on others. El Niño simply doesn't operate on that small a scale to determine whether you'll simply have an area of thunderstorms with flooding rain, or a tornado outbreak. 

Basically, even a record-tying El Niño is not the only "driver" of the atmospheric pattern at any given time.

(MORE: El Niño's Facts and Myths)

The 27 largest, most impactful U.S. tornado outbreaks, parsed by ENSO phase. (Dr. Greg Forbes/The Weather Channel)
The 27 largest, most impactful tornado outbreaks categorized by El Niño, La Niña, or neutral phases.
(Dr. Greg Forbes/The Weather Channel)

One other monkey wrench that clouds any possible trends in tornadoes during El Niños is "tornado inflation", the overall increase in observed weaker (EF0 or EF1) tornadoes over the past several decades due to more widespread spotter networks, advancements in technology (Doppler radar, smartphones, social media), and population increase.

There are, however, at least a few tendencies of note from previous studies.

For example, of the 27 largest, most impactful tornado outbreaks in U.S. history, twice as many January-through-April outbreaks occurred during La Niña (6) as did El Niño (3), according to Dr. Forbes' research.

From May through December, however, 9 such outbreaks occurred during an El Niño, compared to just 2 during a La Niña and another 5 during a neutral season.

A recent study by the National Weather Service in Tallahassee, Florida, found that about double the number of tornadoes can be expected in an El Niño from November through April in central and South Florida compared to a La Niña or neutral season.

Similarly, Dr. Forbes examined 13 previous moderate to strong El Niños and La Niñas, and found 8-9 more November through April tornadoes had occurred in the Sunshine State during each of the 13 El Niño seasons examined than during the La Niña seasons.

Most infamously, Florida's deadliest tornado outbreak occurred during the previous strong El Niño. On Feb. 22-23, 1998, 42 were killed and over 3,000 buildings were damaged or destroyed during a central Florida tornado outbreak, which included a trio of F3 killer tornadoes in the Orlando metro area.

(MORE: 10 Worst Tornadoes of 2015)

Previous studies by Joe Schaefe, former head of the Storm Prediction Center, and others have noted at least a weak correlation with stronger winter and early-spring Florida peninsula tornadoes (EF2+) and El Niños. 

The bottom line is both El Niño and La Niña may have some influence on the occurrence of tornadoes, but it's difficult to separate the many other smaller and large-scale factors in play.

MORE ON WEATHER.COM: South Tornado Outbreak (Feb. 23-24, 2016)

Nick Mobley helps clean up a house owned by a family friend, Wednesday, Feb. 24, 2016, after a storm hit Appomattox County, Va. A powerful storm system swept across the East Coast on Wednesday, knocking out power to tens of thousands of homes and businesses in the region. (Jill Nance/The News & Advance via AP)
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Nick Mobley helps clean up a house owned by a family friend, Wednesday, Feb. 24, 2016, after a storm hit Appomattox County, Va. A powerful storm system swept across the East Coast on Wednesday, knocking out power to tens of thousands of homes and businesses in the region. (Jill Nance/The News & Advance via AP)
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