It's the Most Favorable Time of Year for the Most Violent Tornadoes in the U.S.

Some of the nation's most notorious tornadoes and tornado outbreaks have happened from April through June.

Tornadoes that attain a rating of EF4 or EF5 on the Enhanced Fujita scale, with estimated winds of 166 mph or more, are capable of devastating damage. They can level well-built homes, toss vehicles around and scour the ground.

Violent tornadoes are rare. Tornadoes rated F4, EF4, F5 or EF5 amounted to 0.5% of all tornadoes in the U.S. from 1998 through 2017, according to NOAA's Storm Prediction Center. (Note: The Fujita (F) scale was replaced by the Enhanced Fujita (EF) scale in February 2007. Wind speeds associated with each category were reduced, but the general type of damage expected was not changed.)

That's 131 violent tornadoes out of 24,672 tornadoes in that 20-year period, an average of six to seven violent tornadoes a year.

Some years have few, if any, violent tornadoes.

For example, eight years passed between the F5 Oklahoma City/Moore tornado of May 3, 1999, and the next 5-rated U.S. tornado in Greensburg, Kansas, on May 4, 2007. There hasn't been an EF5 tornado in the U.S. since another Oklahoma City/Moore tornado on May 20, 2013.

Despite their rarity, the small fraction of tornadoes classified as violent was responsible for roughly half of all U.S. tornado deaths over the same 20-year period.

The death toll from just 131 violent F/EF4 and F/EF5 tornadoes (828 lives) exceeded the death toll from the other 24,541 tornadoes combined (813 lives).

Put another way, from 1998 through 2017, every 30 tornadoes weaker than F/EF4 took an average of one life, each F/EF4 tornado claimed an average of three lives and each F/EF5 tornado took an average of 33 lives.

Beware of Spring

We examined NOAA's tornado database from 1950 through 2017 for the 628 violent (F/EF4+) tornadoes to see if there is a peak time of year and found there's a broad peak from April through early June.

Roughly 59% of all F/EF4 tornadoes occurred in that timeframe. April through mid-June encompassed 81% of all F/EF5 tornadoes.

• April 2011 Super Outbreak: 15 violent tornadoes on April 27, alone, including four rated EF5
• May 3, 1999, Plains Outbreak: First F5 on record to hit Oklahoma City metro; F4 strikes south side of Wichita, Kansas
• April 1974 Super Outbreak: 30 violent tornadoes, including six rated F5
• April 1965 Palm Sunday Outbreak: 22 violent tornadoes

The 25 deadliest tornadoes on record in the U.S. all occurred from mid-March through June.

Spring typically provides a prime overlap of ingredients in the southern and central U.S. needed to generate the intense, rotating supercell thunderstorms that can spawn violent tornadoes.

Increasingly warm, humid air streaming northward out of the Gulf of Mexico is topped by cold, dry air aloft and a powerful jet stream pivoting out of the West. This provides the instability needed to generate thunderstorms.

In these cases, wind shear, the change in wind speed and direction from near the ground to several thousand feet above the surface, is intense. This both allows supercells to form and provides the large-scale spin those supercells need to stretch and tilt into tornadoes.

Violent tornadoes are much less common from midsummer through fall and winter, but they can happen any time of year if the volatile setup is in place.

F/EF4 tornadoes have occurred in every month, including winter.

Christmas Eve F4 tornadoes tore through parts of Arkansas and Missouri in 1982 and Tennessee in 1988.

Multiple F/EF4s were spawned in November outbreaks in 1992, 2001, 2002 and 2013.

F/EF5 tornadoes have not been documented in January since 1950 but have occurred in late February (Feb. 21, 1971, in Louisiana and Mississippi) and mid-December (Dec. 18, 1957, in Illinois).

Senior meteorologist Chris Dolce previously detailed where these violent tornadoes are most common: Texas to Iowa to Indiana, and also parts of the Deep South.

While spring brings thoughts of green grass and flowers in bloom, it is also the prime time of the year for the nation's most violent tornadoes.

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