The series of killer tornadoes that piled on death and mayhem in four States in the US (given the moniker as a ‘Quad-State Tornado’) from Friday night into early Saturday morning is competing with a ‘Tri-State Tornado’ that hit three States way back in 1925 as the longest-sustaining ever.

The US National Weather Service is in the process of categorising the cataclysmic events after conducting its own post-event survey. But there are indications galore that the swarm of tornadoes was in the ‘strong’ to ‘violent’ categories, EF3, EF4 or EF5, on the Enhanced Fujita Scale (EF Scale).

Night-time tornadoes

Night-time tornadoes as they descended on the four States are more than twice as likely to cause fatalities than their daytime counterparts, US media pointed out. This is so because tornadoes are harder to see at night, and literally get people napping, leaving hardly any time to make their way to safety.

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The EF Scale is used to assign a tornado a ‘rating’ based on estimated wind speeds and related damage. Wind speeds for EF3 (strong) range from 219-265 km/hr; EF4 (violent) 267-322 km/hr; and EF5 (violent), over 322 km/hr. Pre-monsoon thunderstorms in North and East India is the nearest equivalent.

An EF5 tornado is the strongest designation a tornado can possibly be assigned. They are exceptionally rare and can produce wind speeds higher than 322 km/hr. The last EF5 tornado to strike the US was at Moore, Oklahoma, in May of 2013.

Exceptionally rare event

The country has reported only 19 EF4 tornadoes during December and only two EF5 tornadoes. The last EF4 tornado to strike the US during December was in 2015. The last EF5 tornado to strike the country during the month was in 1957.

The Friday-Saturday event was caused by a volatile atmospheric set-up that was primed to produce violent and long-track tornadoes. Friday had featured unseasonably warm and higher than normal temperatures. This warmth, combined with high humidity, provided ample fuel for the storms.

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An approaching cold front from the North of the country rammed into this setup, setting off a devastating line of thunderstorms. Added to this is the fact that the US is at the receiving end of a reinvigorated La Niña phenomenon, which is known to increase tornado frequency.

Incredible system intensity

One of the 30 deadly tornadoes on Friday-Saturday left a trail of death (estimates vary from 70 to 100) and destruction across the States in the Mississippi Valley, the South-East and the Mid-West of the US, and is considered an extremely rare event this late into the year.

The incredible strength and intensity of the swarm were evidenced in the lofty heights to which debris is reported to have been tossed up in the air. The US media quoted radar observations as suggesting that the twister that hit the worst-hit Mayfield town threw debris over 30,000 feet into the air.

Debris tossed into highs

This is the altitude frequented by commercial aeroplanes. Media reports also said that there were at least eight tornado emergencies issued, reserved for the most life-threatening tornado events.

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Meteorologists attribute the genesis of the tornado that devastated Mayfield in Kentucky to a parent thunderstorm that travelled more than 370 km across the four States over four hours.

Kentucky Governor Andy Beshear described the tornado as the most devastating one to hit the state, estimating that it might have killed at least 70 people or even more than 100.