A 'tornado drought' is growing in the US - why?

Friday, February 9, 2018, 5:33 PM - We might tend to think of tornadoes and severe thunderstorms as mainly summertime phenomena, but the fall and winter months generally hold their own, adding to a year's tornado total.

The past few months have been different, however; in fact, we're seeing a 'drought' as far as tornadoes are concerned in the U.S. While that's certainly better than the traditional kind of drought we're also seeing in the Plains, it's still worth investigating.

How long has it been since we've seen a major tornado, and what's behind the 'shortage'?

A stovepipe tornado near Wray, Colorado, in May 2016. This tornado was rated EF-2 on the Enhanced Fujita Scale. Image: Getty Images.

Tornadoes are among the most powerful, unpredictable and destructive weather systems on Earth. The National Weather Service (NWS) defines a tornado as a violently rotating column of air in contact with the earth’s surface (over land or water), commonly associated with a severe thunderstorm. The most active month is May in Tornado Alley (in the Central Plains), when we can see anything from weak EF-0 to immensely destructive EF-5 tornadoes. We even get tornadoes in the winter! In fact February 5, 2018 was the 10th anniversary of the devastating Super Tuesday tornado outbreak of 5 - 6 February 2008. 86 tornadoes were recorded in 10 states, causing 57 fatalities.

But the last 269 days of 2017 and 2018 have been relatively quiet. With no EF-3 or stronger tornadoes in that time period (and counting), this makes it the longest EF-3+ tornado drought dating back to 1953. This is a good thing - a tornado drought. As we gear up for 2018's main severe weather season, we take a look at the data - and the lack of twisters - below.

How a Tornado forms

Tornadoes are generally fueled by the collision of warmer air near the surface and cooler air aloft - a condition which creates instability in the atmosphere. When strong winds associated with an area of low pressure (such as a thunderstorm) vary with height in the atmosphere, this can generate rotors, or tubes of horizontally rotating air. When those tubes meet the upward-flowing air beneath a thunderstorm (the updraft) they are pulled upward, becoming vertical. When water vapor in the air condenses in the now-vertical rotor, we see a condensation funnel cloud. Many less severe tornadoes are not even visible to the human eye, even though they do touch the ground. Major tornadoes usually become more visible when the strong winds within the funnel lift up dirt and debris off the surface.

Watch Below: What causes a thunderstorm?

While they come in all shapes and sizes, the 'average tornado' has maximum wind speeds of about 112 mph or less, measures around 250 feet in width and travels approximately one mile before falling apart. Some of the most catastrophic tornadoes in recorded history have had winds close to, even in excess of, 300 miles an hour. One example of this was the El Reno, Oklahoma tornado of 2013. This one grew to an unprecedented width of 2.6 miles (4.2 km), becoming the widest-known tornado ever recorded in the United States, packing winds winds greater than 295 mph.

When was the last EF-3 tornado?

Throughout a six-day period, starting on May 15, 2017, numerous tornadoes occurred across portions of the Plains, the Great Lakes, and the Ohio Valley. The most significant activity occurred on May 16, as multiple tornadoes touched down across the Plains states and as far north as Wisconsin. A high-end EF-2 tornado struck the southern part of Elk City, Oklahoma, damaging or destroying numerous homes and businesses, tossing vehicles, and killing one person. Another strong tornado caused damage to homes in the town of Pawnee Rock, Kansas, before it passed near Great Bend, debarking trees and completely destroying farm homes at high-end EF-3 strength. These were the last EF-3 rated tornadoes recorded in the U.S.

When was the last EF-4 Tornado?

On April 29, 2017 two long-lived and particularly dangerous tornadoes ripped through the Canton, Texas, area killing four people and injuring many others. One tornado was rated EF-3 and the other was rated EF-4 on the Enhanced Fujita Scale.

When was the last EF-5 Tornado?

A massive tornado on May 20, 2013 tracked directly through south Oklahoma City and Moore, producing a large swath of catastrophic damage through densely populated areas. Numerous homes sustained EF-4 damage in the Moore, and a few scattered instances of EF-5 damage were also noted.

Below: One year after the Moore, OK tornado

So where have the tornadoes been?

Well, as we talked about above, we need a few ingredients to get any severe weather to form. Heat (instability), lift (a front), moisture, and wind shear (winds changing direction with height). We have had very few instances of all these ingredients coming together this winter.

As most Americans east of the Rockies are well aware, this winter - and, in particular, this first part of 2018 - has been a cold one. The jet stream has been taking a dive too far to the south most of 2018, bringing modified Arctic air all the way to the Deep South. That cold, dry air and Arctic high pressure are just not conducive to getting tornadoes, particularly powerful ones. While we've seen some of the worst of the Arctic air retreat back to the north, we haven't seen much in the way of the milder, more humid air needed to fuel instability and thunderstorms; at least not until the past couple of weeks.

The long range forecast suggests one thing for sure, however - the warm up is on its way as we head into spring and the sun rises higher in the sky. While it remains to be seen how long our strong tornado drought will last, with the official start to the severe weather season just around the corner, it's not too early to start preparing for the drought's end.