Insights & News

Why USA Thunderstorms Can be the Most Severe

28th April 2020

In most countries of the world, a thunderstorm capable of producing more than a low-end (i.e. EF0 to EF2 on the Enhanced Fujita Scale, which I’ve outlined below) tornado is a rare phenomenon, typically seen no more than a few times in a decade.

In the US, however, multiple EF3 strength tornadoes are seen every year. There’s at least one EF4 strength most years and EF5 strength every two or three years. Rarely has one of EF4 intensity ever been (reliably) observed outside of the US.

Of the 62 officially-rated EF5 tornadoes on record, all but 3 occurred within the contiguous US.

Table displaying the wind speeds associated with each category of the Enhanced Fujita Scale

Even an EF0 can bring down tree branches and cause minor damage to buildings. An EF1 can overturn mobile homes, while an EF2 completely destroys them. An EF3 can obliterate entire stories of even well-constructed houses. An EF4 has absolutely devastating destructive capacity, throwing cars around and levelling all but the toughest structures. As for an EF5, well, it can throw trains about 1 mile through the air. I’m not kidding.

So, what gives – why is the capacity for severe thunderstorms ‘next-level’ in the US?

A Very Particular Structure

To understand that, we first need to be familiar with a particular vertical configuration of the atmosphere. This setup is essential for the most severe thunderstorms to occur:

Illustration of the atmospheric setup required for the most severe thunderstorms to occur.

For thermodynamic reasons outlined in my previous blog piece, the warmed near-surface air will become buoyant. As it rises toward the inversion, it will lose some of its heat to its surroundings. If it becomes too cool, the inversion will bring it to a halt.

Typically, it will take many hours before the first ‘parcels’ of rising air (it helps to visualise it that way) have become warm enough to ‘punch’ through the inversion layer and into the relatively cool and dry air above (the ‘free air’). During this time, a great deal of warm, moist air can build up beneath the inversion.

Once a breach occurs, things get downright explosive. The air parcels are much less dense (lighter per unit volume) than free air, so they rocket upward.

Before long, millions of air parcels are soaring to great heights and cooling as they go. This cooling causes the moisture they contain to condense into water droplets, the aggregation of which forms towering clouds.

Note the very shallow layer of cloud surrounding this developing cumulonimbus (towering shower cloud) – this is where moisture has piled up beneath the inversion layer. The tower is positioned above the breach and has likely attained most of its height within the space of half-hour. This Photo by Unknown Author is licensed under CC BY-SA

The difference in wind speed or direction (wind shear) at high latitude then serves to tilt the storm in the vertical. This is critical for severe thunderstorm development because it prevents the storm from ‘choking’ itself (illustrated below).

Diagram showing how wind shear allows a thunderstorm to sustain for much longer

You see, thunderstorms produce rain-cooled (…or hail-cooled!) downdrafts. In a non-tilted storm (i.e. when there’s little or no wind shear), this falls through the updrafts feeding the storm, disrupting them. With such reduced inflow, a thunderstorm rapidly collapses.

Tilting causes the downdraft to fall to the side of the main inflow of warm, moist air. With less disruption to inflow, the thunderstorm can sustain for much longer, sometimes for several hours past initiation.

If there’s strong directional wind shear, the storms will tend to rotate not just in the vertical, but the horizontal too. At this point, a severe thunderstorm can achieve top rank – supercell status.

Why is the Inversion so Important?

It’s a sudden ‘release’ of large quantities of warm, moisture-laden air into cooler air aloft that builds the largest, most severe thunderstorms.

Without an inversion, what tends to occur are many smaller-scale, gentler updrafts of warm air, forming large numbers of smaller clouds. With enough surface heating (i.e. a lot!) or wind shear, severe thunderstorms can develop this way – but not the most severe ones.

Diagram showing how an inversion can lead to fewer but much larger thunderstorms than would occur otherwise.

The USA is no stranger to this ‘lesser’ regime, but sometimes, it can stage the biggest, most dangerous thunderstorms on the planet. Let’s take a look at why this is so.

The Right Ingredients in the Right Places

Due to the Earth’s rotation, upper-level winds are predominantly westerly. As illustrated below, this means that the relative positions of the Rocky Mountains (Rockies) and the Gulf of Mexico are ideal for setting up strong inversions above very warm and moist airmasses.

Map with annotations describing what features in and around the USA allow it to support the strongest thunderstorms on Earth.

The Rockies also tend to obstruct westerly near-surface flows off the Pacific, so it’s not uncommon for the upper level westerlies to coincide with near-surface southerlies (wind shear – check!). All it then takes is some relatively cool air aloft and the stage is set for explosive thunderstorm development to occur, provided the inversion can be broken through.

As if that wasn’t enough, the relatively flat nature of the expansive Great Plains is ideal for the development of very large supercells.

In the mid-late spring season, surface heating is ramping up toward the summer but there’s still plenty of cool air at high altitude above the Pacific (residual from the winter, which takes a long while to warm up).

For this reason, most of the very strongest thunderstorms and tornadoes have been observed in April or May. Check out the contrast in risk coverage between the example risk maps below. While surface heating is stronger in summer, tornado frequency is much lower, due to reduced availability of cool air aloft.

Example risk maps showing much more widespread observed tornado coverage for a week in April compared to a week in July.

The risk in these maps is calculated from all historic tornado observations within the week of the year.
With MetSwift’s revolutionary analytics platform, you can rapidly access tornado, hurricane or earthquake maps and location-specific risk for any week of the year, anywhere in the world (subject to relevance of risk type)!

That concludes this thorough examination of what gives the US its notorious ‘biggest thunderstorm’ potential.

Next week, the view widens… to the rest of the world! You’ll be able to discover how the likes of Europe, Asia and Oceania stack up against the US. Touches down on the web Monday 4th May.

 

James Peacock MSc
Head Meteorologist at MetSwift

Featured Image by Unknown Author is licensed under CC BY-SA

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