Extratropical cyclones – forecasting the sting in the tail

What is a sting jet?

A sting jet is a core of extremely strong winds that occurs within an extratropical cyclone and extends towards the ground. Although it can cause severe damage at the Earth’s surface, it affects a much smaller area than the main storm – typically around 100 km in the maximum width. Additionally, it has a far shorter duration, usually three to four hours.

Most extratropical cyclones don’t have a sting jet. For example, it’s estimated that they occur in around 39-49% of the strongest extratropical cyclones over the North Atlantic.  

What causes sting jets to form?

Sting jets tend to occur in a particular type of extratropical cyclone known as a Shapiro-Keyser cyclone. This type of weather system features low pressure at its centre and forms outside the tropics, causing various weather events such as rain and storms.

On a surface pressure chart, sting jets have a distinct appearance that differs significantly from a typical low-pressure system. These lows develop much more rapidly through a process known as explosive cyclogenesis, during which the associated warm and cold fronts do not merge to form an occlusion. The point at which the cold and warm fronts fail to ‘attach’ is known as the frontal fracture region.

Like all extratropical cyclones, the Shapiro-Keyser cyclone has two conveyor belts: warm and cold. The warm conveyor belt moves parallel to the surface cold front, rising above the surface warn front. As it rises, the moisture it carries condenses, resulting in a cloud head. The cold conveyor belt runs parallel to the surface warm front, then wraps around behind the surface cold front.

The sting jet forms inside the cloud head, in the mid-levels of the troposphere. which is the lowest layer of the Earth’s atmosphere, around three to four kilometres above the ground. Evaporative cooling then causes the jet to accelerate downwards towards the frontal fracture zone and surface. As the jet descends, snow and rain fall into it (the snow turns into rain). The rain evaporates, and the jet becomes colder and denser, causing it to sink to the surface. When the jet hits the surface ahead of the cold conveyor belt, it generates winds that are far stronger than those normally associated with an extratropical cyclone. The wind speed depends on the stability of the atmosphere.

Additional mechanisms may also be involved in generating a sting jet, such as weakened weather fronts and some types of convective instability – the relative importance of these processes remains an active area of research. It’s a complex topic, not least because an extratropical cyclone can produce several sting jets – and a sting jet can create damaging winds in many areas.

Forecasting sting jets

Understanding sting jets is crucial for offshore operations. They are associated with intense and localised weather phenomena, including sudden and extreme wind events. These events can pose significant risks to offshore operations, affecting the safety of personnel, maritime assets, and infrastructure. The high winds associated with sting jets can lead to dangerous sea conditions, impacting vessel stability and navigation. Additionally, strong winds can pose challenges to helicopter operations, often used for personnel transfer to offshore infrastructure. For industries such as offshore wind, the structural integrity of offshore installations may be compromised by the intense winds associated with sting jets. Therefore, a comprehensive understanding of sting jets is essential for companies to implement effective safety measures, optimise operational planning, and mitigate potential risks to personnel and assets operating in offshore environments.

Forecasting the sting jet phenomenon, however, can be very challenging for forecasters. One reason for this is that extratropical cyclones are rather like snowflakes: each one forms under its own specific conditions, which may or may not combine to make a sting jet. Another factor is that forecast models with a finite resolution often struggle to predict sting jets accurately because of their small size.

However, rapid advances in technology continue to improve the accuracy of sting jet forecasting. Some higher-resolution models can now detect small areas of very strong winds within an extratropical cyclone, indicative of a sting jet. It’s also now possible to spot sting jets on satellite imagery – a hook-shaped cloud swirling around the low centre with a sharp point (the ‘sting’) at the end of its tail.

At Fugro, when producing forecasts for our clients, if there’s potential for a sting jet to form, we usually increase the winds above model output and reflect the increased uncertainty regarding the peak wind speed in the confidence section of our report.

To conclude

The understanding and forecasting of sting jets are paramount for the safety and efficiency of offshore operations. These intense and localised weather phenomena pose significant risks to personnel, maritime assets, and infrastructure. While the complexity of extratropical cyclones presents challenges for accurate prediction, advancements in technology are steadily improving forecasting capabilities. By leveraging higher-resolution models and satellite imagery, forecasters can better identify the formation of sting jets and provide timely warnings to mitigate potential hazards. As industries continue to operate in offshore environments, maintaining a comprehensive understanding of sting jets remains essential for implementing effective safety measures and optimising operational planning. Through ongoing research and collaboration, we can enhance our ability to anticipate and respond to the impacts of sting jets, ensuring the safety and resilience of offshore operations in the face of extreme weather events.