A prototype real-time sting jet precursor tool for forecasters

Sting jets are descending airstreams that can lead to damaging surface winds and gusts in autumn and winter storms. UK impacting sting-jet storms include the Great Storm (1987) and the more recent St Jude's day (2013) and Georgina (2018) windstorms. A convective-instability-based precursor to sting jets has been developed by the project team to identify these storms in datasets unable to represent sting jets due to insufficient spatial resolution. These datasets include reanalyses and global ensemble weather forecast model output. Climatological analysis has shown that this precursor is present in about 30% of all North Atlantic storms, increasing to about 40% for explosively-developing storms. A similar analysis applied to end of 21st century climate-model output using the most extreme (RCP8.5) climate forcing scenario found a 60% increase in explosively-developing storms with the precursor. Furthermore, even in reanalyses, storms with the precursor are the dominant cause of storm-related strong UK winds. In reality these winds will be enhanced by the actual occurrence of sting jets. Early detection of storms with the precursor could give advanced warning of enhanced wind risk and so improve national severe weather warnings and thus climate resilience.

The project aim is to develop a prototype real-time precursor tool for use by Met Office forecasters. The existing precursor diagnostic code will be adapted to work routinely on operational global ensemble forecast output to inform forecasters whether or not storms impacting the UK and Europe up to seven days in the future have the precursor. We will work with forecasters to develop suitable graphical outputs and trial the prototype tool during autumn and winter 2019. If the prototype tool is successful then the Met Office anticipate implementing it within their model system such that the sting-jet precursor diagnostic would be available to all their prediction systems.

The proposed implementation of a real-time sting-jet precursor tool for Met Office forecasters has the potential to lead to reduction in the socio-economic costs of severe UK winter storms through improved weather forecasts. The tool will provide improved guidance on those storms likely to produce hazardous surface winds and gusts, and specifically the probability that these storms will produce a sting jet (a descending jet that can lead to the strongest wind gusts over land), several days before they occur. Current models used to provide these medium-range probability forecasts have insufficient resolution to resolve (i.e. represent) sting jets and so forecasters rely on characteristic features in satellite imagery or output from short-range (~1 day) forecasts from the UK-domain high resolution model to inform them that a sting jet is likely to be present in the storm. Once present these features provide, at best, only few hours warning of the sting jet reaching the surface. Hence, the precursor tool has the potential to provide a step change in our ability to provide early warnings of these hazardous storms.

The general public in the UK and Europe are the main beneficiaries of the proposed research. They will benefit from the project legacy of improved early warnings of potentially high impact weather events. Improved weather warnings can save lives and livelihoods.

The benefits to the general public will be realised through the benefits to the Met Office of a tool that can be used by their operational forecasters to anticipate storms that are likely to be associated with extreme, and so potentially damaging, surface winds and gusts several days before they hit the UK and western Europe. We will work closely with the Met Office to implement this tool. The resulting forecast guidance can be used by the Met Office's National Severe Weather Warning Service (NSWWS) and in European storm forecasts.

Improvements to the NSWWS are experienced directly by the general public (who may change, for example, their travel plans when warnings are issued) and indirectly through improved guidance to organisations that have responsibilities for emergency preparation and response at the local level (including emergency services and travel and utility companies). Improved early warnings of hazardous storms will enable such organisations to mobilise staff and equipment more efficiently and so mitigate impacts.

Finally, improved understanding of these hazardous storms and their characteristics (by academics, forecasters and the general public) is likely to result as a legacy of this project. This legacy should improve the UK's future climate resilience to these storms.