Assessing the influence of dynamical factors on seasonal skill of severe winter windstorm predictions

It is known from previous studies that the winter windstorm season is significantly predictable on a seasonal timescale, especially over the British Isles and southern Scandinavia. Winter windstorms are one of the most damaging extreme events for the European continent. Hence, it is important to know that this skill exists as well as to understand how the forecast model reaches this performance to increase the usability of such forecasts.

Here, we link these extreme events to the three most dominant large-scale weather patterns over Europe. A combination of the three leading patterns explains up to 80% of the variability in windstorm frequency and ~60% of storm intensity. A statistical multi-linear model based on these patterns shows similar areas of skill but with lower skill over Europe.

This new investigation uses multiple dynamical atmospheric factors known to be related to windstorms, cyclones, their intensification and genesis. Among the factors examined are jet stream strength and location, Rossby wave source, Eady growth rate and potential vorticity. To understand the influence of these factors on windstorm forecast skill, we apply a three step conceptual approach: first to understand the link between windstorms in observations and hindcasts. Second, we analyse the forecast skill of the factors themselves. In the last step we diagnose significant changes in forecast skill of the dynamical factors between well and poorly predicted windstorm years.

Factors like MSLP, tropical Atlantic rainfall, jet location, PV in 350K, or Eady Growth Rate all show significant results in individual steps but none of the dynamical factors show significant results in all 3 steps. This could mean that an improved representation of factors and their link to windstorms could improve windstorm seasonal forecast skill.