Éowyn, January 2025

Storm Éowyn, on 24 January 2025, was a deep and rapidly intensifying extratropical cyclone that brought exceptionally severe weather to Ireland. Its intensity was notable, with central pressure dropping significantly in the 24 hours prior to landfall, a characteristic often associated with high-impact events. Such systems typically form in the North Atlantic, drawing energy from the temperature contrast between polar and tropical air masses, and are steered by the jet stream.

The classification of Éowyn as a named storm followed the Met Éireann and Met Office storm naming convention, which triggers a name when an amber or red warning for wind, rain, or snow is anticipated. The severity of Éowyn necessitated the issuance of red warnings for wind across multiple counties, indicating a significant threat to life and property. These warnings are based on specific wind speed thresholds; for instance, a red wind warning in Ireland typically indicates mean speeds exceeding 80 km/h or gusts exceeding 130 km/h, though these thresholds can vary slightly by region and exposure. The primary hazard from Éowyn was the extreme wind, with widespread damage to infrastructure and significant disruption to transport and power networks.

The development of Storm Éowyn was marked by a process known as explosive cyclogenesis, or 'bombogenesis', where the central pressure of a low-pressure system drops by at least 24 hPa in 24 hours. For Éowyn, satellite observations and atmospheric soundings indicated a pressure fall exceeding this threshold, with some models predicting drops of 30 hPa or more. This rapid deepening leads to a tightening of isobars and a corresponding increase in the pressure gradient force, driving stronger winds.

The storm's track was crucial to its impact on Ireland. Originating in the western Atlantic, Éowyn moved rapidly eastward, passing close to the south coast of Ireland before tracking towards the UK. This trajectory placed much of the southern half of Ireland in the most dangerous quadrant of the storm, where the wind field is enhanced by the storm's forward motion. For example, if a storm is moving at 50 km/h and has an intrinsic wind speed of 100 km/h in its right-hand quadrant (Northern Hemisphere), the effective wind speed experienced on the ground can be 150 km/h. The precise track and speed of Éowyn meant that the most severe winds affected a broad swathe of the country, particularly coastal areas and elevated terrain.

Forecasting the exact track and intensity of such rapidly developing systems presents a significant challenge for numerical weather prediction models, as small initial errors can lead to large discrepancies in the forecast outcome.

In anticipation of Storm Éowyn's severe impacts, Met Éireann issued red warnings for wind for several counties. These warnings are the highest level in the national warning system, signifying that 'dangerous and disruptive weather is expected, which may pose a threat to life'. The decision to issue a red warning is based on a combination of forecast wind speeds, gust magnitudes, and potential impacts, including the vulnerability of infrastructure and population density.

For Éowyn, the criteria for red warnings were met by forecast sustained wind speeds exceeding 80 km/h (approximately 43 knots or 50 mph) and gusts exceeding 130 km/h (approximately 70 knots or 81 mph) over prolonged periods. These thresholds are designed to trigger protective actions from the public and emergency services. For instance, at 130 km/h, the dynamic pressure on a flat surface is approximately 1.06 kPa (calculated as 0.5 * density of air * speed^2, using air density of 1.225 kg/m³). This force is sufficient to cause significant structural damage, fell large trees, and make driving conditions extremely hazardous.

Worked Example: If a typical roof tile has an area of 0.1 m² and is exposed to a gust of 130 km/h, the force exerted on it is 0.1 m² * 1060 Pa = 106 N. This force, when combined with uplift from aerodynamic effects, can easily dislodge tiles, leading to further damage.

The measured observations during Storm Éowyn confirmed the severity of the forecasts. Met Éireann weather stations across affected regions recorded widespread severe gusts, with some locations experiencing their highest January gusts in decades. Coastal stations, particularly those with exposure to the prevailing south-westerly flow, reported the highest figures.

While specific figures for Éowyn are subject to final verification by Met Éireann, typical high-end gusts during such events can reach or exceed 150 km/h (approximately 81 knots or 93 mph) at exposed coastal sites at 10 metres above ground level. Inland, gusts are generally attenuated by surface roughness but can still be exceptionally high, often exceeding 100 km/h (approximately 54 knots or 62 mph).

For comparison, during Storm Ophelia (16 Oct 2017), a gust of 156 km/h was recorded at Roches Point, Co. Cork. During Storm Eunice (18 Feb 2022), a gust of 130 km/h was recorded at Sherkin Island, Co. Cork. Éowyn's peak gusts were in a similar range, highlighting its extreme nature. These observed values are critical for validating forecast models and for understanding the real-world impacts of such intense storms. The Shear Glass on The Wind Agent would have shown these measured gusts at 10m, alongside the modelled wind at relevant working heights, providing a comprehensive picture of the wind environment.

The impacts of Storm Éowyn were extensive, leading to significant disruption across Ireland. Power outages affected hundreds of thousands of homes and businesses, primarily due to fallen trees and damage to overhead power lines. Transport networks were severely affected, with widespread road closures, cancellations of public transport services, and disruptions to air and sea travel. Coastal areas experienced significant wave action, leading to coastal flooding and erosion in some vulnerable locations.

Emergency services and local authorities were heavily engaged in responding to the storm's aftermath, including clearing debris, restoring power, and managing flood responses. The recovery effort was prolonged in some areas, particularly where damage to critical infrastructure was extensive. The economic cost of such events is substantial, encompassing direct damage, business interruption, and the costs of emergency response and long-term recovery.

Lessons learned from Éowyn, similar to previous severe storms like Ophelia and Eunice, often focus on enhancing resilience of infrastructure, improving public communication strategies for severe weather warnings, and refining emergency response protocols. The Wind Agent's fleet board and alerts would have provided critical real-time information to operational teams during the event, assisting in resource deployment and safety decisions.

The forecasting of Storm Éowyn presented a complex challenge. While numerical weather prediction (NWP) models generally captured the development of a significant storm, the precise timing, track, and peak intensity, particularly the extent of rapid intensification, showed some variability across different model runs and ensemble members. This is typical for explosive cyclogenesis events.

Early ensemble forecasts, such as those from the ECMWF (European Centre for Medium-Range Weather Forecasts) Integrated Forecasting System (IFS) and the NOAA Global Ensemble Forecast System (GEFS), initially indicated a range of possible scenarios, reflecting the inherent uncertainty in predicting such rapidly evolving systems. As the event approached, the ensemble spread narrowed, and the deterministic models converged on a more consistent, high-impact scenario, allowing for the timely issuance of high-level warnings.

However, the exact magnitude of the highest gusts, particularly in localised areas affected by terrain enhancement or convective elements within the storm, remained difficult to predict with absolute precision. This highlights the importance of using ensemble products, such as The Wind Agent's exceedance fan, which provides a probabilistic view of exceeding specific wind speed limits, rather than relying solely on a single deterministic forecast. The Agreement Spine on The Wind Agent would have shown how different models agreed or diverged in their predictions for Éowyn.

Storm Éowyn reinforced several key lessons for operational planning and risk management in Ireland. Firstly, the need for robust and flexible emergency plans that can be activated rapidly in response to red warnings. Secondly, the importance of proactive communication with stakeholders and the public regarding potential impacts and safety measures.

From a meteorological perspective, the event underscored the value of high-resolution NWP models and ensemble forecasting systems in providing early indications of severe weather potential. For operations sensitive to wind, such as crane work, drone flights, or marine activities, the ability to monitor not just mean wind but also gust speeds at specific working heights is critical. The Shear Glass on The Wind Agent, providing height-matched wind data, would have been invaluable for operators needing to understand the wind environment at their exact point of operation, not just at a standard 10m reference height.

Furthermore, understanding the return period of such extreme events, while not a direct forecast, helps in long-term infrastructure planning and risk assessment. The return period for gusts of Éowyn's magnitude is typically several years to a decade for most inland locations, and potentially shorter for very exposed coastal sites, indicating that such events, while not annual, are a recurring hazard in Ireland.