Severe storms in the age of machine learning

Summary

Machine learning (ML) weather models are evolving rapidly, and a key question is: how well do these systems represent the fast-evolving dynamics that define severe mid-latitude storms? We explored this question using Storm Amy, the first named storm of the 2025–26 season for the western group of the European storm-naming initiative. Amy brought disruptive winds across northwestern Europe in early October 2025. We compared the performance of ECMWF’s forecasting systems: the physics-based Integrated Forecasting System Control forecast (IFS-CF, Cycle 49r1), the ML-based Artificial Intelligence Forecasting System deterministic forecast (AIFS Single), and the control member of the AIFS Ensemble forecast (AIFS ENS CF). To put the results in context, we also looked at Storm Éowyn, a more classical winter storm. Capturing Storm Amy’s evolution Storm Amy developed rapidly from the interaction of the remnants of tropical cyclones Imelda and Humberto, southwest of the North Atlantic storm track (Figure 1). By 3 October, Amy’s centre had crossed the northern fringes of Scotland as a deepening cyclone, before stalling north of Shetland and turning towards Norway. All three forecasting systems captured the storm track, pressure pattern, and depth of the low reasonably well for these short lead times. In particular, ML-based systems continue to perform well on these larger-scale flow features. The main differences appear in how well each system captures localised wind maxima linked, in part, to Amy’s strong frontal gradients. Figure 1: Animation of mean sea-level pressure (MSLP) and 10 m wind speed for Storm Amy, based on forecasts initialised on 2 October 2025 at 00 UTC. The analysis is shown in the top-right panel; the bottom row displays IFS Control (IFS CF), AIFS Single, and AIFS Ensemble Control forecast (AIFS ENS CF). Wind and pressure verification Focusing on the passage of Amy over the region in Figure 2a, all models captured the storm’s overall intensity, but some struggled with the timing of its rapid deepening. The AIFS Single initially underestimated the depth due in part to land influence on 3 October, 18 UTC (see also Figure 3c). Although the forecast for midnight improved, the system was still a few hectopascals shallower than the IFS analysis. Later, the AIFS Single recovered rapidly and captured the depth of the low remarkably well during the storm’s matu…