Hauser et al. (2026) Exceptionally poor and good medium-range forecasts of the large-scale circulation over Europe in ERA5 reforecasts

⚠️ Warning: This summary was generated from the abstract only, as the full text was not available.

Identification

• Journal: Repository for Publications and Research Data (ETH Zurich)
• Year: 2026
• Date: 2026-01-01
• Authors: Seraphine Hauser, Steven M. Cavallo, Linus Magnusson, Jonathan E. Martin, David B. Parsons
• DOI: 10.3929/ethz-c-000795734

Research Groups

European Centre for Medium-Range Weather Forecasts (ECMWF)

Short Summary

This study extends the definition of forecast busts to include "exceptionally poor" and "exceptionally good" forecasts, accounting for seasonality, and analyzes their characteristics and links to large-scale weather regimes over Europe using ERA5 reforecasts. It finds a declining trend in poor forecasts and an increasing trend in good forecasts, with distinct large-scale circulation patterns and regime transition timings associated with each skill category.

Objective

• To extend the concept of forecast busts to define "exceptionally poor forecasts" and introduce the notion of "exceptionally good forecasts" using a methodology that accounts for seasonality in forecast skill.
• To identify and compare the characteristics of exceptionally poor and good six-day forecasts over Europe.
• To explore potential links between these forecasts and large-scale weather regimes in the North Atlantic-European region, with particular attention to the occurrence and timing of regime transitions.

Study Configuration

• Spatial Scale: Europe, North Atlantic-European region
• Temporal Scale: Six-day forecasts; reforecast data from 1979 to 2023 (45 years)

Methodology and Data

• Models used: ECMWF Reanalysis Version 5 (ERA5) reforecasts (which are products of a numerical weather prediction model and data assimilation system).
• Data sources: ECMWF Reanalysis Version 5 (ERA5) reforecasts.

Main Results

• A declining trend in the annual rate of exceptionally poor forecasts and an increasing trend in the rate of exceptionally good forecasts were identified, consistent with advances in observations.
• Exceptionally poor forecasts occur more often in the warm season, while exceptionally good forecasts are found throughout the year.
• Mean patterns for poor forecasts are characterized by Rossby-wave trains, whereas blocking over northern Europe (specifically Scandinavian blocking) dominates good forecasts.
• Periods of poor forecast performance coincide with an above-average frequency of cyclonic regimes and persistent no-regime periods.
• Good forecasts show an above-average association with anticyclonic regimes, particularly Scandinavian blocking.
• While the share of cases with a regime transition is similar in both skill categories (approximately 60%), transitions occur significantly later during poor forecasts and earlier in good forecasts.

Contributions

• Introduces a novel methodology for defining "exceptionally poor" and "exceptionally good" forecasts that accounts for seasonality in forecast skill, extending the traditional concept of forecast busts.
• Provides a comprehensive understanding of the large-scale circulation configurations and stages of regime evolution that favor exceptionally poor or good forecasts over Europe.
• Identifies distinct large-scale atmospheric patterns (e.g., Rossby-wave trains for poor, Scandinavian blocking for good) and the critical role of regime transition timing in forecast skill.

Funding

Not specified in the provided text.

Citation

@article{Hauser2026Exceptionally,
  author = {Hauser, Seraphine and Cavallo, Steven M. and Magnusson, Linus and Martin, Jonathan E. and Parsons, David B.},
  title = {Exceptionally poor and good medium-range forecasts of the large-scale circulation over Europe in ERA5 reforecasts},
  journal = {Repository for Publications and Research Data (ETH Zurich)},
  year = {2026},
  doi = {10.3929/ethz-c-000795734},
  url = {https://doi.org/10.3929/ethz-c-000795734}
}