Introduction

Storm Karl affected northern Europe regionally with high-impact weather at the end of September 2016.

This storm event was of interest for forecasting because it illustrated how a transitioning tropical cyclone can reshape the large-scale North Atlantic circulation and affect downstream predictability. When Karl interacted with the midlatitude jet, its outflow altered upper-level potential vorticity and amplified Rossby waves within the Atlantic waveguide. This interaction influenced the development and track of subsequent weather systems over Europe, exposing sensitivities in medium-range forecasts. Intensive aircraft observations, collected during the NAWDEX flight campaign, helped identify where numerical models struggled, particularly in representing diabatic processes and their impact on the jet structure. Karl therefore served as a key case for understanding how tropical–extratropical interactions can trigger forecast errors that propagate far downstream.

Storm Karl was created through extra-tropical conversion of a previous Tropical Storm event. When crossing the North Atlantic it merged with other lower pressure systems and invigorated, resulting in high wind speeds in Scotland and intense precipitation (with local flooding) in Norway.

This storm event was studied extensively during the NAWDEX flight campaign in connection with atmospheric moisture transport. The following table points to some of the published material related to the situation in the North Atlantic during this time period. 

Literature

The table below points to material related to this storm event. This is obviously not an exclusive list, and if we become aware of more published material we will add this to the table. The published article in BAMS gives a good overview of the weather situation in the North Atlantic region during that time. 

Important dates

The dates shown below were obtained from the US NHC Tropical Cyclone Report and from ECMWF ERA5 reanalysis data. They represent important milestones in the development and lifetime of Storm Karl. The first four dates describe events during the tropical storm phase, the following events occurred after the extra-tropical conversion in the North Atlantic and later.

OpenIFS initial experiment data

We provide experiment data packs that allow to simulate the time period of Storm Karl in the North Atlantic with the global OpenIFS forecasting model. The initial experiment data and boundary conditions were created from ECMWF reanalysis data at horizontal model grids N80 and N128, with either 60 or 91 levels, respectively.

The initial experiment data packs and Metview post-processing code can be downloaded here:
https://openifs.ecmwf.int/data/experiments/48r1/2016-09-25_Karl/

The experiment data packs include a model namelist file (fort.4) which contain suggested 6-hourly outputs for instantaneous and derived model fields and diagnostics. 

All OpenIFS experiments begin on 25th September 2016 00:00 UTC, shortly before the extra tropical conversion of Storm Karl. 

The Storm Karl case study is used as the primary case study in the ECMWF training course "A hands-on introduction to Numerical Weather Prediction Modelling" which is held at ECMWF in Reading, UK, approximately every 2 years. 

Licensing and Attribution

The OpenIFS experiment data packages were produced with the OpenIFS Data Hub and their use is subject to the following Data Licensing conditions.