In 2026, there will be a transition from the current Harmonie 43 (HA43) weather model to the new Harmonie46 (HA46) version. This change will have both technical and content-related consequences, such as in data formats and model output. To ensure that the transition goes as smoothly as possible, we would like to inform you in advance about the reasons for this transition, the planning, and the support you can expect from us.
Why is Harmonie 43 being replaced by Harmonie 46?
The switch to HA46 is part of a joint development within the international consortium UWC-West, in which the national weather institutes of Denmark, Ireland, the Netherlands and Iceland (DINI) collaborate.
The new model version offers improvements to a number of known model problems. In addition, it includes a number of technical changes that affect the data formats, such as the transition from GRIB1 to GRIB2 format. With these changes, HA46 is better aligned with the latest insights in numerical weather prediction and the growing need for up-to-date, reliable data for research, policy, and operational applications.
What do we offer during the transition?
- Timely availability of verification reports, test data, and technical documentation (from the first half of Q1-2026). The test data consists of a separate test dataset, a re-forecasting dataset of at least 1 year and a pre-operational data stream;
- Information sessions about the changes in HA46 from the second half of Q1 2026;
What is the schedule for this transition?
| Phase | Period | What will happen |
|---|---|---|
| Preparation phase | Q4 2025 - Q1 2026 | KNMI coordinates with UWC-West and prepares test data and documentation. |
| Test Phase | February - April 2026 | You can prepare and test applications and processes with HA46 test data and provice feedback. |
| Transition phase | May 2026 | Operational transition to Harmonie 46. Everyone will start working with HA46. |
| Aftercare phase | June - July 2026 | We collect feedback and evaluate the transition process. |
Further information
In the first half of Q1 2026, we will provide more information about the test data and validation reports, among other things. We will do this via the newsletter, knmidata.nl and our KDP-community Slack channel.