Technical documentation

Introduction

This dataset presents probability forecasts for weather that may pose a hazard to people or infrastructure in the Netherlands. It is based on model data, specifically the ECMWF IFS (Integrated Forecast System). This is an ensemble prediction system (EPS) of 50 members. It has a spatial resolution of 0.1 degrees latitude/longitude.

To transform the model data into probability forecasts, we define a risk profile and then count how many members of thegi ensemble exceed the specified threshold. For example, if we want to know the risk of a hot day, we set the risk threshold at 30° Celsius and for each grid box and each time step, we determine how many members exceed the threshold. So if 25 members exceed the threshold of 30° Celsius at a given time and place, the probability value will be 50%.

Please note that we do not correct for biases in the model data from ECMWF.

This dataset is not a replacement for KNMI's weather warnings, it simply aims to provide extra information to interested parties. Please refer to KNMI's homepage for official warnings.

Interpretation of risk profiles

Note the time variable in the dataset is backward looking, i.e. a time window of 24 hours labeled as "20260903T12" refers to the time window between 20260902T12 and 20260903T12. All timestamps are in UTC timezone.

Heat

Probability of the maximum temperature exceeding the given threshold (in °C) during the past hour.

Precipitation

Probability of the accumulated precipitation of the past 24 hours exceeding the given threshold (in mm) during the past hour.

Snowfall

Probability of the accumulated snowfall of the past 24 hours exceeding the given threshold (in cm) during the past hour.

Visibility

Probability of fog reducing the visibility to the given threshold (in m) during the past hour.

Wind chill

Probability of the wind chill reducing the experienced temperature to below the given threshold (in °C) during the past hour. In the risk profile name, the minus sign is replaced by the letter "m", so the risk profile Wind chill lt m20 degrees. should be read as "wind chill below -20°C"

Wind

Probability of wind gusts exceeding the given velocity threshold (in km/h) during the past hour.

Dimensions

The radius dimension represents spatial smoothing: the four grid boxes directly next to the grid box under consideration are used to reduce spatial uncertainty. For the ECMWF-IFS model specifically, that means the smoothing radius is 0.1 degrees.

The time_window dimension represents whether we look at instantaneous phenomena (time window set to 1 hour, for example wind or temperature), or cumulative phenomena (rain or snow, time window set to 24 hours).

Overview of available dimensions

The following table states the available risk profiles and other relevant dimensions for each file. The timestamp in the filename corresponds to the forecast reference time as determined by ECMWF.

Filename risk_profile radius time_window
PROBABILITY_HEAT_<timestamp>.nc Maximum temperature gte 28.5 C. 0.1 1
PROBABILITY_HEAT_<timestamp>.nc Maximum temperature gte 30 C. 0.1 1
PROBABILITY_HEAT_<timestamp>.nc Maximum temperature gte 34 C. 0.1 1
PROBABILITY_HEAT_<timestamp>.nc Maximum temperature gte 35 C. 0.1 1
PROBABILITY_HEAT_<timestamp>.nc Maximum temperature gte 36 C. 0.1 1
PROBABILITY_HEAT_<timestamp>.nc Maximum temperature gte 38 C. 0.1 1
PROBABILITY_HEAT_<timestamp>.nc Maximum temperature gte 40 C. 0.1 1
PROBABILITY_PRECIPITATION_24-HOUR_SUM_<timestamp>.nc Cum precip 24H gte 50 mm. 0.1 1
PROBABILITY_PRECIPITATION_24-HOUR_SUM_<timestamp>.nc Cum precip 24H gte 75 mm. 0.1 1
PROBABILITY_SNOWFALL_<timestamp>.nc Snowfall 1H gte 0.1 cm 0.1 24
PROBABILITY_SNOWFALL_<timestamp>.nc Snowfall 1H gte 3 cm 0.1 24
PROBABILITY_SNOWFALL_<timestamp>.nc Snowfall 6H gte 5 cm. 0.1 19
PROBABILITY_VISIBILITY_<timestamp>.nc Visibility lt 10 m. 0.1 24
PROBABILITY_VISIBILITY_<timestamp>.nc Visibility lt 200 m. 0.1 24
PROBABILITY_WIND_CHILL_<timestamp>.nc Wind chill lt m15 degrees. 0.1 1
PROBABILITY_WIND_CHILL_<timestamp>.nc Wind chill lt m20 degrees. 0.1 1
PROBABILITY_WIND_<timestamp>.nc Wind gusts gte 75 kmh. 0.1 1
PROBABILITY_WIND_<timestamp>.nc Wind gusts gte 90 kmh. 0.1 1
PROBABILITY_WIND_<timestamp>.nc Wind gusts gte 100 kmh. 0.1 1
PROBABILITY_WIND_<timestamp>.nc Wind gusts gte 120 kmh. 0.1 1

In one case, for the risk profile "Snowfall 6H gte 5 cm.", the time window is set to 19, this is because the risk profile looks at snowfall accumulated over 6 hours. To get this variable to accumulate over 24 hours, we have to consider 19 time steps.

Change in format

For files up to and including the forecast reference time 202609030000, only the risk_profile dimension is included.

For files after this (starting with the forecast reference time 202609031200), it is necessary to specify the radius and time_window dimensions. The content and interpretation of the file does not change between formats.