UK NWP Index Documentation

The UK NWP Index is a measure of the forecasting skill of limited area NWP (Numerical Weather Prediction) models over the UK.

It is based on UK-specific model forecasts of selected parameters currently out to 48 hours ahead for a selected set of positions verified by comparison with station observations and is based on 36 months of data. A score is calculated for each parameter included in the Index. The individual scores are then combined in a weighted average to form a single value.

The index is compiled from the following parameters

  • Near-surface (1.5 m) temperature.
  • Near-surface (10 m) wind speed and direction.
  • Precipitation yes/no (equal to or greater than 0.5, 1.0 and 4.0 mm over the preceding 6 hours).
  • Total cloud amount yes/no (equal to or greater than 2.5, 4.5 and 6.5 oktas)
  • Cloud base height given at least 2.5 oktas yes/no (equal to or less than 100, 500 and 1000 m above ground).
  • Near-surface visibility yes/no (equal to or less than 200, 1000 and 4000 m).

These are verified at quality controlled Met Office observing sites across the UK at the following forecast ranges: T+6, 12, 18, 24, 30, 36, 42, 48 hours.

All model forecast runs are verified. There are four model forecasts generated per day.

The well-established statistic, the Equitable Threat Score, is used as the basis for the precipitation, total cloud amount, cloud base height and visibility components which calculates the skill of the forecast. It removes any skill that may exist by pure chance.

For other parameters, a root-mean-squared (rms) error skill score is calculated, where we measure the skill by comparing the rmse of the forecast compared to that of a persistence forecast. A persistence forecast is simply forecasting that a parameter will be the same as 24 hours prior (e.g. the temperature tomorrow at 12 UTC will be the same as today at 12 UTC).

Last updated: 12 May 2011