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Scoring for Ireland: How good are our rainfall forecasts?
10 November 2014

Scoring for Ireland: How good are our rainfall forecasts?

Will it rain? When? How much? It is not always easy to provide definitive answers to these questions as rainfall is often a 'noisy' weather element. Weather fronts, for example, can often deliver rain in well-defined bands, the passage of which over the country can be accurately forecast, but in showery conditions it can be very much a hit-and-miss affair with regard to timing, duration and amount. Even when the occurrence of rain is well forecast the influence of the local terrain can markedly enhance or reduce the amount falling at a location depending on the windflow pattern.

In spite of the difficulties, it is important to have some metric to evaluate how well the forecast system is performing; not only does it provide feedback to the scientists who develop and improve the weather models but it also provides assurance to the public that Met Éireann is delivering a high-quality service. Devising a suitable scoring system is not trivial; it must reflect the true skill of the forecast compared to  unskilled approaches based on simply guessing the weather or taking today's weather as a forecast for tomorrow (persistance forecast). The scoring system must address both the occurrence and intensity of the rainfall (i.e. light, moderate or heavy) with the latter categories depending on the local climate.

The score that is used by Met Éireann is called SEEPS (Stable Equitable Error in Probability Space). The composite SEEPS score for Ireland for 1-5 day forecasts are shown in the accompanying graph, together with a 'persistance' forecast. Lower scores are better i.e. more skillful. Note that while there was a significant improvement in 2013, the 'persistance' forecast also improved – a reflection on the natural variability of the weather.

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