Rainfall

Accurate forecasts of rainfall are essential for partner organisations such as the Environment Agency for flood warnings and the Highways Agency for road safety

Unlike temperature, rainfall may or may not happen. Rain can be highly variable, especially when falling as showers; one side of town may get rain while the other remains dry. So for the purpose of verification, forecasts of rain are expressed in terms of the “chance” (probability) of a place being wet or dry.

The Met Office uses a verification method that:

• gives a score (called the Brier Skill Score) that takes account of the probability applied to the forecast;
• takes account of how difficult or easy the forecast was by measuring it against a climatological reference.

A positive score indicates that the forecasts are, on average, better than climatology. A completely accurate forecast would have a score of 1.

High scores are achieved when:

• a high probability of rain is forecast and rain subsequently occurs
• a low probability of rain is forecast and it subsequently remains dry

Conversely low scores are achieved when:

• a high probability of rain is forecast and it subsequently remains dry
• a low probability of rain is forecast and rain subsequently occurs