What is Accuracy?

Accuracy is a scalable measure of how closely the forecast precipitation amount matches the observed precipitation amount.

It is calculated by dividing the smaller value of the forecast and observed amounts by the larger value, then multiplying by 100%.

Accuracy = (Minimum of Forecast and Observed) ÷ (Maximum of Forecast and Observed) × 100%

The result is bounded between 0% and 100%, making it an easy-to-understand metric that can be compared across precipitation amounts and forecast lead times.

Example: If the forecast QPF is 1.00 inch and observed precipitation is 0.75 inch:

Accuracy = 0.75 ÷ 1.00 × 100% = 75%

A value of 100% means the forecast and observed amounts are identical. Lower values indicate a larger difference between the forecast and observed amounts.