Accuracy

The Storm Forecast Accuracy map is updated usually every day following a forecast being issued. It uses a 50 x 50 mile (80 x 80km) grid. The map shows the accuracy of all Storm Forecasts issued since Saturday 1st May 2010.

The data is collected from the UK Met Office ATD (Arrival Time Difference) Network and uses the following basis:

• if lightning occurred and was forecast = high accuracy
• if lightning occurred and wasn't forecast = low accuracy
• if lightning was forecast but didn't occur = low accuracy
• if lightning wasn't forecast and didn't occur = high accuracy

Each time 'low accuracy' appears in one of the boxes on the grid, a "1" is added to the cumulative number value. Grid squares which are consistently accurate in our forecasts remain on "0".

The overall accuracy can then be determined by comparing the number value in each grid square to the total number of forecasts issued. The higher the number value in a grid square, the lower the accuracy. The number value in each grid square cannot, and will not, be higher than the total number of forecasts issued, for obvious reasons. The numbers are then converted using the colour scale into a plot of different colours, as displayed below.

The current map was last updated at 17:08 GMT Mon 08 Nov 2010.