Verification of marine forecasts using an objective area forecast verificationsystem

Many meteorological service providers issue forecasts that contain short summary statements. These forecasts attempt to capture all the expected conditions during a forecast period. The area forecast verification system has been developed to verify these forecasts when they are issued in pre-determined geographical areas. This system verifies forecasts by comparing them with gridded nowcast model analysis fields. Results are presented for the wind speed component of the shipping forecast and the inshore waters forecast. Both of these products are issued by the UK Met Office on behalf of the Maritime and Coastguard Agency. Like most summary forecasts these products contain hand written text. This text is carefully interpreted into a series of invariant sub-periods. These sub-periods are then compared to the nowcast model analysis fields. During each forecast sub-period all available hourly nowcast model analyses are accumulated to create an observation distribution within that sub-period. Verification is performed using a contingency table approach which is specifically designed to reflect the distribution formed by the observations within each sub-period of the forecast. Various performance measures are calculated including the Gerrity skill score - which is weighted according to the priorities of the customer. Shipping forecasts issued during the 12month period ending on 31August2011 had a proportion correct to within +/- 1 Beaufort Force of 0.886 and a Gerrity skill score of 0.823. Inshore waters forecasts issued during this period had a proportion correct to within +/- 1 Beaufort Force of 0.940 and a Gerrity skill score of 0.848. Analysis indicates that both forecasts tend to exaggerate the wind speeds and often neglect to include the lower Beaufort Force categories. (c) 2013 British Crown copyright, the Met Office. Published by John Wiley & Sons Ltd.