Sea-ice forecast skill starts with a baseline

Forecast accuracy does not establish added information unless it is compared with an explicit alternative. This methods note draws on primary ice-edge verification and subseasonal prediction studies to define a fair comparison among a model, climatology, and persistence-based forecasts. A hypothetical Brier-score calculation shows how a model can be skilful against one baseline and unskilful against another. A spatial example explains why correct total extent can coexist with a misplaced ice edge. The note examines issue-time information boundaries, changing climatology, calibration, dependence, and observation uncertainty without evaluating an operational system. Its practical conclusion is a conditional skill statement naming the target, lead, dates, region, reference observations, and baseline, rather than treating a high percentage of correct outcomes as sufficient evidence of usefulness.