Definition
High barometric pressure, typically above 1013 hPa, and persistent offshore winds can lower observed level below prediction, occasionally by more than a metre in shallow shelf seas such as the southern North Sea. Apply roughly 0.01 m per hectopascal above 1013 as a first approximation, then add a further allowance for wind set-down. National authorities issue negative surge warnings; deep-draught vessels and shoal-draught passages should treat these as operational limits.
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