Under exceptional circumstances, either severe extra-tropical storms or tropical cyclones can produce changes in sea level that exceed the normal range of tide. Like Tsunamis these storm surges are also called Tidal Waves.
The most dangerous surges occur when a deep depression moving in from the Atlantic, travels slowly across the north of Scotland from west to east. It causes strong NW’ly or N’ly winds. Surges which occur at Neaps seldom cause abnormally high level but relatively small surges occurring at High Water of Springs can be very dangerous.
A very severe storm surge occurred on 31st Jan 1953. Predicted high waters exceeded by more than 3 metres on the Netherland coast resulting in disastrous flooding with considerable loss of life & property.
Negative Surges :
In a manner somewhat similar to storm surges the level of sea can can also be lower than the predicted level. Again the cause is meteorological. It makes a lot of difference to very large vessels which may be navigating with very small under keel clearance. Negative surges of over 0.6 m occur about 15 times a year in the southern North Sea, 3 or 4 times exceeding 1m.
Warning of onset of negative surges in the southern North Sea is given by a warning service to mariners.