Unusually high or low barometric pressure, or prolonged periods of strong winds can result in variations between actual sea level and the predicted heights.
Differences between predicted and actual times of high and low water are caused mainly by the wind.
Barometric pressure: Tide predictions are computed for a standard barometric pressure of 1013 hectopascals (hPa) or millibars. A difference from the average of 1 hPa can cause a difference in height of 1 centimetre. A low barometer will allow the sea level to rise and a high barometer will tend to depress it. This phenomenon is often described as the inverted barometer effect. The water level does not, however, adjust itself immediately to a change of pressure; it responds to the average change over a considerable area. Changes in sea level due to barometric pressure alone seldom exceed 30 centimetres but, as such circumstances are usually associated with adverse weather conditions, the actual change in sea level is often much greater.
Wind: The effect of the wind on sea level, and therefore on tidal heights and times, is variable and depends largely on the topography of the area. In general it can be said that the wind will raise the level of the sea in the direction towards which it is blowing. This effect is often called wind setup. A strong wind blowing onshore will pile up the water and cause the sea level to be higher than predicted, while winds blowing off the land will have the reverse effect.
Storm surges: The combination of wind setup and the inverted barometer effect associated with storms can create a pronounced increase in sea level. This is often called a storm surge. A long surface wave travelling with the storm depression can further exaggerate this sea level increase. A negative storm surge is the opposite effect, generally associated with high pressure systems and offshore winds, and can create unusually shallow water. This effect is of great importance to very large vessels which may be navigating with small under-keel clearances.