Effect of Weather on the River Level

itchenstorm1-200x300As well as the tide, weather conditions over the English Channel and the local weather in Southampton influence the water level in the tidal River Itchen.  Both the wind and the air pressure can have a significant effect.

The greatest effect from the local weather comes from variations in atmospheric pressure. A change in pressure by 1 millibar can cause a 1cm change in water level, with the sea level being lower under high pressure and higher if the pressure is low. Due to pressure effects alone, a deep depression over Southampton can cause the water level to rise by around half a metre above what is predicted in the Tide Tables.

The effect of the local wind on sea level varies from place to place. For Southampton very strong offshore winds from the north or east can decrease the height of water by up to half a metre. However onshore winds from the south or west do not increase the water level appreciably.  Strong winds can alter the behaviour of the tides near low water and also suppress the “second high water” (see “Tidal Notes and Cautions” in the ABP Tide Tables) but the local wind does not affect the maximum height of the tide. Although, of course if water levels are high, strong winds can produce waves which might then overtop riverside walls.

The increase in water levels produced by the combined effect of weather conditions in the Channel and the local air pressure is known as a storm surge.  If the storm surge coincides with high tidal levels in the River Itchen, local flooding is a possibility.

Sources of Weather Information

march10thmap-150x150The key  to predicting a possible storm surge is a surface pressure forecast. Unfortunately, not all weather forecasts include air pressure predictions.  If you are happy to interpret a weather chart the Met Office surface pressure charts are available for up to 6 days ahead. Otherwise forecasts for Southampton (based on US Weather forecasting models) are available for the next 7 days, pressure is shown in the last column and is shaded deep blue if low pressure is expected. If you are viewing using a mobile device (smart phone or iPad) you may have to hold it sideways (landscape mode) to see the pressure column.

For shorter period forecasts, the BBC coastal waters forecast shows air pressure and winds over the next 24 hours. If you are trying to predict what will happen over the next hour or two the Met office rain radar shows how  rainfall areas are moving and the XCWeather web site shows the most recent wind observations over the country.