Tips for Forecasting high water levels

dsc02239psThe ducks may have wet feet but here are some tips which might keep yours dry!

(a) Every month or so

Check the Tide Table: Look for days during the next few weeks when the predicted tide height is more than 4.5m, ones near or above 5m are most worrying – they will be around the period of new or full moon and be around the middle of the day or midnight… view Southampton Tide Table

(b) One or Two Days before a possible large Tide:

Check the Weather Forecast: Is the air pressure forecast to be low around the time of high tide. The tide will be higher by 1cm for every 1mb below 1013mb, e.g. 970mb, tide will be 43cm higher than in the tide table… view Met Office Weather Charts or St Denys Weather Forecast 


Look at the Portsmouth Surge Forecast: Only available for a day or two in advance, the surge forecast shows how much the tide is expected to be different from the tide table value (including air pressure effects) – make sure you choose the prediction for the time of high tide (shown by a cross on the graph). The surge  for Southampton may be slightly higher than the Portsmouth Forecast… view Surge Forecast for Portsmouth

(c) On the day or evening before:

Prepare and put in place any property level protection measures as necessary. Check your phone and emails for Flood Alerts or Warnings from the Environment Agency.

Re-check the Portsmouth Surge Forecast.  Add the value shown at the time of high tide to the height of tide shown in the tide tables.

If the surge forecast is not available for any reason, check the Coastal Weather Forecast: What is the forecast pressure at around the time of high tide? Remember, the tide will be higher by 1cm for every 1mb below 1013mb and storm conditions over the English Channel have been known to double the surge.  So if the tide height in the tide tables is 4.5m and the forecast atmospheric pressure is 973mb, the expected surge is at least (1013 – 973) = 40cm giving tide height  = (4.5 + 0.4) = 4.9m .  Allowing for effects over the English Channel it could be as high as (4.5 + (0.4 x 2)) = 5.3m.  Note that if the surge forecast IS available, these calculations including the likely effects from the English Channel, have all been done for you… use the surge forecast!

If the Bournemouth tide gauge is working you can Look at water levels at Bournemouth at high water. Since the first high water at Bournemouth precedes that at Southampton by about 2.5 hours it can be used as a warning that the actual tide might be higher than predicted. Experience has shown that taking the excess over the predicted high tide level at Bournemouth and multiplying by a factor of around 1.3 gives a good estimate of the likely excess at Southampton.

DO NOT: rely on using the Tide Gauge at Dock Head as the tide is coming in and try to predict what will happen at high tide.  The difference between the measured and predicted heights will vary wildly due to variations in timing of the incoming tide.

DO: take notice of Environment Agency Alerts and Warnings