A guide to the Sydney Weekend Boating Forecast

Updated October 12, 2007

A point forecast

The Sydney Weekend Boating Forecast is a "point" forecast. That is, it gives forecast values (wind speed /direction and wind wave (sea) height, or swell) for specific locations and times, not a maximum or an average for an area. The locations have been chosen to be representative of the area in which they occur, but the forecast values at these locations may not encompass the complete range of conditions that might be expected at other locations or times. Especially in the enclosed or semi-enclosed waters of Broken Bay, Sydney Harbour and Botany Bay, local effects from surrounding terrain can influence wind speed and direction, and varying water depth and fetch (the distance the wind has blown over the water) can significantly influence wind wave height. See below for further discussion on wind, sea breezes and sea and swell .

The forecast is issued in two forms - a single table (example) and up to three map pages. The first map page (example) has an observation panel, in which observations at the time of issue are graphically displayed on a map of the forecast area, and three forecast panels, in which the forecasts are graphically displayed on a similar map. The second and third page, when issued, each contain up to four forecast panels. The pages on which particular forecast times occur are indicated in the table below for each issue.

Issue and forecast times

Point forecast locations

The Sydney Weekend Boating Forecast has 3 offshore forecast locations for wind speed/direction and sea, 2 offshore forecast locations for swell, and 3 inshore forecast locations for wind speed/direction. (See map.) These latter three each provide the wind value for a wind wave (sea) forecast for a nearby location in the enclosed or semi-enclosed waters of Broken Bay, Sydney Harbour and Botany Bay respectively. The details of the forecast locations are:

Offshore:

Inshore:

Wind

The wind is a continuous succession of gusts and lulls (quiet intervals) associated with equally rapid changes of direction over a range that may exceed 30°. Wind speed and direction also change with height - a phenomenon known as wind shear. Wind speed in forecasts and coastal observations is given in knots (kn) and refers to the average speed over a 10 minute period at a height of 10 metres above the surface. (Wind direction is also averaged over the 10 minute period.) Individual gusts may be up to 40% stronger than the average speed. Sudden squalls may also occur in association with thunderstorms, heavy showers or the passage of a cold front or a low pressure trough and can happen in clear skies (e.g. the Southerly Buster).

The Sydney Weekend Boating Forecast displays forecast wind speed values to the nearest 2.5 knots (2.5 is rounded to 2 and 7.5 is rounded up to 8). These values represent the 'midpoint' of a 5 knot forecast range, e.g. a value of 12 kn should be read as a forecast of 10-15 kn winds; a value of 20 kn should be read as a forecast of 17.5-22.5 kn winds. In the tabular form of the forecast wind direction is given to 16 compass points, on the map forecasts the direction is given by the direction of the arrow.

Coastal waters wind warnings

Various warnings are issued when winds are forecast or observed to exceed certain values listed below. Warnings are issued up to 24 hours ahead, but in some circumstances warning lead time might be greatly reduced so you are advised to regularly monitor coastal reports and warnings.

View any current coastal waters warnings.

Sea breezes

In the warmer months, heating of the land during the day can lead to an air circulation, the lower part of which is known as a sea breeze (see diagram below). The strongest effect is within a few nautical miles of the coast but the circulation can extend in a weaker effect up to about 30 nautical miles out to sea.

Whether a sea breeze will occur, how strong it will be and how far it will extend out to sea and inland depends on the temperature difference between the land and sea and the strength and direction of the prevailing wind. The same factors influence the time of onset of the sea breeze, which can vary from mid morning to late afternoon. The sea breeze dies off as the temperature difference between land and sea reduces and can be replaced by an equivalent, though usually weaker, land breeze at night where the air flows from land to sea.

In the absence of a prevailing wind, sea breezes begin perpendicular to the coast but over several hours under the influence of Coriolis force they gradually turn anticlockwise (back) till, if they last long enough, they become parallel to the coast. In the Sydney area this means that sea breezes (with no prevailing wind) start as easterlies but gradually turn more northerly during the afternoon. The sea breeze combines with the prevailing wind to give the overall wind experienced at a given location. If they are in opposing directions the prevailing wind can weaken or delay the onset of the sea breeze or prevent it altogether.

For further information on sea breezes see:
Sea Breezes on the NSW Coast
More on Sea Breezes

Sea and swell

Wave heights are usually given as 'significant wave heights' which represent the average of the highest one-third of the waves. The sea and swell heights in Bureau observations and forecasts refer to 'significant wave heights.' Some waves will be higher and some lower than the significant wave height. The probable maximum wave height can be up to twice the significant wave height.

The total wave height (as measured by a Waverider Buoy) is a combination of wind wave (sea) and swell. The relationship between sea, swell and total wave height is given by the following formula (where all wave heights are significant wave heights):

Total Wave Height = [(Wind Wave Height)² + (Swell Wave Height)²]^1/2

or refer to the total wave height table.

The Sydney Weekend Boating Forecast gives forecast offshore wind wave (sea) and swell heights to the nearest 0.5 metre, and swell direction to 8 compass points. (Wind wave direction is, of course, in the direction of the wind.) On enclosed bodies of water wind wave (sea) heights are constrained by both depth and fetch limitations. The Sydney Weekend Boating Forecast uses words to describe the wave height on the enclosed or the semi-enclosed waters of Broken Bay, Sydney Harbour and Botany Bay where the expected wave height is less than 1 metre, and reverts to numbers (rounded to the nearest 0.5 metre) when the wave height is 1 metre or more (this applies mainly to the Broken Bay inshore wind wave forecast in the directions where the fetch is unlimited). The terms are defined as follows (note these terms are defined differently when referring to open waters):

Observations

Observations from the Kurnell and West Channel Beacon Automatic Weather Stations (AWS) from the hour before the time of issue are included in the first panel of the first graphic page of each issue. An observation of total wave height (the significant wave height value) from the Sydney Waverider Buoy, located approximately 12 kilometres (6.5 nm) off Curl Curl Beach, is also included in the first panel when available. The Sydney Waverider Buoy records both the significant wave height and maximum wave height values of the total wave height as well as the direction and period of the waves. It is operated by the Department of Commerce's Manly Hydraulics Laboratory for the Department of  Infrastructure, Planning and Natural Resources.

Other observations are available at:

• Coastal Observations
• NSW Waverider Buoy Data - Latest Offshore Wave Heights (Manly Hydraulics Laboratory)
• NSW Waverider Buoy Data - 4 Day Time Series (Manly Hydraulics Laboratory)
• Sydney Offshore Sea Surface Temperature - recorded by the Sydney Waverider Buoy (Manly Hydraulics Laboratory)

Other useful links

• A guide to Australia's marine forecasts and warnings
• Tide Predictions

Safety hints

1. Know the local factors that influence sea conditions and know where to reach shelter quickly.
2. Learn how to read the weather map (pamphlet available).
3. Be aware that the weather map in the morning newspaper was drawn the day before.
4. Always check the latest forecast and warnings before going to sea and know what conditions exceed your safety limits.
5. Beware of rapidly darkening and lowering cloud - squalls may be imminent.
6. When at sea, listen to the weather reports on public or marine radio provided by the Bureau, Telstra or your State/Territory marine safety agency.
7. Be flexible - change your plans if necessary.