About Marine Forecasts

Marine weather forecasts

Marine weather forecast services include:

• Local waters forecasts
• Coastal waters forecasts
• High seas forecasts

Local waters forecasts are for local waters such as bays, harbours and inland waters on which a large number of commercial and/or pleasure craft operate. Local waters forecasts are issued by Regional Forecasting Centres in each capital city generally twice daily and monitored continuously for changes which may occur. Updates may be issued at other times.

Coastal waters forecasts are for areas within 60 nautical miles of the coast (see map for coastal waters areas). Coastal waters forecasts are issued by Regional Forecasting Centres in each capital city generally twice daily and monitored continuously for changes which may occur. Updates may be issued at other times.

High seas forecasts are issued twice daily by the Regional Forecasting Centres in Perth, Darwin, Brisbane and Melbourne for the areas beyond the coastal waters surrounding Australia.

Forecasts of winds

Wind direction and speed are determined by the patterns of highs, lows and fronts seen on weather maps and by local effects such as seabreezes and thunderstorm downdrafts. Closely spaced isobars (lines of equal pressure on weather maps) are indicative of strong winds. That is, the higher (or tighter) the pressure gradient, the stronger the wind speed.

Stronger wind speeds are associated with tropical cyclones, deep lows and cold fronts. Sudden squalls are associated with thunderstorms, heavy showers or the passage of a cold front or low pressure trough and can happen in clear skies (e.g. the Southerly Buster in NSW). The very strongest winds are caused by tropical cyclones, deep midlatitude low pressure systems and tornadoes/water spouts.

The Bureau forecasts of wind speed and direction are average (or mean) values over a 10 minute period at a height of 10 metres. Wind speeds usually increase with height above the sea-surface. When there are expected variations along a coastal area a range may be given, for example 15 to 25 knots.

Forecasts of gusts are not included as routine, however statistically it is estimated that gusts typically exceed the average wind speed by about one third. For example, if the forecast (average) wind speed is 15 knots, and one third of 15 is 5, gusts of around 20 knots can be expected. Gusts are generally associated with showers, thunderstorms and fronts.

Figure 1 Wind trace from an anemograph. This shows a recording of instantaneous wind
speed. The dark blue trace illustrates the variability (gustiness) of wind speed. The higher
wind speeds are typically one-third above the average. Squalls are also evident. These may
have occurred as showers or thunderstorms passed over the recording site.

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Sea and swell forecasts

Because of the apparently chaotic situation of waves propagating in different directions and changing in character as they move, the Bureau of Meteorology uses statistical analysis when forecasting sea (wind) waves and swell wave conditions.

Forecasts of sea and swell in coastal waters forecasts are given in metres and describe the height, which is the average height of the highest one-third of the waves (see definitions and terminology).

Some waves will be higher and some lower than the forecast and observed height.

The Bureau of Meteorology does not forecast maximum wave heights in routine forecasts.

Statistically it is estimated that about one in every 2000 to 3000 waves (three to four times a day) will be approximately twice the height of the significant wave. Forecasts for high seas describe sea and swell using terms such as slight, moderate, rough etc. in place of wave heights in metres. For more details, click on the Bureau's Glossary of Terms, for a description of sea and swell parameters.

Figure 2 Significant and Maximum wave heights at Cape Sorell, west coast of Tasmania. The recording illustrates that maximum wave heights can be twice the significant wave height. It shows a maximum wave height of two and a half times the significant wave height (4am, 3 April 2004). This is sometimes referred to as a Rogue or King wave.

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