Weather  

• Current forecast
• Sources of weather information
• The weather map – synoptic charts
• Forecast content
• Wave information 
• Sea breezes
• Observing the weather
• Coping with weather
• Factors affecting visibility
• Know the conditions

When you go boating there is not much pleasure in doing it in bad weather. You need to be able to cope with unexpected changes in the weather, but even more importantly you need to know the range of conditions you and your vessel are happy with, and the conditions that will keep you at home. Learn how to read the weather map. By following it daily you get the feel for trends in the weather and can use it for planning ahead.

Current forecast

Before actually starting the trip, get the most up to date forecast available.

Read the forecast and warning information pamphlet Marine Weather Services published by the Bureau of Meteorology.

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Sources of weather information

A full range of weather information is available on the Bureau of Meteorology’s website:http://www.bom.gov.au/

The Bureau of Meteorology sends coastal waters forecasts and warnings to the ABC and commercial networks (both city and country).  The stations usually announce the time the Bureau issued the forecast: this is important – older forecasts are less valuable.

Weather and navigation warnings covering the Perth metropolitan waters (within 20 nautical miles) are broadcast on VHF Channels 16 and 67 at 0718 and 1918 hours western standard time by the WA Water Police.

Volunteer sea search and rescue groups operate within normal recreational boating hours. They provide weather information on VHF channels 16 and 67 and/or on 27.88 MHz.  The Bureau of Meteorology provides a limited VHF weather service for some parts of regional WA. For details visit: www.bom.gov.au/marine/images/wa.pdf. 

The Bureau of Meteorology has recorded services via Weathercall for coastal waters forecasts and warnings.

Services include:

The Bureau of Meteorology Weather by Fax provides forecasts, observations and warnings including weather charts (updated three hourly), satellite photos (updated hourly), weather radar reports, the latest warnings, routine coastal waters forecasts and the latest actual reports. Set your fax machine in poll receive mode and dial 1902 935 200 for the main directory.

The Bureau also produces useful pamphlets and booklets including Marine Weather Services, Tropical Cyclones and Wind, Waves and Weather for Perth Waters. These are available online at www.bom.gov.au/info/publications.shtml.

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The weather map – synoptic charts

You normally see the weather map (also called a synoptic chart) on television or in the newspaper. It will usually be older information than you can get from other forecast sources, but it gives a good picture of what is happening and is likely to happen.

Winds

Wind is caused by the movement of air from high pressure to low pressure systems, but it does not flow directly from one to the other. It blows more nearly along the pressure contour lines (called isobars) you find on weather maps. In the southern hemisphere this is in a clockwise direction around low pressure systems and anti clockwise around highs.

The bigger the difference in pressure of the systems, the stronger the wind. The number of isobars, and their closeness to each other, indicate the pressure differences. When estimating the progress of the pressure systems, you can reasonably assume that a low will move the width of Australia in two or three days.

Cold fronts

The line with spikes on it is a cold front – commonly associated with winter lows. Ahead of the front you can expect squalls followed by rain. The wind typically changes direction at the front (shown by the bend in the isobars) and strengthens.

Forecast content

A forecast gives a general description of the expected weather in the forecast period plus outlook including warnings and other specific information.

Strong wind warnings

Small craft warnings of strong winds are issued whenever mean wind speeds exceeding 25 knots are expected.  For any mean wind speed forecast, gusts can be 40 per cent higher, so the very smallest of strong winds will produce gusts of 35 knots.

Severe weather

The northwest of Western Australia is one of the most cyclone prone coasts anywhere in the world. The cyclone season extends from the months of November through to April, but most of the coastal impacts occur between January and March. No vessel should be at sea within several hundred miles of a cyclone.

Wind strength and direction

You are interested in wind speed and direction mainly because of the waves the wind will build. Wave height depends on wind speed, length of time the wind blows (duration) and how far over the water it blows (fetch). Wind off the land will produce smaller waves inshore because of the limited fetch; they will also be steep and close together – a feature of short fetch. If you operate in the shelter of islands you will also get smaller waves than the forecast, which assumes no shelter.

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Wave height depends on the length of time a wind blows and how far over the water it travels; so easterlies and local sea breezes make smaller waves than steady winds off the ocean. Above 15 knots there will be many whitecaps and some spray. Swell waves are independent of existing wind. They are decaying waves usually produced by strong winds in distant blows. Though smooth and harmless looking, they travel very quickly and can create big breakers in shallowing water.

Sea breezes

Local winds such as sea breezes blow for only a few hours over waters close to shore, so the waves they cause are lower than the large scale winds. However, if the sea breeze blows from a similar direction to the existing wind, it will join forces and build bigger waves.

Sea breezes are a near shore event caused by the heating of land during the day. Their strength will be partly dependent on the existing (synoptic) wind. On the west coast sea breezes come from the south west, so they will be opposed and weakened by a synoptic north easterly. The further towards south the synoptic wind, the more it will assist and strengthen the sea breeze.

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Observing the weather

Forecasts tell you the expected weather – keeping your own eye on approaching weather can confirm the forecast or can tell you that something unexpected is about to happen. Or that local conditions look like being worse than predicted.

Keep your senses open to: 

• drops in temperature;
   
• a rising swell;
   
• solid cloud appearing;
   
• barometer falling; or
   
• wind changing.

In fact almost any change can mean deteriorating weather approaching.

Coping with weather

• Have an alternative trip planned for the day in case the weather is unfriendly. You may choose to operate inside the reefs or islands instead of outside.
   
• Use your radio to keep up-to-date on forecasts, the weather can change quickly.
   
• Trim your boat, by moving passengers or using power trim, to suit the direction of the sea. With a planing boat, generally you trim the bow down a little for a head sea, and trim it up for a following sea.
   
• A lowering of speed or a small change in course can make a big difference to the softness of ride.
   
• If conditions deteriorate, put on life jackets.

Factors affecting visibility

• Poor visibility can have a disturbing effect on your orientation, whether you are in open ocean or on an inlet. It also requires you to slow down and keep an even better lookout.
   
• Mist and fog, although not common, occurs, and the Bureau will include predictions of it in both marine and land forecasts.
   
• Heavy rain can seriously reduce visibility.
   
• In summer, one of the commoner influences on visibility can be smoke from bush fires drifting out to sea with easterly winds.

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Know the conditions

The weather could limit where you go, the load you can carry safely, whether you should take inexperienced people with you or if you should stay home. Before you set off, get the latest weather forecast. Knowing the likely conditions will help you decide where the safest and most sheltered spots are.

Volunteer sea rescue groups are based in most coastal population centres and operate within normal recreational boating hours. They all provide weather information on VHF channels 16 and 67, 27MHz channel 88.

For weather information direct from the Bureau of Meteorology phone:

WA Coastal Marine Warnings  1300 659 223
WA Marine Service 1900 926 150
Fax information  1902 935 200
Website: www.bom.gov.au/weather/wa

The Department for Planning and Infrastructure urges all water users to read the forecast and warning information pamphletMarine Weather Services published by the Bureau of Meteorology.

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