A forecast gives a general description of the expected weather in the forecast period plus outlook including warnings and other specific information.
MetEye - for marine and boating
Transcript
[Visual: Aerial view of boats moored in an estuary, cutting to a small boat in choppy water near a breakwater.]
Narrator: Out on the water, the weather can be very different from one location to another, and conditions can change quickly throughout the day.
[Visual: Aerial view of a small boat and it’s wake against the dark ocean.]
Narrator: Checking the forecast before your next boating or fishing trip will help you make safer decisions.
[Visual: A person moving through the Bureau’s MetEye website following the directions of the narrator.]
Narrator: The Bureau’s MetEye tool provides detailed information on forecast winds, waves and weather conditions for waters out to 60 nautical miles from the coast, as well as for some popular inland lakes. To access MetEye, go to the Bureau’s marine homepage at bom.gov.au/marine, choose your state or territory and click the MetEye link.
The first map you’ll see is a forecast for wind speed and direction.
Click and drag the map to your boating area or use the text box above the map to type in the latitude and longitude of your favourite fishing spot.
Once you select your location, a seven-day forecasts pops up. You can also save your location for future visits.
[Visual: Moored boats in a choppy sea and a boat moving in swell.]
Narrator: For boaters, keeping an eye on the wind and wave forecasts is essential.
[Visual: Moving through the Bureau’s MetEye website following the directions of the narrator.]
Narrator: When checking the wind forecast, the arrows on the map indicate the direction of the wind, and the colours indicate the average strength of the wind over a 10-minute period.
You should be prepared for wind gusts 40% stronger than the average wind speed.
To see the wave forecasts, use the button to the left of the map to change the view.
When checking the wave forecasts, check the total wave height and the direction of the swell.
You should be prepared to experience waves up to twice the total wave height, three to four times a day.
MetEye forecasts are available in three-hourly blocks.
To animate the forecast, press the play button above the map. Watch for differences between inshore and offshore weather and waves and see how the weather might change while you're out on the water.
MetEye forecasts are fine-tuned by Bureau forecasters to best represent expected weather.
MetEye also shows real-time weather observations. You can find the current wind speed and sea surface temperatures from around Australia and view the rain radar in select locations.
To see current weather warnings, click the yellow banner above the map.
Here you'll see marine wind warnings if dangerous winds are expected in marine areas.
If you want to know more, FAQ and help sections are available.
[Visual: Boats at a marina.]
[Text on screen: bom.gov.au/marine]
To see how the Bureau can help you plan your next boating trip, visit bom.gov.au/marine
[Text on screen: Australian Government Bureau of Meteorology logo]
End of Transcript
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).
- 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.
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.
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.
Weather definitions
DTMI has developed the following weather definitions to ensure a consistent approach in the use of terminology and to improve awareness and understanding by the boating community of common terms used throughout the DTMI website.
These definitions are based on the Bureau of Meteorology (BOM) definitions.
Wave height
BOM explanation: Wave heights describe the average height of the highest third of the waves. It is measured by the height difference between the wave crest and the preceding wave trough.
Seas (also called sea waves, wind waves)
BOM explanation: Waves generated by the local prevailing wind. Their height depends on the length of time the wind has been blowing, the fetch (the distance the wind has blown over the water), and the water depth.
Maritime explanation: sea waves are generated by the wind currently blowing (the wind you can feel) and are usually short and choppy. They have shorter wavelengths and periods than swell waves and are generally steeper.
Swell
BOM explanation: The regular, longer period waves generated by distant weather systems.
Maritime explanation: Waves generated from distant storms. They may travel over thousands of kilometres. Swell waves are long and smooth and are generally characterised by a wave period greater than 8 seconds. The wave period is the time between consecutive wave crests. Unless there is cyclone activity in the North, the same swell patterns are observed to varying heights and at different times along the coast.
Reference and explanation of the information is available on the Wave data (real time) page.
Deckee definition: The ‘Swell’ measurement on Deckee is not the combined height of the sea and the swell that mariners experience on open water, but ONLY the swell.
Total wave height (also called combined sea and swell, significant wave height, sea state, total wave)
BOM explanation: The combined height of the sea and the swell that mariners experience on open water.
Maritime explanation: The wave climate at any location can be, and usually is, a combination of sea and swell and is referred to as the total wave.
Significant wave height
BOM explanation: Is the statistical basis for all wave heights presented in text forecasts and map displays. The average height of the highest one-third of the waves experienced over time.
Maritime explanation: The average of the highest one-third (33%) of the waves (measured from trough to crest) that occur in a given period. The significant wave height is the average of the highest waves; however, many individual waves will probably be much higher.
Maximum waves (also known locally as king, freak, or rogue waves)
BOM explanation: 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 total wave height.
Maritime explanation: Maximum wave height (Hmax) can be up to twice the size of the Significant wave height (Hs).
Wave period and swell period
BOM explanation: The average time between crests (or troughs) of waves. The larger the time difference, the greater the amount of energy associated with the waves or swells
Wind
BOM explanation: Wind speed over the water is given in knots. When wind is mentioned in forecasts it refers to the average wind over a 10-minute period at a height of 10 m.
Gusts are increases in wind speed lasting for just a few seconds. They typically range 30-40% greater than the average wind speed.
Squalls are a sudden large increase in wind speed (usually accompanied by a change in wind direction) that lasts several minutes and then suddenly dies.