Understanding weather forecasts
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What are the parts of a coastal or marine weather forecast?
A forecast is a description covering the average conditions expected over the next 24 hours over open water. It may be broken down to include information about the following:
Direction such as north-west, is the direction the wind is expected to come from.
Speed is given in knots, and 1 knot is approximately 2km/hr. This is an average speed, so always expect that gusts may be 50% higher. Also allow for funnelling between headlands, causing the wind speed to double.
Sea is a description of the waves formed by the local wind.
A swell comes from either a distant disturbance, such as a cyclone or depression, or develops from wind waves that have been blowing from the same direction for a length of time. Swells increase in height and get steeper when they reach shallow water.
The measures used for swells are:
- low – under 2.0 metres
- moderate – 2 –4 metres
- heavy – over 4 metres
The average visibility in New Zealand is about 15 nautical miles.
This information is typically given when visibility is expected to be less than 6 miles (10km).
The visibility distance are:
- fog – less than 1.0 nautical mile
- poor – 1–3 nautical miles
- fair – 3–6 nautical miles
- good – over 6 nautical miles
All marine forecasts are for up to 48 hours, with the outlook for a further 3 days.
This refers to the size of of significant waves that are generated by the wind in the area.
The approximate wind wave height measures used are:
- calm – up to 0.1 metre
- smooth – up to 0.5 metre
- slight – up to 1.0 metre
- moderate – up to 2.0 metres
- rough – up to 3.0 metres
- very rough – up to 4.5 metres
- high – up to 6.5 metres
- very high – up to 8.5 metres
- phenomenal – up to 11.0 metres (or more)
Most boating areas are covered by the recreational marine forecast.
A situation is a description of the position and movements of highs, lows and frontal systems expected to affect the New Zealand coast within the next 36 hours. It also names those areas affected by warnings.
These are issued for gales, storms or squalls anywhere on the New Zealand coast. A strong wind advisory is issued in recreational areas if the wind is expected to be over 25 knots (about 40km/hr).
If the winds are associated with a cyclone from the tropics this will be mentioned in the warning, but tropical cyclone warnings are not issued in New Zealand.
Winds can be reported as:
- strong – the wind is expected to exceed 25 knots as a steady wind or 33 knots in gusts
- gale – expect 33 to 46 knots as a steady wind – gusts can be 50 percent higher
- storm – 47 knots or more as a steady wind gusts can be 50 percent higher
Wind and tide
When the wind is opposing the tide, expect a much rougher sea.
When the wind is with the tide, expect a calmer sea.