Look at the wind direction (where it blows from) and strength in knots, the gusts, when it is expected to change, and the sea and swell heights. Check the forecast covers the whole time you will be out, and plan for the worst of it.
A marine forecast is written for people on the water, and it covers the things that matter to a boat. Read each part:
- Wind direction is always the direction the wind blows from. A south-westerly blows from the south-west towards the north-east.
- Wind strength is given in knots, often as a range such as 15 to 20 knots.
- Gusts are the short bursts above the average. "15 knots gusting 25" should be treated much like a steady 25.
- Changes: when the wind is expected to rise, drop or change direction. A wind that veers shifts clockwise (for example from south to west); one that backs shifts anticlockwise.
- Sea is the height of the waves made by the local wind, and swell is longer waves from distant weather. Both are given in metres.
- Warnings: a gale warning means winds of 34 knots or more are expected.
In New Zealand, MetService issues coastal marine forecasts for each stretch of coast, and recreational forecasts for the busiest boating waters, such as Auckland and the Hauraki Gulf. Maritime Radio broadcasts the forecasts on VHF at set times. Wind apps and models are useful for detail, but read the official marine forecast and its warnings too.
Good habits:
- Check the forecast for the whole time you will be out, not just the morning.
- Think about the direction as well as the strength: wind blowing against the tide, or onto an exposed shore, makes rough water.
- Expect the local wind near land to be gustier, and in summer allow for an afternoon sea breeze.
- Check again during the day if you are out for long, and watch the sky and the water.
If the forecast is near the limit of what you, your crew and your boat are comfortable with, put a reef in before you leave, or choose another day.
More in the book: Chapter 10, When the wind gets up