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Local conditions: where the forecast stops being useful
Between the weather map and your entry point there's a filter that changes everything
The forecast said fifteen knots. In the bay there were eight and the water was flat; two hundred meters further on, around the point, there were twenty-five and whitecaps to the horizon. Neither place contradicts the forecast: both of them are the forecast, filtered by geography.
In this article you'll see how relief and coastline modify wind and swell at a scale of kilometers, what sea breeze and land breeze are and why they explain the morning calm, how rain and tide decide a site's visibility, and how local knowledge is built that no model contains.
It's the last stretch between the information available and the reality at the dive point. A diver who understands this filter chooses the site well on difficult days and recognizes when a favorable forecast isn't going to hold where they are.
What the model doesn't see
Forecasts work on a grid of kilometers; diving happens in tens of meters.
The first effect is acceleration. When wind is forced through a narrowing (a channel between islands, the mouth of a bay, the passage between two headlands) its speed increases in that throat. It's the same principle by which air accelerates through a funnel, and it explains why a specific site systematically has more wind than everything around it.
The second is the opposite: wind shadow. Behind a cliff or a high island a calm zone forms that can extend hundreds of meters offshore. It's the shelter that makes diving possible on an otherwise impossible day, and also the classic trap: that calm ends abruptly at the edge of the shadow, and an involuntary drift can carry you out of it before you notice you're out.
The third is swell refraction. On reaching shallow water, waves turn and adapt to the shape of the seabed, so the swell wraps around points and concentrates its energy on the headlands, while bays receive an attenuated version. That's why headlands break more than the forecast suggests and coves break less, with the same wave height predicted for the whole area.
The fourth is the local tidal current. In channels and passes, the tide accelerates the water disproportionately compared to the open ocean, with its own timing and direction specific to that place, as you saw in the articles on tides and on currents and drift. That's not in the weather bulletin, and it's often what really decides the diving window.
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