GFS, the forecast that reaches across an ocean

A passage of two or three weeks outlasts most forecasts. Leaving the Cape Verde Islands for the West Indies, the question is not only tomorrow's wind but where the trade wind will blow in ten days' time, and whether a trough will slow the boat halfway across.

The route Saily WR draws rests on GFS, the global model of the American weather service, NOAA. Four times a day it forecasts the wind over every sea in the world, hour by hour for five days, then every three hours up to sixteen days ahead. Being global, it gives a route on any passage, from a day sail across the bay to an ocean crossing.

Saily WR reads it on a grid of a quarter of a degree, about 25 km, fine enough for what decides a long passage: the Azores high, the trade winds and their surges, the lows and fronts crossing the ocean. Free and open for decades, it is the forecast many offshore sailors download first, often over a satellite link.

Say you leave Mindelo for Barbados, two weeks at sea. The first days decide everything: dive south for a steadier trade wind, or hold the direct line? Computed on the morning you leave, the GFS route shows whether the trade wind holds all the way or a trough weakens it in the second week, reason enough to take a more southerly line from the start, where the wind is forecast to hold.

Beyond about ten days, a GFS route says more about the general pattern than about the track: read its last days as a sketch, not a plan. Its grid does not see the sea breeze, the wind funnelled between islands or the calm in the lee of a high island, and it turns a squall into a steady wind. In a tropical cyclone its track is useful, its peak wind usually too low: the hurricane centres' advisories come first.