HARMONIE DINI, 2 km from Iceland to the Adriatic

Sail north of the Channel and the fine models thin out. Among Norway's fjords, around Shetland and the Faroes, off Iceland or across the Gulf of Finland, every headland and sound shapes the wind, yet a sailor there had only models of 6 km or more, which draw a fjord as a smudge and a sound as open water.

HARMONIE DINI fills that gap. The weather services of Denmark, Iceland, Ireland and the Netherlands run it together on a shared supercomputer in Iceland, on a 2 km grid that computes showers and thunderstorms rather than guessing them. Saily WR reads eight of its runs a day, hour by hour up to 60 hours, two and a half days ahead.

Its area opens like a fan, from East Greenland and Iceland to Finland, and south over the Bay of Biscay, the Channel, the North Sea, the Gulf of Lion and the Adriatic. Where AROME and ICON-D2 already reach, it is one more fine opinion to weigh against theirs; further north, around Norway, Shetland, the Faroes, Iceland and the Gulf of Finland, it is the only one.

Say you cross from Helsinki to Tallinn, a day across a gulf barely forty miles wide, with the Finnish archipelago behind you and the Estonian islands ahead. HARMONIE DINI is the only 2 km model there, and it sees how both coasts bend the wind. If its route parts from the global models', tacking down one side of the gulf while theirs hold a straighter line, it is the one to weigh first: the gap tells how much the land changes the wind.

Its two and a half days suit coastal sailing and short crossings, not a long passage: beyond them, the global models take over. The large weather systems come into it from ECMWF at its edges, so a low that ECMWF misplaces enters it misplaced too: for an independent view, look at GFS or ICON. And its area is checked only at the departure and the destination: a route between two points inside the fan may stray into a corner beyond it, off Portugal or south of Sardinia, where it finds no wind and is not to be trusted.