ICON, the German weather service's model

Two forecasts that agree may only share the same mistake. When GFS and ECMWF both bring a front through on Tuesday, a skipper still wonders whether a model built another way would see it too. And among the Danish islands or in the Baltic, where sounds and headlands shape the wind mile by mile, the global grids see little more than open water.

ICON, the model of Germany's weather service, DWD, is built another way: its own grid of triangles, its own physics, its own picture of the weather at the start. Saily WR reads it as three models, each with its own route: the global ICON, ICON-EU over Europe at about 6.5 km, and ICON-D2 at about 2 km over Germany and its seas, fine enough to compute showers and thunderstorms itself.

The global ICON covers every sea, up to seven and a half days ahead from its midnight and midday runs, five from the others. ICON-EU reaches from Madeira to northern Norway and the Black Sea, up to five days ahead from its main runs, two from those in between, but stops short of the Azores and the Canaries. ICON-D2, two days ahead, covers the eastern Channel from Cherbourg, the North Sea up to the Firth of Forth and the southern Skagerrak, the Kattegat and the Danish straits, the southern Baltic as far as Gotland, and the northern Adriatic from Venice to Zadar and Ancona.

From Kiel to Copenhagen, a day and a night among the Danish islands, the wind turns with every sound. ICON-D2 draws its funnelling through the Belts and the sea breeze along the coasts, which the global models smooth away; its route is the one to weigh first between the islands. Beside it, the global ICON brings an opinion on the large picture that owes nothing to the American or the European model: when all of them agree, the forecast stands on firm ground.

ICON-D2 looks only two days ahead, and gives a route only when both ends lie within its area; past them, ICON-EU and the global ICON take over. The global model, read on a quarter-degree grid, sees no detail of the coast. And the route sails ICON's mean wind, not its gusts: under a shower, ICON-D2 computes the squall, but not exactly where or when it will strike.