Several weather models, one route each

No forecast is right every time. One model sees the breeze filling in by the evening, another leaves you becalmed off the cape until morning, and a route computed on one alone hides that doubt from you.

Saily WR now reads four more models beside GFS: the European Centre's ECMWF, Météo-France's ARPEGE and AROME, and the German weather service's ICON, and a single computation gives a route for each model covering both the departure and the destination. Where the routes overlap, the forecast is solid; where they part, the weather is uncertain, and the choice is yours to weigh.

Not every model covers every passage. The global models give a route anywhere in the world, each with its own reach, from about four days ahead to more than two weeks. The regional models, such as AROME around France, join them over their own area only, finer and closer to the coast, but for a few days at most.

Take a summer's day from Marseille to Porquerolles. The routes part: a few keep closer to the coast, others head well offshore. Which to follow? Near the land and in the first days, a fine regional model sees the sea breeze and the coastal effects that a global model smooths away: on a coastal hop like this one, its route is the one to weigh first. On a longer passage, it falls silent after a few days, and it is the global models' agreement that counts.

The further you go, the fewer the routes. A regional model only gives a route when the departure and the destination both lie in its area, and each model sees only so far ahead: past ARPEGE's four days the routes grow fewer, and on an ocean crossing longer than eight days only GFS and ECMWF may still reach the arrival. Where several routes are left, their spread tells you how sure the forecast is; where only one is left, nothing does.