RRFS, NOAA's fine model, over American waters

Along an American coast, the wind of the afternoon is rarely the wind of the morning: the sea breeze fills in as the land warms, and dies again at dusk. Between the Hawaiian Islands, the trade wind is squeezed through the channels and falls away in the lee of the high ground. Off the mainland United States and around Hawaii, Saily WR had only global models, which see a coast as a smooth line and a channel as open sea.

RRFS, NOAA's new fine model, sees these winds at 3 km along the American coasts and at 2.5 km around the islands, and each of its forecasts starts from the showers and storms already on the weather radar. A passage whose departure and destination both lie within one of its areas gets an RRFS route beside the global models', hour by hour up to 84 hours, three and a half days ahead.

It covers three areas: the coasts of the contiguous United States, from the Gulf of Maine to the Gulf of Mexico and Puget Sound; Puerto Rico and the Virgin Islands, from Hispaniola as far as Saint-Barth; and the Hawaiian Islands, from Niihau to the Big Island. From Hispaniola to Saint-Barth its route also sits beside that of AROME Antilles: two fine models from two weather services, and where they agree the forecast stands on firmer ground. Further south and east, Antigua and Guadeloupe lie outside its area.

Round the north of the Big Island, from Kawaihae to Hilo, the routes part off the north-east coast: RRFS's stands well out to sea, while most of the global models' keep close to the shore. A global model sees what the island does to the trade wind only in broad strokes; RRFS sees it right up to the coast. When routes part like this close to land, the fine model's is the one to weigh first, and the gap between them tells how much the island changes the wind.

Its three and a half days suit a coastal passage or an island crossing; for a longer one, the global models see further. What comes in at its edges comes from GFS: a low arriving from offshore is only as good in RRFS as it was in GFS. And RRFS is no hurricane model: in a tropical storm, the National Hurricane Center's advisories come first. NOAA is due to put it into service on 3 November in place of its older NAM model; until then, Saily WR reads it on NOAA's parallel feed, which may run late or be interrupted.