Forecast swell height map

What the swell map shows

The height of the main swell ahead, in feet: the long, even waves a storm raises far away, which keep their shape across an ocean. The lines run the way the swell travels, at its own speed. Played, a hurricane’s swell runs out ahead of the storm toward the coast.

How to read it

Under 3 ft

A small swell.

3 to 6 ft

A moderate swell; good surf on open beaches.

6 to 10 ft

A large swell; heavy surf and strong rip currents.

Over 10 ft

A very large swell; dangerous surf on open coasts.

What it is for

• See where the surf will come up, and when, days before it arrives.
• Follow a hurricane’s swell as it runs out ahead of the storm.
• Judge the rip currents at the beach: a long, building swell drives the strongest.

How it is made

The sea maps are drawn from the weather service’s wave model: the waves the wind raises over every ocean, worked out from the wind forecast, on fine grids along the coasts of the United States (a point about every 11 miles) and a coarser one over the open ocean (about every 17 miles). Each of its runs, four a day, is read whole, laid onto the same grid of latitude and longitude as the radar at 2.8 miles a cell, the finest grid first where they meet, and played forward from the hour it starts.

A frame every 3 hours to 3 days, then every 6 hours to 7 days, from the hour the forecast starts, played from now as a loop of 1 to 7 days. As data: /radar/now.json?show=fswell, the times of its frames.