Forecast wave height map

What the waves map shows

The height of the waves ahead off the coasts of the lower 48, Alaska, Hawaii and Puerto Rico, in feet: the average of the highest third of the waves, what an eye on the water judges. The lines run the way the waves travel, as fast as their energy crosses the sea. Played, a storm’s seas build and roll toward the coast.

How to read it

Under 4 ft

Smooth to slight seas.

4 to 8 ft

Moderate seas; small boats are tossed about.

8 to 13 ft

Rough seas, dangerous for small craft.

13 to 20 ft

Very rough seas.

Over 20 ft

High seas, as a strong storm or a hurricane raises; over 30 ft, very high.

What it is for

• Plan a day on the water, or a crossing, around the roughest hours.
• See a storm’s seas build and spread toward the coast.
• Judge the surf the waves will bring to the beaches.

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=fwave, the times of its frames.