Forecast, every 3 hours
Swell
For
The height of the main swell ahead, in feet: the long, even waves raised by storms far away, and the way they travel toward the coast.
030 ft
Sea map / Forecast, every 3 hours
Forecast wave period map
Over Citizen station EW3506, Illinois, where the map opens. The whole country
The seconds between the main waves ahead: short and choppy near a wind, long and powerful when a swell has crossed an ocean. How to read it.
The rings are the storms the radars detect, the larger ring a severe one; each line runs to where its storm will be in an hour if it keeps its present speed and heading. Rotation and hail are what the radars detect and the analysis measures, not warnings: the official watches and warnings in force are drawn on the map in their own colours and set out in full under Alerts.
What the wave period map shows
The time between the main waves ahead, in seconds. Under 8 seconds they are wind waves raised close by; over 12, swell from a storm far away, which carries more power and breaks harder on the shore. The lines run the way the waves travel.
How to read it
- Under 6 s
- Wind waves: short, steep and choppy.
- 6 to 10 s
- Wind waves and swell together.
- 10 to 14 s
- Swell from a distant storm.
- Over 14 s
- A long groundswell, carrying great power; it breaks hard on the shore.
What it is for
- Tell a local chop from a swell come from far away.
- Judge how hard the waves will break: the longer the period, the more power each carries.
- Read with the wave height: the same height at a longer period makes bigger surf.
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=fwper, the times of its frames.
In the Field Book
- Nor'easterFB-EVT-066
Over a state
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With it
Severe AlertsWeather by stateForecast accuracyInstrument atlas
Beside it
Forecast, every 3 hours
Waves
For
The height of the waves ahead off every coast, in feet, every 3 hours for seven days, with lines that run the way the waves travel.
030 ft