How Right Tide works

Where the data comes from

Waves offshore

NOAA's GFS-Wave model (WAVEWATCH III), read from NOAA's NOMADS service every six hours. For each break Right Tide reads one point in open water, typically 6 to 20 miles offshore, as the local wind sea and up to three swells, each with its own height, period and direction: hourly for five days, then every three hours to seven.

The sea floor

A depth profile drawn straight out from each break: from NOAA's high-resolution coastal elevation models (CUDEM, detail of about 10 feet) in Hawaii and on the Atlantic coast, and from the Coastal Data Information Program's published beach profiles in Southern California. Where a line runs past the coastal survey, NOAA's global relief model (ETOPO 2022) carries it into deep water. Outside Hawaii, most profiles are carried up the sand above the waterline by USGS lidar (3DEP); where none is, a profile continues on its own measured slope when the tide would otherwise cover it, and the forecast records that it did.

Tides

Official predictions from a NOAA tide station (CO-OPS) near the break, applied to every depth each hour. Where a station publishes only its highs and lows, or nothing at all, the curve is derived from them or from a reference station, and the forecast records how.

Wind and weather

The National Weather Service forecast for the beach itself, not the open ocean. Where it has a gap, the wave model's own offshore wind stands in, and the forecast records which one it used.

Buoys

A nearby NOAA buoy (NDBC) supplies the water temperature, which is measured at the buoy and not at the break, and its recent readings are kept with each forecast.

The NWPS wave loop

NOAA's Nearshore Wave Prediction System, shown beside the forecast as a picture for context. It is NOAA's model, not Right Tide's forecast, and no Right Tide number is read from it.

Maps

Coastlines from NOAA CUDEM and roads from USGS, or Natural Earth and the U.S. Census where CUDEM does not cover the whole coast.

How a forecast is made

1. Read the swells offshore. Each wind sea and swell the model states at the break's offshore point, every hour.
2. Time each swell to the break. A swell takes time to cross the last stretch of ocean, and longer periods travel faster. Each one is moved to the hour it actually reaches the break, from its period and the measured depths. The local wind sea is raised by the wind over the water in front of the break, so it is read at its own hour.
3. Account for land in the way. Headlands and islands between the offshore point and the break block part of a swell. Each break's blocked directions were measured from the sea floor and the coastline, and only the share that clears the land arrives. A swell running nearly along the shore reaches the break only in part, and one from behind the coastline not at all.
4. Walk the sea in. Every swell is walked together across the break's own depth profile. Each slows and steepens as the water shallows, bends toward the shore, and loses a little energy to the bottom. The hour's tide raises or lowers every depth.
5. Find where it breaks. A sea starts to break when its significant height reaches about 0.42 of the water depth: a single wave breaks at about three quarters of the depth, and the biggest wave in a sea is about 1.8 times its significant height. A wave that gets too steep breaks sooner.
6. Say it the local way. The breaking height becomes a Surf label in each coast's own convention, because a Hawaii 3-5 ft and an East Coast 3-5 ft are not measured the same way.
7. Rate it. The Surf Rating runs Do Something Else, Doable, OK, Decent, Could be Fun, Fun, Call in Sick and Bombing. It starts from the breaking height, a long swell period can lift it, and the wind caps it: the stronger the wind and the more onshore it blows, the lower the cap. Each day's best hour is its best-rated hour in daylight.

What the words mean

flat

The data is there and the break faces the swell; the ocean is just small.

shadowed

There is swell in the water, but all of it comes from a direction this break does not face.

n/a

The model said nothing usable for that hour. It is a gap in the data, not a calm sea.

stale

The forecast has missed its regular update. It is still shown, with its age.

a dashed day

The model run behind this forecast has not published that day yet. It is still coming, not missing.

Limits

• It has not been checked against the surf. No public source measures the waves at these breaks, so no forecast here has yet been compared with what actually broke. The physics is published science; how close it comes at each break is not yet known.
• Some of its numbers are not tuned. How much energy the sea floor takes from a swell, and the depth at which a sea starts to break, come from published research rather than from measurements at these breaks. The Surf Rating's height bands were set from the forecasts themselves.
• It sees one line. Each break is forecast along a single depth profile, as though the depth contours ran parallel to the shore. What a jetty, an inlet, a canyon or the shape of a reef does to a swell from the side is not reproduced, so jetty and inlet breaks can differ most.
• It has no currents. Rip currents, the tide running through an inlet, and what either does to the waves are not part of the forecast.
• The sea floor is a survey. Each profile is the bottom as it was measured. Sandbars move with every storm, so a beach break can be different today.
• A rating is not a safety call. Call in Sick and Bombing say how good the surf is, not that it is safe to go out. Check the National Weather Service for surf and rip current warnings, and know your own limits.

What Right Tide will not do

• Fill a gap with a guess. A missing input is left blank and recorded with the forecast.
• Hide the model. Where a forecaster adjusts a height, the model's own number is shown beside it.
• Pass off NOAA's pictures as its own forecast.

Heights and depths are in feet, distances in miles, wind in miles per hour and water temperature in degrees Fahrenheit. Directions are degrees true, the bearing the swell or wind comes from.

Credits

Wave, tide, buoy, weather and sea-floor data from NOAA, the National Weather Service and the U.S. Geological Survey are public domain. Map geometry also draws on Natural Earth and the U.S. Census TIGER/Line files.

Sea-floor data for Southern California breaks were furnished by the Coastal Data Information Program (CDIP), Integrative Oceanography Division, operated by the Scripps Institution of Oceanography, under the sponsorship of the U.S. Army Corps of Engineers and the California Department of Parks and Recreation, and are used unaltered to derive each break's profile: https://cdip.ucsd.edu/.