• The NOAA/NCEP implementation of the Wavewatch III wave model for deep water regions (depth > 300m). The Wavewatch III model is a wind-wave generation and propagation model. That is, the global wave forecasts are made based on global surface wind forecasts. [See the NOAA Wavewatch III model web site].
  
  
• The Coastal Data Information Program's implementation of a spectral refraction wave model for shallow water (10m < depth < 300m). This is a propagation-only model (no wind-wave generation). It models the effect of bathymetry (underwater topography) on waves as they travel from deep water towards the coast. [See the CDIP swell model web page]. This implementation is slightly different than the swell version. We also include the propagation (but not generation) of shorter period local seas based on input from the Wavewatch III model.
  

• Every 6 hours, CDIP receives detailed wave forecasts from NOAA for its deep water locations along the California coast.
  
  
• NOAA's WW3 forecasts are used to initialize the CDIP wave propagation model and make predictions of wave heights across the continental shelf to the coast (10m water depth).

• Offshore Wave Height Forecast Plot

  This is the plot displayed at the top of the CDIP Recent-Forecast page. These are forecasted significant wave heights from the NOAA global Wavewatch III model for the two deep water sites off California used in the coastal wave models:
  
  
  • Pt. Conception - (34N 121W, used to make coastal predictions in southern and central California).
    
    
  • San Francisco - (37N 123W, used to make coastal predictions from Monterey Bay to Pt. Arena).
  
  The plot is designed to provide a quick look at whether there is a big deep water wave event on the horizon that may impact the coastline. The storm threshold line of approximately 13 ft. is based on historical storms in southern California. Forecasts of offshore waves exceeding this threshold do not necessarily mean damaging coastal waves will occur (for example the waves can come from the northwest and southern California is sheltered by Pt. Conception). It is provided as a guideline for when you may want to look at more detailed plots of coastal forecasts.
  
  

• Wave Height Maps (San Diego to Pt. Arena)

  These are similar to CDIP real-time swell maps, but include short period local seas. Regions along the coastline are clipped from the larger modeling area and rotated so that 3 forecast days can be stacked on a single plot. The wave height scale on these plots is fixed between 0 and 27+ feet. As with the Southern California swell maps, the time lag for waves to propagate from offshore waters to the coast are not accounted for in these images.
  
  

• Coastal Wave Height Plots (Southern California Only)

  These are also similar to CDIP's real-time swell predictions of alongcoast wave height. They are a plot of the model results along the 10m depth contour. They are not breaking wave heights. Generally, the 10m depth contour is outside the surf zone (area of depth-induced wave breaking). However, when the wave height exceeds roughly 15 ft., 10m is the outer end of the surf zone and the plotted height will be larger than what would actually occur at 10m depth. Nevertheless, it does provide an idea of how much wave energy is reaching the coast relative to other locations. Higher wave energy generally leads to higher water levels and wave runup at the adjacent shore.
  
  

• Coastal Wave + Tide Plots (Southern California Only)

  These plots combine predicted tides and forecasted wave heights in 10m depth (described above) on a site by site basis in southern California. The combined height is defined as the "Potential Flooding Index". They provide a clear view of when forecasted storm waves are going to be coincident with high tides.

• Forecasting Local Seas

  The CDIP coastal wave propagation models do not include wind-wave generation by local winds between the Wavewatch III deep water site and the coast. Unlike the CDIP swell model, we have included the propagation of seas in the coastal predictions using the Wavewatch III forecasts of seas at the deep water sites. For the coastline north of Pt. Conception, the resulting errors should be relatively small during large wave events because the continental shelf is narrow and there isn't much distance (fetch) between deep water and the coast for additional wave generation.
  
  In southern California, the story is once again more complicated given the distance between deep water and some coastal areas, and the existence of numerous offshore islands. For the south-facing coastline in Santa Barbara County the coastal forecasts will most likely underpredict local seas. The CDIP wave propagation model assumes almost complete blocking of seas from the south by the Channel Islands and ignores local wind generation in the Santa Barbara Channel.

Frequently asked questions

• What is meant by 'storm threshold' in the offshore forecast plot?
  
  The definition of a major storm is arbitrary and is based upon historical precendent. In Seymour et al (1984), a list was shown of hindcast and measured large wave events in Southern California during the first 83 years of the century when significant wave heights exceeded 10 feet for more than 9 hours. Seymour (1996) updated this list, but extended the height threshold to 13 feet to qualify as a major storm (because it appeared that smaller storms had been undercounted in the early part of the century when atmospheric data to support wave hindcasts were sparse.) Because the forecasts used here are based on 12 hour update intervals, a modification to the threshold in Seymour (1996) has been made so that wave heights must exceed 13 feet for 12 hours to be counted as a major event.
  
  References:
  Seymour, R.J., R. R. Strange III, D. R. Cayan, and R. A. Nathan].1984. Influence of El Ninos on California's wave climate. In: Proc. 19th Int. Conf. on Coastal Engineering, B. L. Edge, ed., ASCE, Houston, Texas, 3-7 September, 1984, Vol 1: 577-592.
  
  R. J. Seymour.1996. Wave Climate Variability in Southern California, J. of Waterway, Port, Coastal, and Ocean Engineering, ASCE, July/August 1996, Vol 122(4): pp. 182-186