About the 3-Day Wave Forecast Model
These experimental coastal wave forecasts are a joint research effort by:
Office of Naval Research
California Department of Boating and
U.S. Army Corps of Engineers
The NOAA/NCEP/CDIP coastal wave forecast model
The coastal wave height forecasts are actually the combined result of
two wave models:
- 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-diffraction 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.
How the forecasts are made.
- Every 12 hours, CDIP receives detailed wave forecasts from NOAA for
two deep water locations: 34N 121W (near Pt. Conception); and 37N 123W
(off Monterey Bay).
- The forecast data is modified slightly to be more compatible with the
CDIP wave model, and more consistent with recent wave conditions measured
near these two locations.
- The modified 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).
A Description of Forecast Model Products.
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
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
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.
Timing Errors The CDIP coastal wave propagation models do not
account for the time it takes waves to travel from offshore to the coast.
North of Pt. Conception, this "time lag" is relatively small because the
continental shelf is narrow and the deep water Wavewatch III input point
is close to the coast.
In southern California, the deep water NOAA Wavewatch III model location
used to initialize the CDIP model is near Pt. Conception. It can take an
additional 6-8 hours for the waves to reach San Diego when they are
arriving from the west.
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 typical El Nino wave events, the largest
local sea conditions are from the south, produced by the "prefrontal" winds
of the low pressure storm system as it arrives in southern California from
the west. In these situations, local seas from the south are forecasted
in deep water by the Wavewatch III model before they occur in the San
Diego to L.A. County areas because the storm is still moving into the
region. Therefore, the coastal forecast model will predict large local
seas from the south 6-12 hours too early.
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.
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