The Ancient Art of Swell Prediction and Surf Forecast
This surf forecast is valid given that all swell and wind conditions remain constant from origin to destination. Which is rare.
All the data I got for this example are based on satellite photographs taken on July 17, 2007 --it's the middle of Summer here so the swells tend to hit our south shores while the North Shore remains flat like a lake, dormant and silent until November. Remember to check and follow the storm/swell everyday until arrival day. Conditions change.
You need to ask yourself:
Are there any storms generating? Any winds pointing my way? Is the swell generated coming here? When will it get here? How big will it be?
1. Any storms generating? Any winds pointing my way? Is the swell generated coming here?
Then I check to see if the swell is coming my way.
If it is, cool. If it isn't, not cool.
2. Now check out the winds
Look at the red areas for example. Write that down the wind speed using the scale on this picture (30-35 knots per hour, etc).
3. Then I get the coordinates for the center of the swell that I get by looking at the picture. I look at that picture, then use Google Earth or a real map to pinpoint the exact coordinates. You could also wait to see if the storm hits a buoy. That way you'll get more accurate data.
Storm ground zero (off Japan): 29 47'N 144 52'E
note: I estimated that is right off Japan by looking at the greenish and redish zones of a potential swell direction and winds/storm epicenter pictures (see pictures in step 1 and 2 above.)
The place near my house is Barbers Point (Ewa Beach, Hawaii): 21 18'N 158 02'W
4. How big are the waves there? Check the green areas and the scale at the bottom. Don't forget to write down whatever you get from NOAA WAVEWATCH 3 (it's in meters so convert to feet: meters x 3.281 = feet )
answer: about 25 feet or 8 meters as displayed on my picture as of July 17, 2007.
5. Plug in coordinates and wave height.
Using a Great Circle calculator, plug in the coordinates for my area, for the center of the swell that I got, wave height at origin, and nevermind the date.
click on the Calculate Distance button, pay attention to the heading angle and the nautical miles.
In my example, I got:
heading angle: 298
nautical miles: 3102.52
Assume the wind it's blowing my way...
6. Check Table 1. It's a rough relation between wind speed and wave period
Table 1 *based on historic data Wind Speed (knots) | Period Generated * 11 sec 30 13 sec 37 14 sec 42 17 sec 50 20 sec 60 Table 1 (continued) Wave heights at origin | Wave period in secs * 24 to 27 footers 14, 17, and some 12 second periods. 35 feet 17, some with 20 and 14 seconds. 45 feet 20, some with 25 and 17 seconds.
In my case, I determine that the 25 footers I got from Step 4 is within 24 to 27 footers at origin (damn I'm brilliant). Associating that info with table 1 above it gives me 14, 17, and 12 seconds between periods.
7. Arrival date (will use Table 1)
At first you will get results in hours. Just convert that to days. Notice that I divided the Wind Speed by 2 because it roughly loses half it's power when traveling.
Arrival time in hours for each Wave Period = nautical miles / (Wind Speed / 2 )
for the Wave Period of 17 seconds we get: 3102.52 / ( 50 / 2 ) = 3102 / 25 =~ 125 for the Wave Period of 14 seconds we get: 3102.52 / ( 42 / 2 ) = 3102 / 21 =~ 147 for the Wave Period of 12 seconds we get: 3102.52 / ( ~35 / 2 ) = 3102 / 17.5 =~ 177 Table 2 Wave Period (secs) | Arrival Time (hrs) | Days (hours / 24) 17 125 5 14 147 6 12 177 7
You can predict down to the hour if you like but that's good enough for me.
I rely a lot on the tide and the crowd factor when getting ready to surf. It's not really fun for me to surf a 3-4 footer swell ( about 6-8 front face elsewhere) if it's going to be high tide or crowded.
8. How big will it be when it gets here?
Swell height at origin * 0.75^(number of days for the swell to arrive) * 0.75
The result is in feet, front face of the wave.
So for my forecast, it comes down to:
25 * 0.75^(5) * 0.75 = 4.4 feet front face. In other words, about 2-3 feet Hawaiian scale, waist-shoulder high.
Adjust the formula accordingly:
The 25% loss in height is because of wind resistance. Wind conditions which are not 100% consistent. The other 25% at the end of the formula is the loss due to reflection and resistance when the swell hits my area. More than 30 knots against the swell may slow it down around 30-50%, not just 25%. But a wind pushing the swell at 20 knots may not affect the size of the swell in a particular average day.
Also, consider how shallow your spot is, how the swell is affected by local winds and other local conditions, etc. You know your local spot better than I do.
That's it. Repeat all steps above everyday. Conditions change.
My swell will arrive in 5 days carrying about 2-3 feet Hawaiian style (or 4.4 face =~ waist-shoulder high.) That day I'll check also the current wind conditions and the crowd factor. It might be blown and packed.
You can get photographs for your region wherever you are.