Where does the data come from?
#1 Where we have human forecasters making observations & issuing reports/forecasts...
Surfline forecasters update the water temperatures daily. Water temperature at surf spots is estimated using nearby buoys, satellite data, reports from local fishing piers, etc. We regularly update a more in-depth forecast that can be found here: East Coast & West Coast (US) water temperature forecast.
#2 Where we don't have human forecasters making observations & issuing reports/forecasts...
We use satellite data from NASA, updated once per day.
Was the surf colder than we predicted?
If data is coming from a satellite survey (#2 above), the satellite is averaging temperature over an area larger than 1 km2.
In most cases, this provides a reliable guide to temperatures encountered in the surf.
But there are circumstances where this isn't the case and there is a large temperature difference in a very small area. The likely culprit: COASTAL UPWELLING.
In some areas more than others — good fishing grounds typically are found where upwelling is common — this natural phenomenon is responsible for freezing cold water in the surf that isn't picked up by the satellite. Simply due to the fact it averages temperature over a larger area than the cold pocket.
Wind blowing in specific directions pushes warmer surface water offshore, in turn drawing up cold water from the depths to the surface, right next to the coast.
Note: direct offshore wind doesn't equal surface water moving directly offshore, due to the Coriolis Effect.
The deep water tends to be rich with nutrients, great for diverse marine life! But awful for the surfer with a thinner wetsuit.
Inaccurate data is still inaccurate data, so we want to know about it! Submit a request with the request type "Forecast Accuracy" so we can have our forecast engineers take a look. Attach screenshots of the forecast you're referring to.
We might not be able to do anything about it, but your feedback having actually been at the location is always valuable.