Why are the West Wales reports sometimes different to the Pembroke Buoy?

We've been asked this question a couple of times recently. The answer is valid for most comparisoms between the NWW3 data and the buoy but the examples I give are for Freshwest and the Local buoy.

The surf report at the top of the page is based on the latest NWW3 data not on the buoy reports (it mentions this on the left on every forecast page "Current conditions reports are based on the latest forecast conditions, not on wave buoy observations.") - there will be discrepencies with the pembroke buoy as it is located very nearshore and the swell here is already experiencing decay against the offshore swell data. This report serves two purposes - it provides a 'virtual buoy' for the majority of surfers who aren't fortunate enough to have a buoy directly offshore and it provides and essential tool for comparing what you see now with what the forecast is calling for the next few days so that the size and strength of a coming swell can be called.

Whilst, of course, we want to maintain a good reputation our site is based on delivering the data 'as is' so that surfers can make the call based on their interpretation of it, which will always require a good deal of local knowledge. To forecast accurately you need to consider additional near shore decay. What we're reluctant to do is to go in and put in a crass fix to the data which will not increase the true accuracy of the product. The south wales forecast does take into account the swell shadow from Ireland and a degree of Bristol Channel decay but as offshore data (up to 30 miles) it won't be perfect for an on the beach assesment - exactly the same for all our forecasts.

As a forecast it's always been most useful to spot a new swell event arriving. For this is has been in testing very accurate - a switch from a 3ft@6sec to 7ft@10sec is always worth knowing about - We've been at pains to suggest that even the most experienced forecasters are then going to struggle to work out exactly what size this is going to be when it materialises on the beach and this is why we link to webcams and 'on the beach' reports aware that what we provide will only ever be part of the picture when it comes to making that final call for a dawn session. The buoy is your best bet for these last minute calls either way updated every hour against every six for the forecast report. These trends are best appreciated from the graphs at the top and bottom of the pembroke buoy page. The top graph is the last 24hrs buoy data:

and the bottom on the next graph the next three days NWW3 forecast data:

Right now they correlate 10secs to 8secs and 2ft to 1.5ft which is pretty tight and the trends look very similar.

It's also worth considering the fact that a call on period and height at the buoy is not an exact science - its a measurement of a range of energies in the spectrum and a call on which is dominant. If you look at a current spectrum in the NE today:

You'll see that there are a really strong mix of waves at around 6-8 seconds. In this instant the call on the buoy is 6.8 seconds which we would round to 7secs but you can see that the forecast model would have to shift that peak by a tiny tiny amount to call it 6, 7 or 8 seconds. With two swells running it gets even more complex as both the model and the buoy try to call the dominant one:

I guess what I am saying is 'here are a bunch of tools' and 'here is as much explanation of them as I can give' - I won't apologise for the fact that the sum total isn't a perfect or simple to use solution to long term forecasting but we will continue to make it as close as we can without dumbing it down. We've long set out our stall not trying to impress the wannabe two week a year guys but provide surfers like you with a little extra knowledge to get it right....