I have been asked many times where I get my surf forecasts from.

I devise them from the Met. Office Atlantic synoptic charts primarily, current buoy data and a glance at the current wave model for the North Atlantic (to back up the synoptic chart). 

Forecasting surf for Pen Llyn is fairly straightforward as we only get good surf from within the small wave window between south Wales and southern Ireland. (draw a line from Porth Neigwl to the tip of south Wales and extend it out as far as the tropics. Now do the same from Porth Neigwl clipping south east Ireland and extend that out toward the Americas. All waves over 7 seconds period here (I will get back to this) will have been formed within or close to this cone and, on final approach, will have come through this St. Georges Channel gap. 

To start with there is no such thing as an 'accurate forecast’. The expression is a fallacy. A forecast is, effectively, an educated guess at what may happen. With surface weather (our driving force for waves) the variation from a forecast increases dramatically with every hour out from that forecast so that by 36 hours the difference could be so huge that a stationary observer could easily experience the opposite wind direction from that forecast. Wave forecasting is actually a little less variable because waves, once formed, travel indefinitely in straight lines and a lot slower than some weather systems. Long distance swells can be forecast beyond 5 days if the ocean is big enough. For example, California gets most of it’s summer surf from the Antarctic Roaring Forties and these swells take around a week to stream up through the Pacific after they have been formed, so they can be forecast with a reasonable degree of accuracy. Europe, unfortunately, is bordered by a much smaller Atlantic Ocean and a storm near Newfoundland may generate a huge swell that will only take 3 - 4 days to hit Ireland, UK, France, Spain, Portugal and Morocco. European weather is also less predictable than California so surface conditions, when that swell arrives, could be wildly different here if we forecast beyond 2 days. So, for the UK especially, treat any app/forecast etc, beyond 36 hours, with a pinch of salt.  Also let me clarify that the present, future and past tense of 'forecast' is forecast (not forecasted or any other abomination!). 

Now, to get on to forecasting surf.

The fundamentals: Waves are produced by wind. The stronger the wind blows then the bigger the waves produced. The longer the wind blows for, the bigger the waves produced. The greater the distance the wind blows in the same direction, the bigger the waves produced.

Distance and length of time come collectively under the term ‘fetch’. This is a global rule but to get back to forecasting for Pen Llyn, we need a strong wind blowing at us, over a large sea area somewhere out from that gap between south Wales and southern Ireland. It can be any distance out, the further away the better usually, so hurricanes moving from the Caribbean to the Azores are slap bang in our swell window. (Hurricanes are actually often very small diameter in the scheme of things so swells are usually a little disappointing from these). 

The UK Meteorological Office produce the best synoptic charts for the North Atlantic with a current chart update every 6 hours or so. These are where I get almost all my forecast information from. A basic grasp of Isobars and fronts unlocks all the information in these charts. Again, current charts are best and forecast charts are to be treated with caution. A quick look at a current chart, and possibly a 24 hour old chart to compare, will give me an idea of what wind is blowing in which direction, where and for how long. This then gives me an idea of how big a sea is being produced and which direction it is mainly running. A glance at the current wave models will confirm this. 

A very basic guide to reading synoptic charts: Think of the isobars as contour lines. The tighter together they are the steeper the pressure gradient and the stronger the wind. The wind blows anticlockwise around low pressure. It is not exactly in the direction of the isobar lines with anything from 10 to 30 degrees of skew toward the centre of the system. It blows clockwise around Highs with a similar skew outward from the centre. The standard chart has a perspective key in the top corner allowing for the curvature of earth being drawn flat. It basically shows that the further south you look at the map the isobars don't need to be as tight as further north to represent the same wind strength. Fronts are shown as dark lines with either cemi-circles (warm fronts) or triangles (cold fronts) or a combination (occlusions). The wind will back and freshen ahead of these and veer behind.  

Bigger swells travel faster than smaller swells. For example, a moderate swell travelling at 30 knots will travel around 800 miles a day. If you see strong wind, aimed at us, in our swell window and you confirm that a big sea has already been produced by looking at a wave model, then you should be able to time when that swell will arrive.

Local weather forecasting, an essential factor in wave quality, is much harder to forecast beyond 12 hours and forecasts need to be checked constantly to get the best conditions. 

Light wind locally and warm sunshine are best for wave quality. We, unfortunately, have big tides in western Europe with ours moving as much as 5m from low to high sometimes. This has a big effect on wave size and quality with waves at low tide often a third of the size compared to high tide. (All commercial surf forecast websites/apps ignore this). 

Wave period: This is the distance from crest to crest of successive waves (usually measured in the time it takes for them to pass a fixed point). Bigger swells travel faster and have a greater distance between crests so there is a big difference in swell size with only a modest increase in swell period. Waves travelling faster pack more punch so a 1m swell at 15 seconds is a lot bigger and more powerful than a 1m swell at 7 seconds. The 7 second swell may look lazy and barely break, it will be frustrating to surf often filling out and backing off whereas the 15 second swell will break hard from the start and continue with power along it’s length. It will also hold you under water a lot longer. 

1m at 7 seconds may produce waves here around waist to chest high at optimum tide times. 1m at 15 seconds will probably be overhead and breaking hard. 

So, in summary, learn to read synoptic charts and ignore apps.