# Ten Day Forecasts and Forecast Probability

### Probabilities

Every surfer who's used any computer generated surf forecast regularly in the past ten years or so has learnt one fairly obvious fact: forecasts can change and the further you look into the future the more likely this is to happen.

You know the story, you spot a new swell right at the end of the seven day forecast and cross your fingers, as it draws closer the forecast updates and it either sticks around, changes, or sometimes even disappears completely. You know this and we know this, but magicseaweed is the first and only free surf forecast service to now try to put some science to this phenomena.

Against almost every three hourly forecast you'll now see a new number, the 'probability'. This gives you a great idea of how likely the forecast is to change. You'll notice that for almost all locations this is as high as 100% for up to three or four days from now. This shouldn't come as too much of a surprise, if you compare your MSW forecast to a local wavebuoy you'll find that it's really very very accurate over this time range.

What you'll also notice is that towards the end of the forecast (particularly now we've extended the range to a full ten days) is this becomes generally less certain. But remember not all swells are created equal, that local wind swell might be showing only a probability of 60-70% for the weekend, but a nice long range groundswell has probably already been created by a storm somewhere, so you'll find this sort of swell can be up to 100% certain seven or more days from now - this is information that simply wasn't available to the surfer before.

Another phenomena you'll spot is a swell showing at 90-100%, but the first day of the new swell looking a lot less certain. This means the forecast is absolutely confident the swell will arrive, it's just a bit less sure as to exactly when, these little issues will iron themselves out once the swell gets to within three or four days from now, but if you're making plans make it it's really helpful to know when things are most likely to change.

Bear in mind that this is still a forecast, it's a complex series of mathematical process designed to predict the future and that's a hard thing to do. It's worth repeating that - we DON'T have a crystal ball, we can't actually compare a future forecast with the future, a forecast of 100% certainty simply means that the computer is so confident of the situation that no amount of pushing will get it to forsee a different future - it DOESN'T and CAN'T mean that there's an actual 100% certainty of the weather responding exactly as predicted. So even a 100% certainty doesn't mean that tomorrow's forecast, with new information, can't change. As with any forecast this data is a guide to help you plan a session, not a cast iron guarantee.

Nonetheless this new tool should really change the way you're able to make plans around our data.

This probability only applies to incoming swell, we'll add probabilities for wind and weather shortly.

### Ten Day Forecasts

We've extended most of our detailed long range forecasts to a full ten days. The first seven days are exactly the same data you've been used to using. The extra three days come from a slightly different forecast method, and to run a forecast this far out we've compromised with the level of coverage - you'll see that beaches in the same area share the same data and that the forecast is run on this regional level. The size predicted may vary slightly by comparisom to the data for your local spot (Although ocean swell isn't produced locally and regions are selected so that all spots will be affected by similar swells).

Nonetheless our testing has shown this forecast a reliable and accurate indicator of the arrival of both easier to predict long range groundswells and even spotting short period wind swell up to 240 hours in the future. Coupled with the new 'probability' rating you're better placed than ever to start making plans to travel up to ten full days in advance. Magicseaweed is proud to be the only free surf forecast service offering this level of detail and length of forecast and with the new probabilities showing occasions where the forecast is hard to rely on we hope that we're giving you this information in the most honest way possible...

### Multiple Swell Warning

We've come a long way in the last decade understanding swell and period and getting used to accurate long range forecasting. But our ongoing work here at MSW it to figure out how to best present complex ocean conditions as simply as possible and sometimes that simplicity can show less than the whole truth.

In many locations it's very ususal to have more than one definded swell running at the same time, your MSW forecast shows you EITHER the most powerful swell likely to make waves at the beach OR a predicted combined conditions if there is more than one swell that looks like it'll make ridable waves.

However there are issues. We've noticed this season in Indonesia that there have been regular south swells running in conjunction with the usual south-westerly swell. We show the combined effect of this incoming swell, but on the beach in Bali, for example, this more southerly swell hasn't made it to the reef and the forecast looks like it's overcalling, a quick check of the full breakdown shows the real situation.

In other locations this measurement of the effect of combo swells is much more useful - so to give you a warning when this is the case we've added this symbol everytime a combination if swells is running. Click the 'Full Swell Breakdown' tab at the top of your forecast and you can instantly see the exact mix of swell likely to make surf.

N.B. Our star ratings DO take this into account, giving you a rating for the best looking individual swell NOT the combination.