I understand the information it gives, but I'm puzzled by the red-amber-green rating that it gives to conditions.  Does anyone know how these ratings are calculated?

Met service and swell-map are done on the same forecast, and over the past couple of years I've concluded it's abit of a cr@p one.  I tend to look at 4-5 forecasts to get a better idea of exactly what is to be expected.

Check that you are looking at the boating forecast and not the surfing one. Been caught out myself on that

I use swellmap for Manukau bar crossings and like any forecast it is a guide more than anything. I look at both the harbour entrance and the shelf break. I look at the wind forecast and use the tide times to determine when to cross. Tidal flow is important too. For instance I wouldn't want to cross an hour or two into an outgoing tide on a 4.4m tide especially if there is any sort of swell running. Swell and wind direction all play a part too Kevin. The period between swells also comes into play and the longer the period the more energy is stored up in those waves. Some think the bigger distance between waves gives you more room for error but I see it as more potential to get a big green wall of water standing up either behind you or in front of you. Either way that is scary.

So I guess you can see that it isn't only the wind that they base their rating on. Having said that, I take the factors into account and take those ratings with a grain of salt. I've been out when it's three and been shunted around when it's six.

Most importantly I check for waves breaking at Puponga Pt and if there is significant white water on the western side that's a good rule of thumb that the bar will be a little messy. Two weeks ago, everything looked reasonable but we went right up to the shallows and sat and watched. There was a lot of white water across the south channel and were in two minds as to whether to cross. When the tinny we watched smashing its way out turned around and came back in (same size as our boat with only 2 pob, we made the call to fish inside. Pretty disappointing as we were on a mission, but at least i still have my boat in the garage and sitting here able to pass on my thoughts.

If we were an hour earlier (it would have been a bit dark) the bar would have been fine. The wind picked up pretty strong that day and despite crap fishing in the harbour, two weeks later I have no regrets.

Remember back to when I crewed on your boat on the Coastguard course? They said there to stick to a couple of websites and learn the quirks of each one. I'm a strong believer in that.

The Answer is it don't 

Some forcast worst case scenario like Metservice and Swellmap, while others forecast best case scenario. One thing that stands out is fishermen always like to go off best case scenario and complain when the worst case meant they called off a fishing trip when they could have gone, I'm sure most would agree that erring on the side of caution is a good thing. Met service use a system of grading. which is found on the warnings map. It runs by a percentage of deemed accuracy, from a 30% chance through to a high chance which I think is 70%... People seem to forget that all of them are a prediction. Isobars are great but you have to understand the ISO charts and also if the system is deepening or receeding, otherwise it's like sticking your finger in the air in the morning and expecting the weather to stay the same all day...