Annoying thing about forecasts.

[Graham Easterling]

They always equate wave size to wind strength. There is a connection obviously, but some huge variations.

So today's warning for SW England mentions large waves. The Atlantic swell size is small, it has a very short period, so very little energy. Jut a harmless wind wave. 

However, big things on the way. The bottom row shows predicted wave energy at Sennen, in kj. Anything under 10,000 is fairly unexciting, it's currently a low 1,000.

During the strongest winds in the early hours of tomorrow the wave energy is forecast to be 15,000, then as the wind drops it increases to 28,000 in the afternoon. This weather observer will be at Sennen!!

However, that's nothing compared to the forecast wave energy of 63,000 late Sunday. That's a massive energy wave if it comes about. A 10m wave (about as big as it gets) combined with a long period. Now that deserves a warning, just 63 times the wave energy at the moment.

Graham

Penzance

[Len]

Met Office giving 5.3m max wave height for here in Wembury 21.00 Sunday.

Len

[Graham Easterling]

The MetO always show a shorter period than the more specialist sites. They seem to take some sort of average of the different swells, which is extremely misleading.

Breaking the swell forecast for Wembury (00:00 hours Monday) down, there are 2 concurrent swells that are worth mentioning.

1.  4.5m 15sec

2.  1.1m  7sec

Because of this huge difference in period, well over 90% of the energy is in the 15 sec swell, the shorter one is the wind wave, rather than a true swell. The specialist surf /swell sites will show this 'dominant' 15sec period (and then break it down) The MetO version gives some sort of weighted average, when no forecast waves actually have that period.

The dominant swell isn't always the largest as a long period smaller swell can have more energy. Different large swells getting in synch give the rather misnamed 'freak' waves. If you know the different swell heights and periods then the incidence of freak waves can easily (though not be me!) be calculated. This is why the largest swells are typically 50% bigger than the significant swell height.

It's a very interesting subject!

Graham

Penzance