|Buoy Location 59°07.3'N, 11°42.5'W|
In this instance, "Significant wave height recorded is four times the RMS value of the water level above the average level of the water surface measured over a 17½ minute period. The factor of 4 applied to the RMS value is because the waves are trochoidal in nature. (Waves at sea, especially those growing under the influence of the wind, tend to be short-crested, i.e. the wave crests project further above the mean level than the troughs are below it.) The ‘average’ wave period, again over a 17½ minute sample, is the average of the periods over 7 successive 2½ minute samples (each determined from the number of wave cycles in the sample)." - WMO
Okay, that's not just some single wave calculation. For all the non-math nerds, what this essentially says is that the number represents a sampling of waves over a period of time, and the "root mean squared" (RMS is a calculation typically used to find peak magnitudes for a set of numbers) value of those wave heights was multiplied by a factor of 4 to account for the shape of the waves (wind driven in this case) and the buoy's heave sensor sampling rate. So what we're talking about is not a 19 meter rogue wave, but a brief series of waves that statistically calculate to be of that average height.
Those are monster waves. I've been in 40ft seas in the Pacific and can say it was an amazing, if somewhat frightening experience. Not one I'd like to repeat.