In terms of trade wind well, how long do you need winds to blow to get it going, how much fetch, how fast the winds? In my mind I see trade wind swell as large fetch with moderate, low winds where the back end of the fetch is creating swell and as it moves it continually gets "pushed along" by the winds in front of it. When we get it, how far away is it typically being generated? I've heard of a Bermuda high-the high this week is further east though. If the period of a trade wind swell can't get much higher than 8-9 seconds it would make sense that the swell is being generated 1000nm or so. Assuming that on Sunday 2 ft at 8 second swell from SE is being generated from the window below on Thursday: the winds seem to back off in that area around Thursday.
I know a 1000 mile fetch at 15 mph takes aboot 24 hours to produce rideable wind swell down here, if it is pointed in the right direction - say from Nova Scotia. (the Bahamas block alot of trade swell action here, but if you were on the Out Islands - Elbow Cay or Green Turtle, you'd have a nice punchy chest to shoulder high sloppy swell over some reef). Cocoa will get it, maybe in the knee to waist high range and very mushy due to all the sand and gradual slope. NSB will get it, chest to shoulder at the inlet.
With that 1000nm of fetch it seems that initial pulse of swell would be coming from about 290nm out if it was 8s period. That given the 1.5 nm/hr rule for swell period. When the wind dies or switches would you expect to see swell for the next 3-3.5 days given the trailing edge is 1000nm away? If the middle section of the swell was 3ft@8 I would think the end of the swell event would be more like 2ft@8. I guess the question there is does swell decay get negated with it being pushed along by the winds in front of it while the end of the swell after the wind switches will just decay without any reinforcing winds.
24 hours is plenty of time for a good pulse of trade wind swell to be generated. Considering the velocities are usually under 25 knots, a fully developed sea is developed within 24 hours, so longer times wont make the swell much bigger, but it will make the pulse last longer. Yeah...if the trade wind swells are no more than 3-5 feet, and are being generated by a fetch area no more than a few hundred miles from the coast, periods of 8-10 seconds are the norm, and if the periods at the coast are in the 8-9 second range then the swell is probably being generated no more than a few hundred miles away. We had a SE nice trade wind swell in the mid atlantic today that was in the 4' @ 9 sec range, and based on the charts, it was generated about 300-400 miles to our south east. I'm not sure that sunday swell at the coast would be generated on a thursday a few hundred miles offshore. According to this, even 8-9 second swell travels a lot faster than that.....like over 2000 miles in 72 hours:
Good work Mitchell! Usually from what I've seen last few years, the initial pulse is the longest period and then it eases off as time goes by. But if there is a redoubling of wind, or a tightening of the isobars or an overlaying opposing ridge of high v low in a big open area, things could ramp up as the fetch continues and becomes more focused and the intensity increases. A stalled front late spring/early summer - say Memorial Day ish, could produce a good head high wind swell up and down the least coast - it's been know to happen. Usually sloppy but fun.
Yep...longer period energy travels faster, so if an area of fetch out to sea creates a bunch of wave energy with different periods, the longer period stuff reaches you first followed by the shorter period stuff that travels more slowly, and also decays in size a lot more.
Thanks for that graph. I don't think the 2000nm in 72 hours is right. If you follow the blue line that crosses 2000nm at 8 seconds it's more like 155 hours or 6 days. This upcoming Sunday it seems there are remnants from, what I can guess and am trying to understand, the flag below. The flag is about 720 nm away. Looking at your graph above that would be slightly under 60 hours or about 2.5 days to reach SE NC. If the swell is coming from that ballpark of an area it makes general sense. The winds seem to weaken on Thursday in and behind that area. If you assume swell generation stops on Thursday I would think it would makes sense the 2.1@7 on Sunday morning is from near or past that flag.
