Tuesday, July 04, 2017

Tuesday 7 4 17 morning call

Action shot of the day reflects the actual conditions. Hatted lady was going to nose ride this thigh high bomb, but she changed her mind and is now lifting her back foot to stomp a stylish cut back. Don't be afraid to lift your back foot and place it all the way back and on the rail to achieve sharp turns on your longboards and SUP boards. And shortboards too.

3am significant buoy readings
South shore

4.6ft @ 7s from 83° (E)
2.4ft @ 14s from 114° (ESE)
6.2ft @ 8s from 80° (E)           
2.6ft @ 13s from 121° (ESE)

6.8ft @ 8s from 84° (E)           
3ft @ 13s from 120° (ESE)
For a change, I reported also the windswell readings at the outer buoys, because I'm gonna discuss directions again. My theory is that when a buoy is hit by multiple swells, those all influence the reported directions of each other. I didn't read that anywhere, it's all my original thoughts based on observations, so I could be wrong.

Today we have 5-6f of 7-8s windswell from around 80-84 and 2.5-3f 13-14s from 114-120. I believe that the higher windswell makes the direction of the smaller ground swell appear to be a little more east than it really is. Important detail: the windswell doesn't change the direction of the ground swell, it only changes what the buoys perceives the direction to be!

Without local buoys that are not hit by the windswell (specifically, Lanai and Barbers), the only way to verify if I'm right or wrong is to have a look at the fetch that originated the ground swell. Below is the map of June 27, showing a fetch generating waves from a directional band of 125-130. So the windswell induced "deviation" seems to be around 10 degrees in this case.
I've seen much bigger in the past and there's too many variables and approximations involved to be 100% sure of that, but for sure it's a thing to keep in mind.
And while I was writing that, I just thought that 125-130 seems a direction with an unusual amount of east... what was the shadow line from the Big Island again? Didn't remember it, no problem, I went and retrieved it from the (epic) post Buoys to Maui travel times and Maui's shadow lines. As shown also by the picture below, that shadow line is around 160! No wonder that despite 3f 13s of open ocean swell we only get thigh high waves!

And today they might be even smaller, as the Pear Harbor entrance buoy went from 2f 14.3s at 4am yesterday to 1.3f 13.3s this morning. I'm not checking that buoy at all, after I read what Pat Caldwell wrote about it being severly influenced by the surrounding bathymetry (plus Surfline seems to have given up adding it correctly to its summary page), but it still is an indication of a decreasing size. Nonethless, I'll probably drive to Lahaina anyway because I like the mornings over there, so check the webcams and my beach report later.

North shore
4.6ft @ 7s from 68° (ENE)

That's what's on offer on the north shore today, together with 14(8-21)mph from 75 (great direction for sailing on the north shore) at Hookipa at 4.45am.

Wind map at noon.

North Pacific completely dormant with only the windswell fetch.

South Pacific not offering much more either with only a below average SSE fetch.

Looks like plenty high clouds to our east, but not quite over us.


scott werden said...

Wind swell affecting wave direction: I got curious about how NDBC buoys work and spent a few hours reading various papers on their instrumentation, calibration and analysis techniques. It is pretty interesting stuff. Well, to me it is.

My general impression is that the buoys are pretty solid in design and I would be surprised if they could not correctly account for wind swell. There is an inherent assumption that all waves are linear and thus do not interfere with one another so there is some possibility that a non-linear wave is being formed by the local wind. There also is some known spectral leakage and spreading, but this does not appear to be what you are observing.

I did notice one thing - the directional-spectral density as reported by Surfline does not always compare well to the raw buoy data I looked at. In particular, the SE component at 51002 was quite broad (120 to 136 degrees) this morning and Surfline only reported it as 120 degrees. Note also that Pat Caldwell was calling for a SE swell out of 130-140 for Monday and today. But maybe he is also being fooled by an errant buoy!

cammar said...

Thanks for your contribution.
I started observing these (allegedly) discrepancies at the outer buoys only since the Lanai and Barbers buoys went offline. Before I wasn't paying too much attention to them. As soon as one of the two will come back online, we'll know if I'm right or not. If I am, the local buoys will show less east in the direction of a southerly ground swell than the outer ones. Cheers.