Wednesday, November 18, 2015

11 18 15 morning call

Note: I've pasted into this post all the info I later added in other posts that are related to swell directions and shadowing angles. Thanks to its label NW buoy to Maui travel time and shadowing angles, this post can be accessed quickly from the label menu.


Long awaited new WNW swell on the rise, let's jump into that.

Below the graph of the NW buoy. As you can see, it went from 2 to 10f in 18h (blue line) and that is a steep rise caused by the proximity of the source.
Notice also how it started from a straight west direction and only later it added a bit of north (lower section of the graph).
6am reading is 10.2ft @ 12s from 296° (WNW).
Last thing I'd like you guys to notice is how quickly the period went from 15s to 12s. Short distance of travel and not particularly strong winds in the originating fetch are the reason for that.
But the size is there, the questions are: is it gonna make it to Oahu and Maui, when and how much of that energy will be lost because of the shadowing islands?

To answer that, I made the effort (you guys lucky that I'm injured) to launch google earth and find some angles. Below is the straight line that connect the NE tip of Kauai with Pipeline. The direction is around 295. For Haleiwa it's around 298. You might think that anything less west than 295 doesn't get blocked at all, but it's not that black and white. A swell from 300 will still refract and bend on all the upstream islands and the result of that in terms of energy loss and direction after the refraction is really hard to quantify.

Here's the article on surfline that explains a bit of that and the travelling time. For convenience, I'll copy it here.

51001 buoy is 170 nm WNW from Hanalei Bay, Kauai and 255nm NW ~300 from Waimea Bay, Oahu.

For a NW swell around 300degrees on the 51001 buoy...

Wave period--Wave speed--Arrival time on North Shore

20sec--30kts--8.3 hrs

17sec--26kts--9.5 hrs

14sec--21kts--12 hrs

11sec--17kts--15 hrs

A more Northerly swell would arrive a little sooner on the North Shore and a more Westerly swell a little later. However, a WNW swell between 270-295 degrees has to contend with the Kauai shadow and will result in 50-70% less energy getting to the North Shore.

Hope this helps some, aloha,

Jonathan Warren
Surfline Forecaster/Meteorologist

First let me point out a little mistake that I believe is in there. If the NW buoy is at 300 degrees from Waimea, that's the direction of a swell that will take the longest time from the buoy to Waimea. Anything more north will take less time, but also anything from more west will take less time. It's a simple geometrical concept. But that's a very unimportant detail, since there's not that many swells from less than 300 and when there are (like today), there's more important things that happen to them, like refraction.

The article says "if it comes from 270-295, the Kauai shadow will result in 50-70% less energy.
We have a wonderful opportunity to verify if that is right! If it is, we should expect the Waimea buoy not to go up more than 7f today.
But at the same time, long period energy refracts and bends around land points much more than short period ones, so there really isn't a magic formula for that.
It's more your local knowledge and observations that will have to guide you.

"What about Maui??" I hear you screaming.

There you go, below you have a line from Hookipa to the tip of Molokai and the angle says 305.
I apologize if in the past I said 320, I guess I never really measured it with google earth.
That 320 was more out of local knowledge and personal observations and that is the confirmation that even a swell from, let's say 312, will feel the influence of Molokai (at least the part that travels the closest to it), get refracted and lose some energy.

I included Kahoolawe in the picture so you guys understand why I always blame it for blocking the energy of the south swells. It sits perfectly south of the coast line between Thousand Peaks and Lahaina. A straight south swell will get into the narrow corridor between Kahoolawe and La Perouse and hit Maalaea. A more westerly direction will hit the lahaina coast better.

But where is the Pauwela buoy? Below is the answer, thanks to google Earth: 6 miles offshore of Hookipa.
As you can see, the Molokai shadow for the buoy is 299. Hookipa's is 305. It's not a huge difference, but it is a difference. That means that every time there is a swell that is more west than 300, the buoy will show more energy than the one that will eventually hit Hookipa. Let alone the rest of the coastline towards Kahului.
Peahi's line instead sits at 298, so what the buoy reads is what is going to get there. Quite convenient, I'd say.

Below is the shadow line for Honolua that sits at 335 degrees. Anything more west than that, will have to bend around Molokai to hit the bay. The bigger the size and the higher the period, the more that will happen.
I also drew some lines representing a swell coming from 360 instead to show how it will travel unrefracted till the bay and then, if it's big enough, start bending around the coast line. If a straight north swell is big enough, the wrap will be felt all the way down to Puamana. Same can happen for swells coming from the NE quadrant, as long as they are big/long period enough.

And if a swell from 335 or even more west than that is big/long period enough, you can imagine it doing an S turn: first refracting on Molokai and changing its direction to more straight north and then refracting on west Maui and changing its direction to more west again. All this, of course, happens to the expense of the energy of the swell, but it's pretty amazing that waves can do that.

This is the shadowing angle for a very popular beach: Kanaha. The angle there seems to be 319 and is due to the West Maui Mountain more than Molokai. So, a swell coming from 325 for example, should feel any influence from either one.
A swell coming from 320 will feel some influence from the WMM instead.
I've watched for years big breakers off the golf course in Waiehu in such occasions, because the reef bends the sets towards itself leaving Sand Piles pretty much flat. All the next breaks down the coast towards Kahului instead will have plenty waves.
Anything more west than that, will still eventually get to Kanaha (and the Waiehu coast) depending on size and period. You can imagine Molokai doing a first part of the refraction for the swells between 310 and 320 and Kauai and Oahu doing adding more refraction from swells more west than that. In other words, good luck at trying to predict the size of the waves at Big Left on a 10f 15s swell from 305. That's when you just get in the car and go check. For sure you can expect less consistency than if it was hitting straight without refraction.

Let's talk also about the Kihei coast and the westerly swells.
Google Earth shows the shadow line from Lanai to Kalama park in Kihei as 273 degrees. Anything from there to straight west, doesn't get blocked/refracted and will have a more direct impact.

A little better angle applies to Ahihi Bay: 283.

But don't forget that a the bigger the size and the period of a swell, the more the waves have the ability to refract around land points. The photo below shows that as long as the swell is 290 or more west, the south point of Lanai will refract energy that has not been refracted by Ni'iahu. But if a swell is big and long period enough, even if it comes from directions more north of 290, it could still refract first over the south point of Ni'iahu, change its direction into 290 or more west, and then refract again over Lanai and hit Maui. I remember one coming from around 300 that provided double overhead waves to Kihei.

It all depends on each single swell and there's no mathematical/geometrical rules you can apply that work all the times.
I've see too many times very similar swells doing very different things.
That is also because if the swell has a direction at the NW buoy, that doesn't mean that the swell hitting the south point of Ni'iahu will have the exact same direction. It is possible that it will be more west there and that the waves will be bigger than you would expect based on these information I'm providing.
So this is just a reference to try to guess when it's worth to get in the car (or check the webcams).

And for the very rare ESE pulses, here's the shadowing angle from the Big Island to La Perouse: 161. I like even numbers, so I'm gonna try to remember 160.

And since this post is becoming the place where I' storing all the shadowing maps, here's the ones for Kahoolawe and the south swells.
As you can see, the tight opening that Maalaea has to the east of Kahoolawe is only between 165 and 185. And you can't even consider all of those 20 degrees "unaffected" by the uninhabitated island. Directions close to the edges will still be refracted and diminished by it. Best direction would be 175.

Lahaina has a much better "view", as its angle goes from 185 to 232. As a consequence, it likes a bit of west in the south swells.

The Samoa buoy (S on the map, F stands for Fiji instead) is located at 14.265 S 170.494 W which is at an angle of 202 degrees SSW of Maui and at a distance of 2,222 nm. Below are the travelling times of swells coming from that direction.

20sec--30kts-- 74hrs (3days)

17sec--26kts-- 85hrs (3.5 days)

14sec--21kts--106 hrs (4.5 days)

11sec--17kts--130 hrs (5.5 days)

As you can see, there's a narrow path between the New Zealand's north island and Fiji that is unobstructed. The direction of that path is 210. The biggest south swells for Hawaii, usually come from just east of New Zealand, an area which is at 200 south of us.

I found the time to do some investigation on the wave speed topic I mentioned yesterday and found this article that seemed to be the most reliable. I got to it following some links on this other one, which is a really good read if you have time and want to learn more about waves.

From it, I'm gonna quote this sentence: "It's matter of experience and choice as to what size, period, and swell direction each of us prefers, but if you know what the swell conditions are before you set foot in the water, you'll be a lot better prepared."
That is pretty much what this blog is all about.

Let me also point out this page with the world wide great circles, from where you can get the Hawaii great circles for the north pacific and the Hawaii great circles for the south pacific. The great circles allow you to see from what direction a swell from a fetch in a certain position will come and they take into account the curvature of the Earth, which instead gets distorted in the two dimensional maps that we commonly use.

Below is a table that has the following columns:

- P is the period
- S is the speed at which swells travel  in knots (nmph)
- T is the time to cover the 383 nm that separate the NW101 buoy from Maui
- GP's is the approximation of that time if you use the new and improved GP's rule of thumb, which you can easily remember because it goes: 13h at 19s and viceversa.

Everything in between (actually everything between 12 and 21 seconds), you can just interpolate by adding and subtracting 1h to the time and 1s to the period. The maximum approximation you get in the 12 to 21s range is 0.7 hours (42 minutes), which is plenty precise for me, seen the non precise nature of these things.

  P               S                T               GP's
10s  =  15.6   =  24.5  =  22 n/a   
11s  =  17.16  =  22.3  =  21 n/a
12s  =  18.72  =  20.5  =  20
13s  =  20.29  =  18.9  =  19
14s  =  21.84  =  17.5  =  18
15s  =  23.4   =  16.4  =  17
16s  =  24.96  =  15.3  =  16
17s  =  26.52  =  14.4  =  15
18s  =  28.08  =  13.6  =  14
19s  =  29.64  =  12.9  =  13
20s  =  31.2   =  12.3  =  12
21s  =  32.76  =  11.7  =  11
22s  =  34.32  =  11.2  =  10 n/a
23s  =  35.88  =  10.7  =   9 n/a
24s  =  37.44  =  10.2  =   8 n/a
25s  =  39     =   9.8  =   7 n/a

Those travelling time are most correct for a swell that comes from 303 degrees, which is where the NW101 buoy sits compared to us. Any difference in the direction will introduce a difference in the travelling time, which for our purposes I would tend to consider negligible. Just remember that both for more northerly and more westerly swells, the time will be less than what the table says, but the energy hitting the buoy will not be exactly what we receive, so it's not worth to put too much effort into calculating the exact arrival time.

1 comment:

Tradewinds said...

Amazing report!