Ocean Science 101: SoCal’s North Pacific Swell Window

Since we have started moving into the fall/winter season I have had a few people ask me about the W-NW swell directions...in particular they were curious about the swell shadowing that occurs in Southern California as swells start to track in from over the 300-degree mark.

A couple of notes on swell directions

Just for a quick refresher...when I am talking about swell-direction in "degrees" I am referring to the degree range on a navigational compass. (So North is either 360- or 0-degrees, East is 90-degrees, South is 180-degrees, and West is 270-degrees). You measure swell direction from the direction a swell is approaching a fixed point (we measure wind the same way). So if swell is coming from a storm to the NW of your position then the swell is a "NW swell" even though it is heading SE. It sounds confusing at first but once you think about it for a second it makes more sense. Here I drew a picture...



On the more technical level I use True-North directions as opposed to Magnetic-North (what an actual magnetic compass would register). Most maps and swell models are based on True-North since it is a more consistent value and not subject to the Earth's slowly shifting magnetic fields. Since I like to have consistency in my data, and I am just too plain lazy to constantly do the math, I use the True-North.

Sometimes, thanks to the difference between the 2 Norths, you get some confusion on what sort of swell direction a beach is exposed to. This confusion is usually compounded by some of the buoys and swell models that "average" swell-directions giving you a skewed degree reading thanks to the wider mix of swells, rather than the pure swell direction that is creating the surf. This can make it seem like a swell is coming in from a steeper swell angle when it isn't even geographically possible. This isn't a huge issue normally but I get bummed out when you guys miss surf or burn gas checking spots that won't be breaking...so I try and make sure that we are all on the same page with swell directions.

Ok back to the North Pacific swell shadow...

There are actually two components that block NW swell...

1. is Point Conception, the massive headland that forms the Northwest corner of the Southern California Bight.

2. is the Nearshore Islands, which include the Channel Islands, Catalina Islands, San Clemente Island, Santa Barbara Island, and San Nicolas Island.



Generally Point Conception blocks all of Santa Barbara, Ventura, and North LA while the Nearshore islands soak up the rest of the swell heading toward the South Bay, Orange County, and San Diego.

NW swell (295-310+)

I went back and picked out a few historical images from the CDIP to give you a visual on the swell shadowing. This first image is a NW swell (295-310+) with most of the energy coming in from above the 300-degree mark.



At first glance you can see how much energy is being shut out of SoCal, it has nearly 10'+ of swell (bright green color) showing through the outer waters and less than 4' showing at most of the areas along the SoCal coast (blue and dark blue colors).

When you start to focus in on the details a little bit you can see some minor windows where swell is leaking through the shadowing.

Ventura and the South Bay are both seeing some energy slipping through the gap between Point Conception and the Channel Islands.

Further south you can see a bigger chunk of swell making it through the area between the Channel Islands, San Clemente Island, and Catalina that is heading toward San Diego, particularly Southern San Diego.

One thing to remember with NW swells is that swell-period is really important...the swell in this picture is a longer-period swell, probably close to 16-17 seconds...and as the swell periods get longer you see more refraction, or wrap, off of the sea-mounts and shelves that hang out in the nearshore waters off our coast. Basically the edges of the swell shadow get "fuzzy"...there isn't as clear a delineation between the areas that will pull the swell. The swell hitting Ventura in this image is a great example, as well as the areas of North San Diego.



You can see a slight upward curve to the swell as it wraps around the nearshore islands for both of those areas. If the swell period was short, say around 8-10 seconds (or lower), there would be no curve to the swell...and the edges of the swell shadow/window would be very sharp...sometimes to the point where you can have surf hitting one spot 100-yards away and nothing at the spot you are sitting at.


WNW swell (285-300+)

Here is an example of a WNW swell...and while most of the raw energy is above 300+ degrees it is showing plenty of energy in the 285-300 direction range.



First off ignore the sudden change in the color for the different swell heights, I am not sure while the CDIP changes the colors as more swell hits but it is aggravating...if you don't check the scale you can look at the map and think that you are seeing a waist high swell when in fact it is like 10-foot...anyway.

As you can see on the map as the swell direction starts to shift more westerly you get a lot more energy making it into Southern California.

The WNW swell direction still has some shadowing issues but not as much as we would see on the NW swell. The spots that did well on the NW swell do even better on the WNW'er...so Ventura, The South Bay and South San Diego will be the biggest but you start to see some decent energy moving into parts of Southern Santa Barbara/North Ventura, areas of Orange County, and North San Diego as well. There are still a few blue gaps but they are much, much smaller.


W swell (260-280)

Finally here is an example of a pure W swell...this one doesn't have any northerly components of swell, just pure W energy. (Again the colors change so make sure to read the legend).



West swells are one of the "magic" swell directions...it is one of the few swell angles that can pass energy almost directly into SoCal without interference...which is one of the reasons why the swell heights on this model are so big.

You can see that the winter top spot like Ventura, the South Bay, and San Diego do even better on the W swell, pulling in more energy than the other regions. At the same time there is plenty of swell making it into almost every other region with a couple of shadowing exceptions behind the bigger nearshore islands like Catalina and San Clemente island.

It is worth noting that on swells this big sometimes it is better to look at where a swell is "not" hitting. This swell in particular was producing double-overhead+ surf at the standouts which puts the wave heights out of most people's comfort (and safety zones). Big W swells (if the weather is good) are good swells to hunt around for better shaped surf if you don't mind sacrificing some size and consistency.


To finish up...

The NW swell shadow is a pretty straightforward feature in hunting for surf in SoCal...but you still need to be aware that it is there. A good rule of thumb is that as swells approach the 295-300+ degree range your surf spot options get more and more limited. Once the swell goes above 300-degrees most spots will shut down completely unless the swell period is super long so don't spend a lot of time driving around as the swell gets steeper. Oh and make sure to do a few special wave dances so that we get mother nature to kick out plenty of W swell this winter!