Tragedy has struck again...Bodega Bay

Longer period swells are correlated with calmer water.  Here's why:

Storms cause swells.  The storm itself is a jumbled, nasty, choppy sea state.  The longer the storm blows in the same direction (called "fetch") the longer period energy it will create.

Swell decays as it travels.  Like a pitted surface being sandblasted, the smallest bumps disappear first.  As a swell moves, the bigger, longer period energy survives more, while the shorter period chop gets cut down.

Long period swells usually come from far away, like the Aleutians or middle of the Pacific, because (a) there's enough ocean there to generate big fetch; (b) the long period energy is what's left of faraway storms after the travel decay; (c) long period energy moves faster than short period energy, so it separates and arrives first without additional short period energy chopping it up.

Short period swell tends to be more locally generated, like from the Cape Mendocino gradient often present in spring.  There isn't enough fetch to form long period swell, and the close distance to the swell generation wind means the periods haven't had space to decay or time to separate.

Synopsis: Longer period swell generally means you're farther from the storm, so it has time to smooth out.  Pure, unmixed long period swell also tends to come in fall/early winter or mid summer, when we have relatively high pressure sitting over the coast for good conditions.  Mixed swells (many hitting at once) will jumble things up.

Stormsurf is a fantastic swell model site.  I use it religiously.  However, its forecast tends to slightly undercall swell from a North Coast perspective (it centers on SF, which gets less pure swell than the Sonoma/Mendo coasts) and it undercalls south swells.  The NCal Stormsurf page includes tabs for specific locations - make sure you're clicking on those to check them.  Surfline and MagicSeaweed are your other options.

Think of it as a box.  If a 5" by 5" box of lead weighs 20#, the a 10"X5" box is going to weigh twice as 40#

Now, swing each box on a rope into the side of your wife's car.  Which will make the bigger dent?  they are both only 5" tall, why is that 2d dent so much worse?  Same thing with waves.  10 foot at 20 seconds can be a wave that is only 10 foot high above the surface, but 100 foot wide, in terms of where the wave actually begins and actually ends.

I've always looked at it like this.  There's a storm way out there with wind that generates chop.  The chop becomes directional with the wind pushing; and start to clump up, making wind waves.  These waves are essentially, potential energy, and over a distance will coalesce into each other, and  become more coherent, or "regular."

At this point they are on their way, even with no wind blowing.  They are going to travel the deep water which offers little resistance until they reach a bottom shallow enough to drag the bottom of the wave.  Increasing shallow bottom will slow the bottom (under=water) part of the wave enough so the face steepens, angle increases, and the wave breaks.

Here is a short piece that describes it better than I can:

http://oceanservice.noaa.gov/education/yos/resource/JetStream/ocean/waves.htm

Here is an ever better description from surfing POV

http://www.surfline.com/surfology/surfology_forecast2.cfm


Bushy

It all sounds swell, deep conversation, lasting a long period.

It all sounds swell, deep conversation, lasting a long period.

Rich just likes the part where they "hump up."

It all sounds swell, deep conversation, lasting a long period.

Rich just likes the part where they "hump up."

*groan* 

  Two more tidbits. There was just a large storm near New Zealand in the news. That will perk up the attention of surfers thruout the whole Pacific, including surfers that have south facing beaches in Cal. This is what surf forecasters do.
  Mark Foo died on his first wave at Mavericks in 1994. He surfed that same swell when he was in Hawaii a day or two before that. Then got on a plane and flew here. There's buoys everywhere so it is easy to predict. I heard about the high surf warning from the channel 5 weather on wednesday. It hit Friday just like NOAA said it would.   

Another cool website. Earthnull.

  Two more tidbits. There was just a large storm near New Zealand in the news. That will perk up the attention of surfers thruout the whole Pacific, including surfers that have south facing beaches in Cal. This is what surf forecasters do.
  Mark Foo died on his first wave at Mavericks in 1994. He surfed that same swell when he was in Hawaii a day or two before that. Then got on a plane and flew here. There's buoys everywhere so it is easy to predict. I heard about the high surf warning from the channel 5 weather on wednesday. It hit Friday just like NOAA said it would.

That storm looks extremely powerful - seas above 55 ft - but it's aimed more westerly than northerly.  Chile is going to get pounded.  Magicseaweed has two forecasts just on the edge of their range for the North Coast - 2.5@19 186 deg and 1.3@26 182 deg.

2.5 @ 19 will be head high at OB/north Pt Reyes/Salmon Creek/open beaches, probably 4' at Bolinas and the shallower south Pt Reyes beaches, anywhere from 2-6'+ in Santa Cruz.

1.3 @ 26 will be imperceptibly small, but that size at that period suggests we could be looking at 4+@20 or 5-6@17 when the real energy hits.  That would be a fairly large south.     

Swell is hitting this weekend and Monday.  3.5@22, 4+@20.  That means 8-10 foot breaking waves at south exposures.

However... there's another 3@15-16 south swell overlapping it, which means 4-5 foot breaking waves.  These can combine to form 12'+ plus breaking rogue waves at times. 

Moral of the story: not a good weekend for kayak fishing unless you really know the reefs and have a protected, north-facing spot.

I was reading the swell all wrong. I too, like some others thought it's better to be out there on 5@17 than 5@10.
appreciated the thread.

Well, if you're fishing offshore and launching from a protected spot...then longer period translates into a smoother surface. Once I get outside, I prefer a long slow predictable/consistent rolling swell...compared to a short choppy unpredictable swell.

To get a sense for how forecasts translate into actual ocean conditions, you have to look at a whole matrix of variables. And swell periodicity is just one of those variables. In my own personal matrix, I would rank it 5th or 6th most important factor after swell size, wind strength, swell direction, wind direction, and tidal fluctuation.

Well, if you're fishing offshore and launching from a protected spot...then longer period translates into a smoother surface. Once I get outside, I prefer a long slow predictable/consistent rolling swell...compared to a short choppy unpredictable swell.

To get a sense for how forecasts translate into actual ocean conditions, you have to look at a whole matrix of variables. And swell periodicity is just one of those variables. In my own personal matrix, I would rank it 5th or 6th most important factor after swell size, wind strength, swell direction, wind direction, and tidal fluctuation.

All great points Pat. I think people sometimes forget tidal influence on your drift that makes for a more efficient day.

Alex

  Long interval swells are what kills people. Because there is a long time between sets. That's what a sneaker wave is. 5' 17 seconds could cause a 15' face on a breaking wave. When they come in NOAA reports warn about a long interval swell. That's a hint. If anybody is still confused, ask a good surfer. They drool when they see 5' 17 seconds. That's another hint.

i could see the concern on long period swell.
if i was fishing the pound zone, or surf launching but if im outside the break whats the issue.

i could see the concern on long period swell.
if i was fishing the pound zone, or surf launching but if im outside the break whats the issue.

If you're actually outside the break, the main issue is getting back in.  Didn't some guy get killed last year trying to make it over the Dillon Beach bar in a large swell?

If you only think you're outside the break because you don't know where the outer reefs/boils are and you didn't see a larger set break out there, then you have another issue.

If you're diving, long period swells cause a lot of surge.

If anything goes wrong outside the break, your problem is thoroughly compounded by the ocean conditions.