NOAA dominant period

Yesterday’s Southern California NOAA forecast:




My question is, given the same Combined Seas Height, what difference does it make to a kayaker what the Dominant Period is? Why would anyone pay attention to this number?

Waves Shape /Spacing
Yesterday off of Scripps pier there were 10 ft swells at 6 seconds, so although the dominant period in the report is a bit longer the actuals were a bit shorter; the shorter the dominant period the more local wind driven the swells are and the more breaking wind waves you will encounter. If you try to paddle off the beach in short period swells that are large you will take a real beating. Definitely not a time for an inexperienced paddler to be out on the ocean. Long dominant periods like 18 and 20 seconds mean big powerful surf from a great distance, a long time between the waves to paddle out but if you get hit by a large swell with 20 second period it will have enormous power. Typical numbers around here are 2 -4 ft swells at 10 seconds. Reasonable beach launches and landing if you have a little bit of instruction. Sign up for the surfzone class from Aqua-Adventures.

Waves are Dynamic not Static
Waves combine and create bigger waves all the time.

A forecast is an “approximation” a snapshot in time,

Period is huge factor
4 foot swells at a long period are not a problem for most with some experience, and that is not an uncommon height along much of the US coast.

As seadart said above, 4 ft height at a short period is steep and much more likely to have a sharp breaking top. It’ll capsize a lot of folks who would be OK if it was long swells. It can also make it impossible for paddlers who are not comfortable on edge to be able to turn, maybe trapping them somewhere offshore that they don’t want to be.

Part of paddling open water is to learn what the norm is for a given situation. Any variation shorter in time or steeper in height or wind speed is cause for caution.

Actually pretty accurate
The NOAA predictions are based on hundreds of buoys and and satelite observations. THey are usually roughly accurate allowing for local effects that can cause a lot of variation. That’s where local knowledge and experience comes in.

If the NOAA site says 10 ft swells at 8 seconds, good idea to take it seriously.


– Last Updated: Apr-15-12 10:25 PM EST –

Clarification - NOAA gives decent forecasts but actual
conditions "may" be tougher than what you planned on.
I concur that "locals" know the area best, ask them

NOAA numbers
Very rough estimations -

Significant Wave Height is, roughly, twice the average wave height.

1 in 10 waves, roughly, will be at the Significant Wave Height and above.

What is the point? - if you go for a paddle, and the waves don’t seem very big for the forecast, don’t go cursing NOAA, just wait and see.

Also, when surfing waiting is the name of game - the bigger waves will come, you only need to wait.

Wikipedia has a decent overview

“Why would anyone pay attention to this number?”

I always thought it’s surfers who pay attention to the period as much as wave height.

both touring and surfing
on wide open water a longer period is gentle and depending on wave height barely noticeable, but very short periods especially with bigger height make for an exciting trip.

Closer to shore a longer period is associated with bigger surf for some given height. I also find that longer period swells tend to result in longer lulls between sets which is important because you can get fooled into thinking an area is safer than it really is.

open water
I don’t do much surfing but do go into open water. I don’t have a feel for the numbers. Is 8 sec short or long?

Personal comfort level…
It really comes down to what you are used to. Again, if you are already paddling open water you should have a sense of what you are accustomed to by checking the marine reports before you go out and looking at historical info after you have gotten off the water.

If this is not a habit you have developed yet, you should start doing so now.

8 seconds at 2 feet is usually not an issue, at 3 feet it can get somewhat interesting and at 4 feet more so. It comes down to the paddler and their own experience whether they can paddle as interestingly as the water they are on.

some folks in a group
get rattled when they period and amplitude of the waves makes their buddies disappear from view between waves.

So to figure your comfort level its a good idea as Celia notes to make a record.

It’s on the short side

– Last Updated: Apr-17-12 12:07 PM EST –

Less than eight seconds with swells bigger than 4' and it will be pretty bumpy. It sounds like you only paddle out through the jetties.

Watch the daily posts on the NWS forecast site when you go out and you can get a feel for conditions from the predictions.

Make sure you learn how to do surf landings in bigger waves on unprotected beaches, if you have to make run for shore. Storms do come up from time to time, and you don't want to be stuck outside the channel in huge waves, as you probably know the channel mouths can form huge haystack standing waves and you could be in real trouble if you don't know how to land in heavy surf. There have been several rescues of "experienced" kayakers at the channel mouths during unexpected wind/wave events who did not have the skill to get back to shore.

Pitch pole - Pearling -

– Last Updated: Apr-17-12 1:43 PM EST –

Waves coming from the rear can be nasty

Could quickly become a "yard sale" at the beach

pitchpoling not that bad for the paddler
It looks more dramatic than it is since the paddler is just rotating on a much smaller axis than the ends of the kayak.

While most paddlers like long intervals between waves, remember the longer the interval, the fast the wave is traveling. You need this knowledge when paddleling out through the surf zone. Time it wrong and you’ll take a pounding. I generally stay on the beach for anything 8-ft or more. Ideal is 4-6 for surfing, 2-4 for cruising along the coast.

MB Channel
Yes, I only go out the channel. Only if the lifeguards report “slightly rough” or less, and only if it looks ok to me. I usually go north to Crystal Pier and back from the Bahia, usually alone. I have done this paddle many times over many years.

I don’t have any intention of learning to land on unprotected beaches in large surf. Too old.

This the channel by Mission Beach?

– Last Updated: Apr-18-12 8:04 AM EST –

It's the nearest one I could find to a Crystal Pier in CA. Then an open paddle north to the Pier.

If you have been doing this paddle for years without having found out why short period can be a problem, you would probably find out that the lifeguard rating of slightly rough is associated with fairly long periods and lower wind speeds. (and that you have avoided sudden squalls)

Mission Bay Channel
Sudden squalls in San Diego are exceedingly rare. The open ocean is not usually too rough, except in storms. Other than surf launching and landing from the beach, which I almost never do, the worst conditions to be found in San Diego waters are for about 50 yards before and beyond the channel entrance. Even when the lifeguard report for the channel is “rough for navigation”, the open ocean is often not that bad. When the lifeguards report “hazardous for navigation” or “closed to navigation”, the channel should be avoided. However, the lifeguard report does not generally mention swell period; the NOAA report is required for that.

I have been transiting the channel for perhaps 25 years, once or twice a month, and regularly get the lifeguard report. Only recently I started getting the NOAA report including wave period, but I don’t know what to do with that info. I also don’t know what to do with barometric pressure.

Barometric pressure
Re the conditions, it appears that the open ocean area is relatively consistent there. Otherwise you’d have found out what short period means before now. :slight_smile:

Barometric pressure is most important in its changes. High pressure means relatively good weather and increasing pressure means improving, very low pressure means bad weather and dropping pressure means bad weather approaching. An extreme drop in pressure is time for ships to haul off to safety, fast.

Take a look at the swirls of clockwise (high) and counterclockwise (low pressure) swirls on the weather channel or whatever. You’ll start to see how they interact with each other at the boundaries.