onshore wind vs offshore wind swell height?

[benjl]

Hey guys

I've been following several different websites surf forecasts / reports for some time and they are usually very closely correlated except for when it comes to off-shore wind swell and size. I follow one website that usually gives a forecast such as wind directions, swell direction and height of waves and wave faces (m's) whereas the other website gives an actual report of the local conditions as they are each morning and talks about the wave height in feet.

Usually they are very similar ie. if one website says the set faces are going to be appx. 2M then the other website usually says the waves will be about 4ft etc.

Where there seems to be a difference is when the wind is offshore? I remember once the surf forecast website said the faces were going to be about 2.5m but yet the actual surf report website said the conditions were only going to be 2-3ft. Given how big the swell had been that week during the 'on-shore' winds, i couldn't figure out how the wave faces would be smaller despite the swell getting slightly bigger. I turned up to the beach and the report was right- the waves were clean, smallish and nice. Not the big dumping 4-5ft waves that had been there a few days earlier when the swell was supposed to be slightly smaller and on-shore.
Even yesterday the set wave face was supposed to be 2.1m although the surf report website said that the conditions were 1-2ft +.


I thought if anything off-shore wind peak the wave faces up and make them bigger than what the swell height would be if they were on-shore?

Any more experienced thoughts on this to help grow my knowledge when analysing surf reports?

Cheers

[jaffa1949]

Let's get down to it straight away. Most websites with forecasts are pretty generic and rarely cover the specifics of a given beac.
Swell has some bearing but only if it is within the radius that effects a given beach , Often I can see 5 to 6 metre swell from the beach but they are angled away so my beach is flat!
Often under these circumstances the swell is being generated by an offshore wind and the stronger it is the bigger it gets heading out to sea.
Nearshore swell generating systems often have a big blunderous surf attached to the onshore winds, because a the major factor for quality swells is missing .
And that is FETCH, for a truly great swell which can follow a Rhumb or great circle line the storm system is a long way away , the disorganisation of the immediate storm swell converts over distance to a ground swell with set period and wave heights from a clear specific direction. Ground swell arrive independent of local conditions and are enhanced or diminished by the local weather.
Then your coastline determines how the waves fit in.

Put on top of all this your local wind strength and consider lee and windward sides of headlands.
Now about the height question, on shre winds tend to push a wave up from the back but crumble down from the face, in extremes they jus blow the whitewater to the beach and sand blast anyone on the beach.
Offshore winds will steepen the waves to a point holding the breaking section up also making it harder to penetrate down the face for a take off. At extreme wind level any rising swell is blown out to sea or at least diminished severely.

The short answer, read the prediction observe your local beaches and understand how those predictions work for them. That knowledge will get you in the water more often than the report and surf cam watching hordes!

I often score great surfs with only 4 to 5 guys ( always the same ones) who know the surf not the report.
Takes about 3 hours for the word to trickle out and I've been and gone

[oldmansurfer]

Over a distance onshore wind builds the waves bigger. Over a long distance offshore winds reduce the size of the waves. Over a short distance reverse the effect. The reason is that over a long distance wind pushing the back of the wave adds power and wind pushing back against the front of the wave reduces the power of the wave and the size but once the wave gets to the beach the on shore blows the top over and off shore holds it up.

[waikikikichan]

Let me put it in a perspective you can understand being an American ........ let's talk Football ( sorry Jaffa, not what we call soccer ).
Imagine a goal line stance on the 1 yard line. The wave is the running back. The line backer is the off shore wind. If the defensive line has a gap, the running back can just walk in for a touchdown. But say the linebacker makes contacts with the running back and holds him up. He might straighten him out, but the running back doesn't "grow". But the wave is much stronger than the wind, it just changes that timing of how the lips goes beyond vertical and "breaks".
Now let's talk about a blocker pushing down a tackler from the back. That foul is called Clipping. On-shore winds are like pushing a tackler down from behind. The tackler falls forward onto his face faster than if he was running on his own pace.

[oldmansurfer]

Think of it like this, the wind is blowing in one direction across a lake on one side little ripples develop but since the wind continue to add energy to the little waves they grow bigger. How big they get depends on how fast the wind is blowing and how far across the lake it blows. When they reach the opposite shore they are waves that break in the shallow water. Wind is what makes the waves and this is why there are huge waves when there are hurricanes. The stonger the wind and the longer the distance the wind blows on the water the bigger the waves are. But once you get to shore the wave starts to stand up and get taller as it reaches shallow water and starts to break. With wind blowing on the back of the wave the lip falls over quicker makking it a little smaller when it breaks but if the wind is blowing into the face of the wave it hold the lip up and allows the wave to be a little taller before it breaks. Typically waves are generated far away from shore and the wind conditions on shore are different from those out at sea where the waves are generated But offshore winds that go way out to sea will affect the wave size by reducing it and onshore winds will increase the size of a swell way out at sea. But most of the time the wind on the shore is related to the land heating up duing the day causing upward current of heated air rising up over the land which pulls air from over the sea causing onshore wind but prior to that the land may be cooling down pulling air from above and creating offshore wind so in the morning it is offshore and switches to onshore in the afternoon. This is only if there aren't other factors so the wind on a shoreline is often just from the land heating and cooling and doesn't affect the size of the wave other than making it stand up taller or making fall over quicker. How it affects the wave depends on if it is breaking and for how long (or distance) it is blowing on the wave.

[jaffa1949]

Let me put it in a perspective you can understand being an American ........ let's talk Football ( sorry Jaffa, not what we call soccer ).
.

Sorry walkikikichan the football I used to play at a high level wasn't soccer either, but it is the second most popular code of the football sports played around the world! Rugby Union
Uncle Jaffa was a Front Row