We move to the time of year around the autumnal eqiunox where the earth, moon and sun are all perfectly aligned.

The result ? Unusually higher than normal Spring tides.

With low pressure resident this weekend...circa 980 -1000 mb north to south and accompanying possible gales. Coastal flooding could be an issue.

Here in Folkestone a high tide of 7.64 metres is expected just before noon. Normal Spring tides reach 7.0m to 7.2m. Combine this with the unsettled atmospherics and we have potential coastal flooding impacts possible.

One to watch, especially southern and western coast dwellers. Including the Severn Bore.

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Good shout Steve.
Yes we are going into a period of exceptionally high tides over the weekend into early next week. Two severn bores are predicted as 5 star, the biggest rating. Coupled with strong westerly component winds on occasions should push extra water up the Bristol Channel. Expect people will have to park their cars carefully if they are going to watch!
Elsewhere, coastal topping/flooding at least is likely to be a feature.
Could see a few headlines being made and written over the coming days on this high tide/wind combination.

I thought you posted about seeing a prog-rock electronic music group this weekend in here by mistake. 

Being serious, that looks like something I will have to mention in my own weather page today or tomorrow. 

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Hi Steve,

When I read the first part of your post this morning, something didn't ring true, but I didn't think that I knew enough to comment at the time. So I thought that I would polish up my knowledge of astronomy (I used to be very keen on it as an amateur many years ago), and do some reading on the physics of ocean tides. Perhaps I could present what I've found.

At the equinoxes, the axis of rotation of the Earth is perpendicular to the line joining the Earth and the Sun, so as the Earth rotates, all points on the Earth's surface have equal length day and night.  The equinoxes don't in themselves have anything to do with the Earth, Moon, and Sun being in alignment. The unusual height of the spring tides this weekend, associated with the New Moon is most likely due to the fact that the syzygy coincides with perigee (the point in its elliptical orbit where the Moon is closest to the Earth). Because the moon is closer, the gravitational attraction on the ocean waters is greater, so the tides are higher. The coincidence of syzygy and perigee is often called a 'supermoon', and the associated tides referred to as perigean spring tides.  If there was a correlation between unusually high tides and the perfect alignment of Sun, Moon, and Earth, we would expect to see those tides coinciding with eclipses (lunar or solar).

There seem to be a few sources that promote the idea that tides are higher at the equinoxes, but I was rather sceptical about this, and decided to read further.  The physics is highly complicated, with several astronomical factors (sometimes described as harmonic constituents of the tidal wave-function) influencing the generation of tides in different ways.  I haven't been able to come to a definite opinion, but at the moment I would suggest that any effect of the equinox is likely to be an order of magnitude less than the effect of the Moon being at perigee, and possibly pretty negligible.

If you (or any other reader) wants to investigate further, the Wikipedia article on Tides is a good introduction, but doesn't go into any effect of the equinox. I have also read the following internet sources, and there may be others I've not discovered.

https://faculty.washington.edu/luanne/pages/ocean420/notes/TidesIntro.pdf

https://tidesandcurrents.noaa.gov/publications/tidal_datums_and_their_applications.pdf (Pages 3-13 'Tidal overview')

https://academic.oup.com/gji/article/168/3/999/2044447

In the last one, an academic paper, it does say that peaks in solar semi-diurnal tides occur at equinox, but if anyone here can understand the sentence further down that explains why this is, they're smarter than me.

Angus

 

 

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Thanks for the input and links Angus.

I find the whole subject quite fascinating and will research this further when I get a little time at the weekend. 

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Good shout Steve.
Yes we are going into a period of exceptionally high tides over the weekend into early next week. Two severn bores are predicted as 5 star, the biggest rating. Coupled with strong westerly component winds on occasions should push extra water up the Bristol Channel. Expect people will have to park their cars carefully if they are going to watch!
Elsewhere, coastal topping/flooding at least is likely to be a feature.
Could see a few headlines being made and written over the coming days on this high tide/wind combination.

 

 

I was hoping to watch the 5 star on sunday evening,unfortunately most of the places I watch and film it during the daytime bores aren't great for filming when its dark,i may try at the Severn Bore Inn,which I think has floodlights.The last 5 star bore back in march which coincided with the spring tides and equinox was a huge disappointment and anti climax,river levels were too high and too much fresh water coming down river from rain which fell over several days.The resultant bore was not much better than some 2 star bores I have seen.Im also hoping to watch the 4 star bore the following morning,weather looks calmer and touch wood,should get some drone flights with some surfers trying their luck.

 

The river level may lessen the bore,but at least the wind is coming in from the right direction and also lower pressure which can increase the height of both the bore and following tide.

 

https://www.youtube.com/watch?v=ZZ9ey-05tcM&t=440s

 

That's one of my drone videos of the severn bore

 

 

 

 

 

 

Thanks, Steve. It certainly is very interesting.  Perhaps you might post something on what you've found out, and we could possibly help each other develop a clearer understanding.  The article in the first link that I gave seems to me to be the best explanation that I've found so far, though I do have some queries about one or two points in it.  If I was in a class where it was being presented, I would be raising my hand at one or two points and asking 'Excuse me, I'm not sure I follow this, shouldn't it be ....?'

[ETA] I've looked back at that first link, and the relevant explanation starts at page 4.  I don't actually think that it is a particularly good explanation, and I could possibly do better myself.  If you can find a better one, I'd be glad to be shown it.[End edit]

However, it does give an understanding that allows you to read these comments: https://astronomy.stackexchange.com/questions/16732/why-are-tides-biggest-during-the-equinoxes/17816 and assess them for credibility.  I don't think any of them have nailed the answer with clarity and completeness.

An example of a seriously muddled and flawed explanation can be found in the modules of this course: http://freedomsailingscotland.com/5a-causes-of-tides/ where, for example, the second sentence of the module on Equinoctial Tides (click on the list on the RH side) can only have been written by someone who didn't have the faintest clue about what they were writing.

The problem, as it seems to me, is that the subject is so complex that an accurate explanation is beyond the comprehension of most people, and a simple explanation is so inaccurate as to be practically wrong. (Rather like the weather, I suppose...)

 

 

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