PDA

View Full Version : Sea state and the UK Inshore Waters Forecast??



bluedragon
19-08-07, 15:25
I made a comment in the Bristol Channel section of the Forum earlier today on the topic of sea sate, but I think it's worth wider discussion. My gripe is that the Inshore Waters Forecast uses the "offshore / deep sea" definitions of sea state and applies them them to shallow inshore waters with strong tides, overfalls, headlands, sandbanks, etc, and thereby lulls us into a false sense of security. The definitions are as a follows:

Smooth: Wave height less than 0.5 m
Slight: Wave height of 0.5 to 1.25 m
Moderate: Wave height of 1.25 to 2.5 m
Rough: Wave height of 2.5 to 4.0 m

Now we all know that 1.0m wave height with a long period in open water is barely noticeable. Bring that same wave height inshore and steepen the gradient by tide or nature of the bottom and it can be anything but "slight"!! It can be downright rough, or even dangerous to small craft. How often have we gone out with a sea state forecast of slight - moderate and feel like we've just done Cape Horn! Sea state is not just a question of height, but period and character of the waves (cross seas for example). I really doubt the Met office have thought this through. If not maybe we are better off without these sea state "forecasts" or at least only use them with a good dose of scepticism. I'm sure the Met Office could do better than this and perhaps use terminology that is more suited to those of us that sail the inshore waters in small craft. What do others feel? Or is it just me?

MoodySabre
19-08-07, 15:34
The BBC coastal forecast as here (http://www.bbc.co.uk/weather/coast/coastalforecast/regional_forecast.shtml?8)
uses the description "wavelets" that I can't find a definition of. Certainly sound more friendly than waves /forums/images/graemlins/smile.gif

Wind direction and seabed profile certainly count for a lot close inshore where a quartering sea and wind aft can make it very uncomfortable even though "F3 sea state slight" might seem ideal.

roly_voya
19-08-07, 15:40
The problem being that there is so much local variation. For example around here (Pembrockshire) with a SW 3-4 I would expect a forcast sea state of moderate. Along the N coast it will be flat, of the heads on turbot bank mod/rough plus the sound will nbe anything from slight to horrendus depending on state and direction of tide because the wind also gets accelerated through them so the forcast 3-4 could become 4-5 againt a 6kn tide. Back the wind only one piont though and the islands become a lee, add neap tide and you are down to 2-3 with 2.3kn current, not much more than choppy.

So unless there is time to give 4 sea forcasts just for pembs has to be the expected sea state where there is no tidal or other local effect and then you apply local knowlage

captainslarty
19-08-07, 15:54
My advice is to dl the synoptics, take note of the forecast, make variation yourself for the area you are sailing. It really isnt too difficult.
Apply headland variations, fetch, bottom contour etc.
Be happy

bluedragon
19-08-07, 17:14
Yes, that's the problem. We're berthed temporarily in Milford right now. Had a rough passage from Tenby two weeks ago in a "slight" sea. The problem I'm hearing / seeing is that there are a lot of newcomers to boating that would logically think "slight" meant something just above "calm"...also to those new to an area. I understand it would be difficult to have four different forecasts for say Pembs. but I still feel inshore sea conditions are being played-down. If you take the wind forecast, it's usually the opposite - take the worst expected for the whole area and apply it to all. My comments in many ways apply to the whole marine weather service we get from the MCGA, Met Office and CG. It's really the bare minimum in terms of useful local data IMO. A lot more could / should be done.

MarkGrubb
19-08-07, 17:58
I'm not sure the role of the met office should include holding our hands. I think the information they give us adequate and the onus should be on the sailor to seek local knowledge before undertaking a passage.

A quick glance at the chart and tidal stream atlas is likely to indicate whether headlands, fast streams, shoaling and rough sea beds etc. are likely to be encountered. If so, then even the most basic RYA training or books tell us that these will modify the wave height, and a prudent mariner conscious of his ability would seek local knowledge - I'm sure the local RNLI station would be glad to offer advice. Those with more experience would not doubt more confidently make their own assessment and plan the passage accordingly.

I sympathise with your frustration and no one likes a rough passage, but even for newcomers I think lessons and skills like these are best learnt by experience.

shmoo
19-08-07, 18:07
The problem is that the forecast needs local qualification. That is, quite rightly in my view, to be found in almanacs, sailing directions, pilots and the like. There are lots of places you wouldn't want to be in F5 or 6 even if the forecast did say "moderate": most of them are listed the sort of publications above or by swirly whorls on the chart.

You wouldn't really expect something like "moderate, except over the Deben bar between 1700 and 1900 when it will be awful, or lively, depending on your outlook"

PeterGibbs
19-08-07, 19:06
Yup, the answer is to go to a local station. We have to accept tha the BBC covers a very wide are in its forecasts, more than any other provider in Europe. It also still has a "shipping forecast" ie big chunks of sea mentality, which can be less than helpful in marginal conditions, as posters are complaining.

In the SW, the Sennen cove site is most useful - ie used by fishermen. We could do with more sites like this.

Otherwise the French meteo service covers swell/wave height and complements UK sources. It's also v good for wind direction - altogether a more small boat friendly service. Try it.

There is a site that gives more detail on the UK - the GB wind map - very good for winds and sea state. Worth a bang too.

PWG

bluedragon
19-08-07, 19:23
[ QUOTE ]

A quick glance at the chart and tidal stream atlas is likely to indicate whether headlands, fast streams, shoaling and rough sea beds etc. are likely to be encountered. If so, then even the most basic RYA training or books tell us that these will modify the wave height, and a prudent mariner conscious of his ability would seek local knowledge - I'm sure the local RNLI station would be glad to offer advice. Those with more experience would not doubt more confidently make their own assessment and plan the passage accordingly.


[/ QUOTE ]

Yes, that's what I do, but bearing in mind that a large part of our coastline comes in this category (ie. local conditions that are different), why then put out a forecast at all that is useless to most / many?? Either get it right for the locality or don't bother. Just give wind information and let each determine his/her own opinion of sea state. It's the misleading element of this I'm really getting at.

PS - I knew exactly what we were in for on our Tenby - Milford trip and had waterproofs, lifejackets and harnesses on. Another yacht just behind us was in the "newcomer" category and didn't have any of this ready and did learn a hard lesson. But what created that sense of complacency?...in part the CG inshore waters forecast.

ChrisE
19-08-07, 20:07
I'm sorry that you've toshed a couple of times but think that you're on a loser when blaming the CG or the Met.

As others have said you need to be able to interpret what is being said. Eg, a SW4 on the ebb on The Bridge on the Western approach to the Solent is a bit of sod for small craft but the same SW4 within the Solent is just above a ripple but both are within the sea area Beachy Head to Lyme Regis. By the same token those conditions on The Bridge are fine for my R38 but probably a bit too much for Wayfarer.

If you can't/ won't care to use a bit of nouse to interpret the forecast and calibrate it to your craft then quite frankly who can?.

shmoo
19-08-07, 20:21
[ QUOTE ]

a SW4 on the ebb on The Bridge on the Western approach to the Solent is a bit of sod for small craft


[/ QUOTE ]

Exactly - and the forecast is not the place to warn about this because it always happens that way.

Sailing directions are the right place for this info:

[ QUOTE ]

The Bridge (5038'N., 139'W.), a dangerous reef, extends
up to about 1.2 miles W of Needles Point. A lighted buoy,
equipped with a racon, is moored in the vicinity of the seaward
extremity of this reef.
The Bridge is marked by ripples in calm weather and by
distinctive overfalls in rough weather. During S gales it is
marked by a well-defined line of broken water.


[/ QUOTE ]
(ENROUTE) SAILING DIRECTIONS 2004
ENGLISH CHANNEL PUB 191

bluedragon
19-08-07, 20:39
OK I give up...end of discussion. Worth a try anyway... /forums/images/graemlins/smile.gif

jonathankent
19-08-07, 22:01
Don't forget, the inshore forecast actually covers up to 12nm offshore, therefore conditions are liekly to be quite different out there than they are along the edge of the coastline.

Our local one (Lyme Regis to Lands End incl. Isles of Scilly) covers such a large and varied area that you have to come up with your own forecast - you can take the wind, but then have to predict the sea depending on wind direction/force.

ChrisE
20-08-07, 09:47
Very magnanimous, if I might say .....

wanderlust
20-08-07, 10:08
If you know all this then why do you not take that in to consideration? I cant see what the issue is. You cant expect the Met office to tell you what you should already know

Brian_B
20-08-07, 15:26
Today's forcast
Start Point to Lands End including the Isles of Scilly
Coastal forecast for Start Point to Lands End including the Isles of Scilly
Note the 51 knots but sea state Moderate and described as F3-F7.
2007-08-20 1200 - 1759
Pressure - 1006 mB R
Temp max/min - 17/13 degrees C
Wind speed - F4-6 becoming F3-7
Wind direction - NW
Max gust in knots - 42 becoming 51
Sea state - Moderate

Longshanks
21-08-07, 09:41
Exactly. What utter nonsense this is. Guess it's come from the BBC? Useless to man or beast IM(not so)HO

Wanderlust - I don't think you read the original post properly?