Which weather forecast to believe

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JamesS

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Sunday - ready to go off for a week in the Solent.

The weather forecast linked to this site reads 'SW 6 to 8 locally 5'

The met office inshore waters site reads 'W 5 to 7, increasing 7 to 9'

Both forecasts for the area Selsey Bill to Lyme Regis and presumably both from the same source.

Who do you believe?
 
Both! They are basically saying the same thing, its going to be windy! From the west - south west. But were they issued at the same time? One may be more recent than the other.
 
Yes, you are right it's going to be windy!

From what I can see, both issued at the same time and same source. It may be that there has been some editing somewhere down the line.

Cheers
 
I think it always pays to believe the worst one and you wont get caught out to often. Ever since the Michael Fish 'Hurricane' weather forcasters tend to err on the pessamistic side.
 
None of them and all of them. When I did a Met course years ago I was taught that with luck I would be right about 60% of the time, with modern aids that may have improved but not by much I think.
The problem is that a depression can easily speed up so the weather and wind shifts arrive early, or it can stall so they arrive late. It can also be deflected when it hits the land, sometimes a low will ge deflected up the Bristol Channel.
You alo have to remember that the forecaster is working on data that will be between 6 and 12 hours old although satellites may have improved that, he then needs time to make his forecast, and it then has to be sent out, so it is already a few hours out of date when you get it. Two people will not necessarily interpret information the same way, and indeed there are different computer models which give different answers, The shipping forecast covers very wide areas whereas Southampton covers a localand much smaller area which again makes a difference.

I would agree that the forecast for this weekend is windy and you can expect the force to be about right. I usually allow for up to two forces above the forecast if it is up to six, and for the winds to be up to six hours early or late.

In the end, listen to what they have to say, look out of the window and make your own mind up.
Its your call

Chris Stannard
 
As a sailor you should not take too much notice of the actual wind speeds and other forecasts 'results' which are broadcast publicly.
Instead study the synoptic charts, note changes and movement of Lows and Highs and make your own decisions. Many is the forecast gale or strong wind during summer months in English latitudes that never turns up. Of course, it can also work the other way round.
 
As has already been said, the two, practically speaking, are not all that different at all.

A check I make is to look at the Marine Buoys actual,

http://www.ndbc.noaa.gov/Maps/United_Kingdom.shtml

reading the last 24 hours history that comes with each, and then looking at the buoys further into wind to see what's happening there.

Piers du Pré
MBM Cruising Club enthusiast
www.dupre.co.uk/fsPlaydeau.htm
 
Both forecasts are very similar but with a bit more detail on the Met office forecast. The inshore waters forecasts for the Solent have been spot on this weekend. It was forecast to be very wet and windy last night (which it was), decrease a bit this morning (when it was blowing Force 6 gusting 7 in the central Solent) and then increase this afternoon (and rain lots) which I think it has.

With either forecast, if you feel comfortable in the Solent in a gale you would head out and if not, you wouldn't.

Richard
 
The inshore waters forecast usually seems to be deduced simply by knocking one force off the shipping forecast.

One would normally expect the "increasing 7 to 9" part of the forecast to specify approximately when it's expected eg. "later" meaning in 12-24 hours. It may be that the two forecasts cover different periods eg. if the first forecast covers 12 hours, and the inshore forecast covers 24 hours and the increase is to happen "later", then that would explain the difference perfectly. Not having seen the forecasts to which you refer, I can't confirm whether that's the case.
 
The thing to remember with wind forecasts is that the forecaster has calculated a 'mean wind speed' from the pressure gradients expected in the forecast area and period.

As only a relatively minor change in pressure gradient can increase a F7 to a F9, forecasting is is not a precise science - although a good forecaster should be able to tell if the PG is likeley to vary from what the computer is saying.

But its the business of 'mean wind speed' that is most confusing - and its nothing to do with that head wind that seems always to set in whichever way you plan to go!

Mean wind speed is the average wind speed expected across an entire area. It takes little or no account of gust speeds, and in an unstable airstream such as the one we are experiencing in the South at present , it has very little meaning, except as an indicator of the speed at which the varying weather local regimes will come through. Looking out of my window now the wind speed can be little more than 15 - 20 kts at most, but 4 hours ago it was averaging 40kts, with gusts in excess of 50kts. Judging by whats up wind, it will be back up to 40 - 50 kts within an hour or two.

So to ask which forecast is 'right' F6 -8 or F7 -9 is unrealistic, and asking more of the forecasters than they can give. In any case what realistically is the difference at sea between a top end F8 and bottom end F9 ? a matter of only a few knots.

The fact remains that if the forecast mentions F8 or 9 then it is going to be very rough and very windy and the prudent skipper makes his decisions based on the ability of his boat and crew to stand up to violent conditions.

And with the big storm cells that we are experiencing, if we do go out we should be prepared to have to cope with gusts much higher than the forecasters are giving. Solent CG in these conditons often give a gust forecast. Even these should be not be taken as any guarantee, as local topography such as the high ground of the IoW can modify and increase gust speeds quite disproportionately. Anyone who lives or works near tower blocks will have experienced this effect locally ashore.

'Pity the poor forecaster'
 
While it might not apply to your particular problem a common mistake when listening to forecasts is to compare Sea Areas as used in the shipping forecast and the inshore waters. Example: Sea Area Plymouth covers a far greater area to the south than, say, St Catherine's Point to Land's End in the 0540 Inshore Waters on BBC R4. While gales might be expected off the French coast a gentle F 3 could be forecast for the English coast, but both are within in Sea Area Plymouth. Guess which one is in the shipping forecast?
 
James, the only forcast to believe is yesterdays, as for today and tomorrows I always take the worst and use that as a basis for what "might" happen.
 
It's not really a matter of believing one or the another. They're both indicating much the same . Taking into account that they're both probably accurate only within one or two points of the Beaufort scale , they both point to a strong wind , possibly reaching F9 , and I would take action based on the worst possible scenario. my action in this case being to open a good book and turn on the radio or possibly to put on a pair of walking boots and keep them very firmly planted on terra firma.
 
A contradiction

Reading these replies, a recurring theme is that forecasts are so difficult to make accurately that they should be taken with a pinch of salt anyway.

Elsewhere on these fora I have sometimes heard that modern yacht design doesn't need to be as bulletproof as in times past because with modern weather forecasting there is no need to be caught out in a blow.

Am I the only one to feel a contradiction here?

One incident where I was about to set off on a solo trip up the Channel the year before last springs to mind. The Met Office website that evening was predicting F0-2 for the next 5 days. 12 hours later the wind was already doing F6/7 and soon increased to F10, causing widespread damage including to my boat even though she was in harbour.

Luckily I had waited until morning before setting off so didn't set off. Isn't modern forecasting wonderful?
 
Re: A contradiction

I think you're winding us up? You can remember the day and time last year? Go to:
http://www.wetterzentrale.de/topkarten/tkfaxbraar.htm
and find the synoptic chart for that day - F10 should be unmistakable. You can then see the next 5 days.

The "inshore" forecasts on this site are not the same as the Met office inshore forecast. This forecast splits the South coast at St Catherines Point. Wherever it comes from, I don't think it's useful. With SW wind funneling up the channel you're going to get markedly higher wind strength at North Foreland than St Catherine's Point.

FWIW the Americans, bless 'em, can now ring an offshore weather buoy and extract current conditions. So next time you're sailing in the Atlantic and hear "brrring...bbrrring" coming out of a buoy, you'll know what it's for!

After the disastrous Sydney-Hobart race, the Aus Met office remarked that their wind estimates were mean and there could be sustained gusts higher than forecast. There weren't excusing themselves, just stating a fact as few people seemed to appreciate that (and they were being heavily criticised).


<P ID="edit"><FONT SIZE=-1>Edited by ParaHandy on Mon Apr 29 16:10:04 2002 (server time).</FONT></P>
 
Re: A contradiction

It wasn't last year, it was the year before. If I remember correctly it was around October/November 2000. I may be completely wrong but looking at the records that you pointed out, I have a suspicion that I may have looked at the MetWeb site on 21 October 2000. I would need to check logbook (it's on my boat) to see the date I eventually sailed to Portsmouth to confirm at least the month.

Look how the winds developed the next day and for example on 25 October 2000. Is there somewhere on that site that gives actual recorded windspeeds?

The storm was the one that hit the front pages of all the newspapers as causing flooding and other damage across the UK and caused an awful lot of global warming debate. It took a few days for the storm to work up full strength, so I'm not saying it was that strong the next morning. Damage caused to my boat in Brighton were 2 burst fenders, damaged toerail and two severed mooring lines (luckily I'd doubled them up before leaving the boat or it would have been worse).

I had checked Met Office website (using the 5 day forecast on the paying MetWeb section of the site) in the afternoon, planning to sail Brighton - Portsmouth next morning early. Forecast was basically for no winds for 5 days, and I thought I'd have to motor the whole way. Set the alarm clock for early morning, but didn't need to. By that stage the wind was howling in the rigging enough (I estimated a good F6, probably gusting F7) to tell me I wasn't going. The wind continued to increase and the waves were shooting high into the air over the harbour walls, so I decided to push off home. A few days later the gale/storm reached its peak. I don't have details of windspeeds so can't be certain about the F10, but judging by the damage done, it must have been something like that.
 
Correction

Looking again, it seems more likely that I may have checked the MetWeb site on October 26th 2000, not the 21st. See how the weather developed up to around the 30th (still within a 5 day forecast). Still, unfortunately I can't be sure of the date without doing a fair bit of research.

OK, so 5 day forecasts are not supposed to be accurate, but the weather had in effect gone seriously pear-shaped within a few hours of me checking the site. I wonder if there are archives of what was published on MetWeb?
 
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