Valid weather forecasts?

Jane Farraday

Jane Farraday

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As a sailor I'm always looking at the weather forecasts. Generally speaking they don't seem very reliable. What does anyone think is a reasonable length of time to forecast the weather for, with any reliability?
 
Jane Farraday said:
As a sailor I'm always looking at the weather forecasts. Generally speaking they don't seem very reliable. What does anyone think is a reasonable length of time to forecast the weather for, with any reliability?
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Whoop! Whoop! Whoop! Can of worms alert!

Whoop! Whoop! Whoop! Mysterious newbie alert!

Seriously though . .. the 24/48 hour inshore forecasts in Scotland are generally not too bad in terms of weather progression, though people in the Solent seem a lot fussier. The thing with the inshore forecast over 24 hours plus the 24 hour outloook is that in unsettled periods the weather may well arrive in the sequence predicted but the timing can be well out. We plan passages with that in mind. UK coastal forecasting is probably among the most difficult in the world.

Further offshore the GRIB s are as good as anything and often surprisingly accurate up to 5 days. I think the GRIBS are as good as it gets for long passage planning, but after 4-5 days you are very definitely on your own.

It is unlikely that weather forecasting will ever be 100% accurate, and the wise sailor plans accordingly.

- W
 
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Hello, yes I'm the new girl round here. I have no idea what Webcraft is on about. I'm just so keen to get on the water. Every week I watch the weather forecast and so often it's wrong. Am I looking at the wrong sites? I'm just interested in getting on the water and doing some sailing.
 
Jane Farraday said:
Hello, yes I'm the new girl round here. I have no idea what Webcraft is on about. I'm just so keen to get on the water. Every week I watch the weather forecast and so often it's wrong. Am I looking at the wrong sites? I'm just interested in getting on the water and doing some sailing.
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This has been a somewhat controversial subject in the past - many on here hold the Met Office in low esteem. Never mind about what GRIBS are etc just now - here's a few links to sites we use get you started. You should be able to easily find out what GRIBs are though - I am sure you are not a 'dumb blonde' and know how to use Google.

Inshore Waters Forecast

Shipping Forecast

Windguru

Magic Seaweed (Excellent GRIB-based site)

Guys, poor old Jane needs some friends - looking at her profile her only pal on here at the moment seems to be the Crimson Vicar. I expect Lovezoo will sign up in a minute though.

- W
 
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Would you pay more for a better-or kinder- forecast? What sort of accuracy (to, say, one decimal point) might be deemed acceptable ?

Yer takes what yer gets or forever sit on the beach, sorry.
 
Wow, where do we start with that one?

Firstly, the Shipping Forecast and Inshore Waters forecast are ALWAYS your first port of call. Then, make your own observation of the weather, and if this doesn't match up with your expectations fro the forecast...something is wrong...be aware!

Choose two or three websites and stick with them. There are several discussions on here about sources of weather data. My three top picks for UK sailing are:

1. Weatheronline.co.uk
2. BBC Weather
3. Met Office

(Oh, and mine is at weatherweb.net but that would be advertising wouldn't it?)

Now, don't be tempted to search around the web until you find a) the forecast you want to see or, b) the site that looks 'sexiest'

As for accuracy. For planning things are generally okay 3 to 5 days ahead, and by planning I mean "I wonder if it will be okay to sail this weekend?". For actual sailing 24 to 36 hours is about your limits.

You also need to educate yourself about the weather. It's a huge subject and those who make the most of the weather are able to make some basic predictions themselves (plug for my Weather Whys DVD). ALWAYS REMEMBER a forecast is exactly that, a forecast. It is not a absolute statement of future conditions. Things will change, the forecast will change and you should expect this to happen.

Hope that helps for starters.

Simon
 
Two points everyone should take into account:

1) The size of the grid on which the weather forecasting models operate is quite coarse - and hills ans so on less than twice that size can't be taken into account. So, in many areas of the UK, the correct area forecast will be over-ridden by local effects below the scale at which the forecast operates. The forecasting scale varies, but is of the order of 10-20 miles.

2) Weather is a chaotic system - it was, in fact, the first system where the phenomenon of chaotic behaviour was observed. Tiny errors in initial data will rapidly cause large variations if the forecast. Generally speaking, forecasting works out to the 3-5 days Simon mentions, but beyond that, it becomes increasingly problematic. Under some benign circumstances (settled summer weather) the forecast horizon is further out; under other conditions - like now - the forecast horizon may not be as far ahead as 5 days!

My favourite weather forecast is the one from the Lake District: If you can see Helvellyn, it is going to rain - if you can't, it is!
 
Good question, especially at this time of year!

If you compare Windguru with Magic Seaweed for, say, Plymouth you'll get quite different forecasts for the week ahead! Confused?

Then look at the Met Office site and it really is only short term with any detail. Long term (ie over 5 days) is rather vague.

Learn to understand the pressure charts (which the Beeb seem to have abandoned in dumbing down). If you can see what is developing out in the Atlantic, where most of our weather comes from, that will be the best indicator for the next 3 to 4 days but as mentioned above, go nowhere without checking the 24 - 36 hrs updates.

Just looked at weatheronline and weatherweb, both much better with winds and surface pressure charts.
 
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Well Jane Farraday, was any of that information of any help to you?

What sort of boat do you sail and where, if you don't mind me asking?

- W
 
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It's a shame that the start of this thread was so much like a troll, as I think it could be interesting.
In the past I have always looked at the Met office forcasts and found them to be reasonable. Last year I started to look at GRIB files as well. I don't try to do my own forcasts from the GRIBs, I just look to see 'how much weather there is around'. That is to say, if there are a lot of systems around, the forcast may not be too reliable. The fewer lows and high, the greater the reliability of the forcast.
I am having some problems finding GRIBs this year as the site I used from the Netherlands has gone offline recently.
Allan
 
Thanks guys for the sites, I'm trying them. It just gets so frustrating when you've been working all week and looking forward to a weekend sailing and trying to plan things. I've had times when I've gone down to the boat and ended up not being able to sail because of the weather. Then in the week, when you're working, the weather is perfect for a sail! That's life though isn't it?

Anyway, thanks again. BTW I am only interested in the weather for sailing, not sure why someone thought my post was Troll like, and I still don't know what Webcraft was on about. There seems to be something strange about these forums, which I've not met before.
 
Hi Allan,

Interesting you are starting to use GRIB, and sounds as if you are doing the sensible thing by getting to understand how to use the data supplied as a GRIB file before using it operationally.

There are several sites around from which you can download the GRIB data. What GRIB software are you using and for what areas/what info were you downloading? I can try and point you in the right direction.

Best wishes,
Simon
 
Jane Farraday said:
T

Anyway, thanks again. BTW I am only interested in the weather for sailing, not sure why someone thought my post was Troll like, and I still don't know what Webcraft was on about. There seems to be something strange about these forums, which I've not met before.
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I would have thought that someone from Wonderland would be well at home here - we try to accommodate all types, don't we guys? Mortals and supercools....

PS, If there was a serious point to your question, you had lots of advice above and could look up more on the search facility. Local radio is your best guide to the short term outlook.

Let me try to be helpful in another self-help sort of way; the UK sits on the boundary between the Atlantic temperate zone, mostly consisting of low pressure systems driven along by the famous jet stream, and the continental high pressure zone. Looking at the charts in the papers (and the skies) will tell you which is in residence, and which will win out in the next few days. The rest you know - deep depressions moving in, say, 890 mb or less and we're in for a blow. Highs of 1020 or more and winds will be kinder, the weather more settled. The more the difference in gradient between the two systems, the more wind will be generated.

My colleagues will recognise that in simplifying matters, I'm trying to be helpful. There's an awful amount of detail behind the daily picture, but it always comes down to one decision - to go out or not? Most times that can just be a matter of comfort.

PWG
 
PeterGibbs said:
I would have thought that someone from Wonderland would be well at home here - we try to accommodate all types, don't we guys? Mortals and supercools....

PS, If there was a serious point to your question, you had lots of advice above and could look up more on the search facility. Local radio is your best guide to the short term outlook.

Let me try to be helpful in another self-help sort of way; the UK sits on the boundary between the Atlantic temperate zone, mostly consisting of low pressure systems driven along by the famous jet stream, and the continental high pressure zone. Looking at the charts in the papers (and the skies) will tell you which is in residence, and which will win out in the next few days. The rest you know - deep depressions moving in, say, 890 mb or less and we're in for a blow. Highs of 1020 or more and winds will be kinder, the weather more settled. The more the difference in gradient between the two systems, the more wind will be generated.

My colleagues will recognise that in simplifying matters, I'm trying to be helpful. There's an awful amount of detail behind the daily picture, but it always comes down to one decision - to go out or not? Most times that can just be a matter of comfort.

PWG
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Fair advice.."The rest you know - deep depressions moving in, say, 890 mb or less and we're in for a blow"
If there's any chance of 890mb at sea level I for one will be hiding in a basement somewhere ;)

Graeme
 
simonjk said:
Hi Allan,

Interesting you are starting to use GRIB, and sounds as if you are doing the sensible thing by getting to understand how to use the data supplied as a GRIB file before using it operationally.

There are several sites around from which you can download the GRIB data. What GRIB software are you using and for what areas/what info were you downloading? I can try and point you in the right direction.

Best wishes,
Simon
Click to expand...
At present I only use GRIB files to see if the features are tracking as they have been predicted to track. In the future I hope to be able to do some of the forcasting for myself, using GRIB data. The site in the Netherlands I used to use seemed to publish scans of Met office GRIBs with predictions for up to 120hrs ahead. One thing I have done is saved the 3day predicted GRIB a couple of times in the week before I any distance and then compare them with the actual situation on the day they predict. I have found that if they are close to being correct, the next few days forcasts pretty good.
Allan
 
Hi Allan,

Thanks for that. I wonder if you are actually using GRIB files?

GRIB files are data files which you download and then decode into a piece of software which displays wind and pressure etc... The Met Office do not actually distibute GRIB files 'freely'.

Are you actually referring to the Met Office charts with the fronts marked on them? These can also be obtained now from here http://www.weatheronline.co.uk/cgi-...=3&VAR=modtemps&HH=51&BASE=-&WMO=03534&ZOOM=0

Best wishes,
Simon
 
Yes Simon, those are what I was talking about. I understood that the charts on your site were derived from the GRIB data.
Allan
 
Aha,

No Allan, those charts are initially from model data, then the forecast draws the fronts over the top.

GRIB data is actually compressed files of data which you then 'unpack' on your computer and view using special (but free) software.

Hope that helps,
Simon
 
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