XC Weather Disclaimer

[Seatrout]

Yesterday I ventured out with a friend and her two boys 12 and 14, neither of whom had been on a yacht before.

Preparing for the trip to sail them back to the island they came over from in the morning, I checked XC weather among others. 15mph gust 28mph reducing to 13 and 25 respectively was the wind forecast. I set off reefed in with the idea of later taking the reefs out.

Instead of decreasing, the wind increased and INCREASED! At the point we had a 47knot gust over steady 35 knots it was time to return. Took the main down and sailed ion a run with a 2/3 reefed jib doing 9 knots!

Now while it was rather exhilarating for me, the youngsters, although saying they enjoyed the day, were rather nervous at the point we turned back.

So I had a look at XC weather on return and now they had a wind indication which more reflected the conditions. So it is not a forecast, but a late response to what actually is happening. Their disclaimer http://www.xcweather.co.uk/Info/Disclaimer which basically says 'We are rubbish! Do not make any plans on what you read here because we are not a weather forecast site."

I shall place less reliance on the site as I have had up until yesterday.

 

[Heckler]

Yesterday I ventured out with a friend and her two boys 12 and 14, neither of whom had been on a yacht before.

Preparing for the trip to sail them back to the island they came over from in the morning, I checked XC weather among others. 15mph gust 28mph reducing to 13 and 25 respectively was the wind forecast. I set off reefed in with the idea of later taking the reefs out.

Instead of decreasing, the wind increased and INCREASED! At the point we had a 47knot gust over steady 35 knots it was time to return. Took the main down and sailed ion a run with a 2/3 reefed jib doing 9 knots!

Now while it was rather exhilarating for me, the youngsters, although saying they enjoyed the day, were rather nervous at the point we turned back.

So I had a look at XC weather on return and now they had a wind indication which more reflected the conditions. So it is not a forecast, but a late response to what actually is happening. Their disclaimer http://www.xcweather.co.uk/Info/Disclaimer which basically says 'We are rubbish! Do not make any plans on what you read here because we are not a weather forecast site."

I shall place less reliance on the site as I have had up until yesterday.

I use it as a medium term, ie a week ahead, indicator. It gives a flavour of what is going to happen.
Stu

 

[AngusMcDoon]

It's difficult to be precise when a cold front goes through - especially like yesterday when there are 2 close together chasing each other. The best a forecaster can say is that it is going to be unstable and gusty. Forecasting downwind of mountains is also hit and miss, which a lot of the Clyde is. The wind often comes down the glens in lumps and 5 minutes later is gone.

The Met Office on the other hand just choose a large chunk of coast and give a bland forecast covering a long time period and state the worst possible case for all that area over all that time. Usually it's just an exaggeration of what actually happens as they are just rse covering all the time.

 

[NormanS]

I find XC to be a very useful Forecast, but, like any othe forecast, it can only give an idea of probable conditions. It's unfortunate that you had these conditions for the boys' first sail. I hope it didn't scare them off sailing.
A possible clue in the forecast is when they show a big difference between "wind" and "gust". In gusty conditions, it's notoriously difficult to predict the strength of gusts.

 

[Lakesailor]

"Weather Forecast Wrong" not a headline really.

I use weather sites to get a feel for the coming weather.
The summer has finished. http://www.accuweather.com/en/gb/windermere/la23-1/september-weather/327462?monyr=9/1/2013

 

[Woodlouse]

I use weather sites to get a feel for the coming weather.
The summer has finished. http://www.accuweather.com/en/gb/windermere/la23-1/september-weather/327462?monyr=9/1/2013

I love statements like this.

I look forward to 6 weeks of sunshine.

 

[jakeroyd]

I use
www.weatheronline.co.uk/marine/weather?01&CONT=ukuk&LANG=en
This site has extensive maps of gusts which I have found quite reasonable for accuracy.
As others have said any prediction is just that.
Use them as an indicator only.
Many use the gfs model which neans they are all reformatting the same data.

 

[Croak]

There was some odd weather yesterday afternoon and evening, winds above forecast, and just a bit odd.

 

[lpdsn]

I shall place less reliance on the site as I have had up until yesterday.

To be fair, XCWeather, like quite a few other sites, is simply a presentation of the GFS model output. If you look down at the bottom right, just below the 'forecast' it'll tell you which model run it was.

You need to use your own skill and experience to turn it into a forecast. You can of course simply listen to the MetOffice forecast on the VHF but that has no guarantee of accuracy either.

I prefer to use the model and add my own judgement. Doesn't mean I'm always right, but I feel better inform than just listening to the VHF.

 

[Momac]

I use xcweather and find it accurate , particularly so when looking a day or two ahead. I think that's as good as it gets in the UK.


It's no really surpising to find there is a disclaimer - seems like a very sensible idea to me.

 

[Lakesailor]

I love statements like this.

I look forward to 6 weeks of sunshine.

We can hope.

I am looking at Sunday 8th. Looks OK for our first competition of the season. But the Saturday looks a bummer. I'll keep looking and see the pattern changing as the week progresses. Hopefully in my favour.

XC Weather shows a much better outlook, so I'll go with them http://www.xcweather.co.uk/forecast/penrith

 

[franksingleton]

This is a buoy that has been rounded many times in YBW forums. Remember that XCWeather, Magic Seaweed, most of the tablet apps, zyGrib, Ugrib etc are all direct output from the same NOAA GFS. This is a pretty good numerical weather forecast model. Like all such, it is a useful tool but is not and can never be precise. There are many reasons for this.

The grid length used – 27 km, although the output is only at 0.5 degree lat/long.
Uncertainties in the initial data.
The atmosphere being in a continual state of flux.
Couples with all of these, chaos.

At http://weather.mailasail.com/w/uploads/Franks-Weather/bookcover.pdf, I have tried to explain how to make best use of the weather information available to use. But this is not due out until 2 January.

Mediterranean sailors might like to try the Turkish Met Service page http://212.175.180.126/DENIZ/DTS/sea.php. This is simply output direct from ECMWF and should be as good as it can be given the current computer power and data sources. ECMWF uses the maximum amount of data possible and analyses the weather on a grid of 0.125 degrees.

Those not in the Med might like to ask the Met Office (or their national weather service) to make the same data available for those west of Gibraltar.

 

[prv]

This is a buoy that has been rounded many times in YBW forums. Remember that XCWeather, Magic Seaweed, most of the tablet apps, zyGrib, Ugrib etc are all direct output from the same NOAA GFS.

I pay Windguru a few Czech blats each year for "pro" access, which includes forecasts based on "WRF 9km" instead of GFS. I don't really know what this is - can you enlighten me?

Pete

 

[franksingleton]

I pay Windguru a few Czech blats each year for "pro" access, which includes forecasts based on "WRF 9km" instead of GFS. I don't really know what this is - can you enlighten me?

Pete

With some slight variations, Predictwind for one. All these companies start with the GFS output at a 0.5 degree spacing. They interpolate to get a finer scale analysis but use no more data. The do use detailed topography. They then run a numerical weather prediction model. The usual choice is the Weather Research Forecasting meso-scale model. They do this over a limited area using a shorter grid length. For Windguru this is probably around 10 km...

I do not know the area used by Windguru. That used by Windfinder is shown at http://weather.mailasail.com/w/uploads/Franks-Weather/windfinder-area.png. Windguru will, no doubt, use similar sized areas. Meso-scale model area boundaries have to be continually updated from the parent global model. In this case, the GFS 0.5 degree data.

This immediately sets a limit on how good the forecasts are and how far into the future. If you are near the area boundary, forecasts may add little to what you can get from the GFS. Depending upon how mobile or settled is the weather, then even near the middle of an area you may get little improvement over the GFS even at such short time scales as 2 hours.

There is nothing wrong with the model, it is probably as good as any. My big hang-up with these “unofficial” models is that they do not analyse current weather on the scale that they predict it. National Met services such as the UK, France, Iceland, Norway, Poland, Denmark, Spain, Italy (Tuscany), Croatia and Greece all se a vast amount of data from terrestrial and space based observing systems for their fine scale forecasts.

Some points to bear in mind with any objective forecast

1. Any model can only represent weather and topography on a scale of about 4 or 5 grid lengths. With a 10 km grid, a model can only define weather on a scale of around 40-50 km.

2. This means that winds may well be a force higher than the models predict.

3. If making serious sailing decisions, always look at the GMDSS forecasts. This is particularly so when visibility is forecast, I have seen some pretty hairy forecasts from Météo Consult forecasts pinned up in French marinas.

A long reply, I know, but simple questions about meteorology can rarely be satisfactorily answered in a few words.

 

[aquaholic]

So I had a look at XC weather on return and now they had a wind indication which more reflected the conditions. So it is not a forecast, but a late response to what actually is happening. Their disclaimer http://www.xcweather.co.uk/Info/Disclaimer which basically says 'We are rubbish! Do not make any plans on what you read here because we are not a weather forecast site."

I shall place less reliance on the site as I have had up until yesterday.

Did you check the inshore waters forecast? Would be interesting to see if that was forecast.
I never rely just on one site, always check the inshore, and use Windguru and Meteo Consult along with the BBC Coastal forecast.
And XC to see if I am going to get wet.

 

[oldvarnish]

Frank seems to have confirmed my long-held view that all these websites use the same data and vary only in the method of presentation; so to prefer one over the other when it comes to forecasting the weather is pointless.

What you are really preferring is the way they look.

Your best bet, surely, has to be the UK Met Office when in UK waters. They have that magic ingredient called 'human experience' - which no computer model has managed yet.

 

[franksingleton]

Frank seems to have confirmed my long-held view that all these websites use the same data and vary only in the method of presentation; so to prefer one over the other when it comes to forecasting the weather is pointless.

What you are really preferring is the way they look.

Your best bet, surely, has to be the UK Met Office when in UK waters. They have that magic ingredient called 'human experience' - which no computer model has managed yet.

You are absolutely right and I have said that on many occasions on these forums.

I never go to sea without knowing what the relevant national service is saying. The forecasts are necessarily brief and broad brush. They cannot and do not give a detailed description of the weather over the next two days. They are more of a warning system than anything else.

When it comes out have a look at http://weather.mailasail.com/w/uploads/Franks-Weather/bookcover.pdf. I do practice what I preach! That is a combination of GRIB files, GMDSS forecasts and experience couples with commonsense.

 

[Spirit of Glenans]

National Met services such as the UK, France, Iceland, Norway, Poland, Denmark, Spain, Italy (Tuscany), Croatia and Greece all se a vast amount of data from terrestrial and space based observing systems for their fine scale forecasts.

Ireland's national weather service, Met Eireann also.

 

[franksingleton]

Ireland's national weather service, Met Eireann also.

Sorry. I did not make it clear that these were countries that made their fines scale out available on the internet. Eg
http://en.vedur.is/weather/shipping/coastal/
http://www.dmi.dk/en/hav/
http://www.yr.no/kart/#lat=64.2929&lon=12.2307&zoom=6&laga=vind&proj=3575
http://www.puertos.es/content/prediccion-del-oleaje-mapas

Does Ireland do anything similar? The UK Met Office app, whilst great for rain is pretty useless for sailing use.


What we would really like is a weatern European version of http://212.175.180.126/DENIZ/DTS/sea.php. I believe that there is no reason why not. It is output staight from ECMWF and on the Turkish met service website. It looks very useful and makes me wish I was back in the Med.

 

[westernman]

I use windfinder - but as has been said above, the basic data is the same.
However, you should always look at a grib file as well - as this will give a good impression as to what is going on (particularly if you can animated it over a period of 48h or so).

You will find that windfinder/xcweather etc, may just be a grid or two out on the forecast. This is particularly so when local geography may have an impact - (e.g. the Pyrenees). So have a look a couple of grids on the grib file each way.

If you see a force 8 two grids away, and a force 4 on your grid and XCweather predicts a force 4, then you are on dodgy ground relying on only seeing the force 4. You might get F4, or you might get F8 or anything in between.