Yachting and Boating World forums: weather question

Hi Emnick,

Firstly, thanks for watching Weatherweb.TV. The more the merrier, it's got quite a following now, in no small part thanks to all of you here, so thank you.

Various state weather services around the world run computer simulations of the atmosphere. These are what we refer to as 'models'. There are global models (covering the entire world) and regional models (which are 'tuned' to certain areas, i.e.e Europe).

Much of the data you see on the web is raw model output. That means that the model predicts the stated conditions based on the mathematical formulas it uses to predict the weather.

Now, one does have to be very aware of this when viewing forecasts, especially on the web, or receiving text messages on weather. By their very nature models can only be as accurate as the reolution at which they 'see' the world. For example, the GFS (which is the American model and probably the most common you will see on the web) has a resolution (at best) of 0.5 degrees. That means that it is making a prediction for points 30nm (about 45km) apart. Now, this needs to be taken into account, because what models at this resolution don't see are things like the Isle of Wight, Anglesey etc.. They also 'see' mountains and hills as blocks, rather then the delicate contoured land masses which we see.

Certain companies do claim to run higher resolution models. What these companies do is take in the raw data from something like the GFS and then apply some mathematical techniques which improve topography and interpolate to improve resolution. However, you must be cautious because even though the topography may be improved, any underlying errors in the forecast remain, and may even be amplified.

What human forecasters do (and of course I am biased) is add their experience and knowledge. They look at a variety of cimputer models, and by knowing the biases of each can interpret the guidance from them, hopefully improving the forecast quality and recucing errors to a minimum.

Weatherweb.TV forecast charts are output from the raw GFS, although forecast do draw the fronts on ourselves. I show this rawer quality GFS as I believe this is as good as the model gets, and that any higher resolution is to give a false impression of it's accuracy.

Hope that answers you question?

Simon

--------------------
Simon Keeling MSc, FRMetS
Have you read my book? 'The Sailor's Book of the Weather'
www.weatherweb.tv
www.weatherweb.net