WeatherWebb

Making the weather more accessible

Why do weather forecasts vary so much?

on March 18, 2015

There are many different sources for checking the UK weather, whether it be reading the paper, checking an app, watching the weather forecast on the TV or listening to the radio as shown below, but it’s been noticed recently that the forecasts can often be very different!

Example of weather forecast sources

Example of weather forecast sources

The 4 main weather forecasting companies in the UK are the Met Office, MeteoGroup, MetDesk and WSI, which supply forecasts and data to many of the other companies. For example the Met Office supplies the BBC with data and MeteoGroup supplies Channel 4. In the last few months, more and more people have been asking why the weather forecast differs so much depending on where you look at it. For example Liam Dutton from Channel 4 tweeted at the beginning of March that the forecast varied depending on which app you looked at, as shown below. In fact on Saturday 7th March it was a gorgeously sunny day and the maximum observed temperature was 16.1C in Greater London!

Liam Dutton's tweets

Liam Dutton’s tweets

The reason the forecast varies so much between the sources is because different companies look at different weather models. At MeteoGroup we have access to a number of models such as those shown below and then the forecasters analyse these models at the start of a shift;

  • ECMWF
  • EURO4
  • UKMO Global
  • KNMI (HiRLAM)
  • GFS
  • WRF

It is likely the other companies have access to these or at least some of these models as well, but they are likely to weight their forecast on a specific model. For example, the Met Office spends a lot of money developing their own models, such as EURO4 and the UKMO Global model, so they are likely to use that model on a more regular basis. However, by only looking at one or two models you decrease how accurate your forecast will be as because the weather is a chaotic system; if the starting conditions are wrong then it is likely the forecast will be wrong.

A key example of when this happened was on Tuesday 17th and Wednesday 18th February 2015 when the Met Office/BBC were forecasting it to be largely cloudy and cold across England as that’s what their models suggested. However, MeteoGroup looked at other models such as ECMWF which suggested it would be largely clear and warm with plenty of sunshine. In fact both days saw spells of sunshine across England and it was an unseasonably warm day. It became very obvious that the Met Office forecast was wrong when the BBC weather presenter showed the current satellite image with largely clear skies at the beginning of their forecast, but then went to the forecast for the same time and it was completely cloudy… It can often be difficult for the lead forecaster to decide on the forecast scenario if the models contradict each other!

It is important to remember that weather forecasting is a science and that it is on average only 87% accurate. The models we have access to now are incredible, but as I mentioned before, the weather is a chaotic system and therefore it can always change, potentially just because a butterfly flapped his wings.

If you’re after a new app as you’re fed up of the forecast being wrong, I’d recommend MeteoGroup’s app WeatherPro!!

2015-03-18 13.49.45


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