Sunshine and showers? – a challenging forecast

30 06 2011

“And today we will see a mix of sunny spells and scattered showers” says the weather forecaster on the TV, as you get ready for work. 

This is certainly a phrase that you will hear used quite a lot during the summer but, despite its easy tone and clever alliteration, it is one of the most challenging forecasting tasks that a forecaster will come across.

Sunshine and showers – on a countrywide scale this is relatively easy to visualise, and taking a broad brush we are likely to be spot on.  But what really matters, of course, is whether you get soaked by one of the showers or if you stay dry and warm in the sunshine.  Now, that is more difficult to predict and where the challenge really begins.

Let’s look at some statistics.  The UK covers 246,610 km sq.  The average shower covers around 1 km sq and may cover about 64 km in an hour as it travels across the country before dying away.  So the average shower only covers 0.026% of the UK landmass in its 60 minute lifetime.

So, the crux of the matter for the forecaster is which 64 km sq of the UK is going to see that shower, and when.

This kind of detail is quite difficult to forecast, but is exactly what most of us are interested in during the summer. There is a massive difference between a cool, showery day and a sunny, warm day when you are out and about.

Now let’s be honest, we are good at forecasting severe winter storms, which is a vital part of what we do. We are also excellent at predicting a change from cold and snowy to dry and sunny, or in this instance to ‘sunshine and showers’. In other words we are very good at forecasting the overall picture up to a week ahead, and in winter that’s all that matters to most people.

However, in summer it’s the detail that really matters. In summer when the weather is relatively quiet it’s the local effects of the hills and valleys, land and sea and subtle variations in heat and moisture that dominate what weather we will see.  Will the showers be inland or on the coast? When will the grey mist and fog clear to let the hot sun through? In summer these differences can make or break your picnic.

Yes, our four-day forecast is overall as good as our one-day forecast was 30 years ago, but we know that everyone wants to know what the weather will be like in their postcode when they walk out of the door in the morning, and advances in the science and technology at the Met Office are now tackling this issue.

As a result, we now provide local weather forecasts for around 5,000 different towns and cities across the UK. These are updated more frequently through the day, providing  a fuller picture of what the weather is doing. These forecast are available on our website; through your mobile phone or smart phone, or via our widget on other websites – keeping you up-to-date with the weather.


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