Cold weather ahead? Long range forecasts predict tumbling temperatures

One of the most high risk comments a weather presenter can make is one based on a long range forecast.

Frequently, a measured, fully caveat-ed few words can morph and mangle into a screaming tabloid headline, particularly when it is to do with cold weather, and particularly this side of Christmas.

Long range forecasts (up to 30 days ahead) need to be treated differently to our usual time period of five days which focuses on detail and tends to be quite location and timing specific.

The fact remains that even with modern forecasting ability and the 215 billion observations received and processed each day by the Met Office, the further into the future we look, the less easy it is to accurately predict the exact movement of air around the earth.

If the timing or exact track of a system is out by just fraction, the weather on that day can be radically different.

Any long range forecast in huge detail such as "it will be raining in Norwich three weeks from now, with a temperature of 10C and light north-westerly winds" is almost certainly not worth the paper it's written on!

That said, credible long range forecasts do have their uses, particularly at this time of the year.

Tracking movements of weather and giving a broad indication of likely conditions helps councils gauge stocks of grit for icy conditions, or in the summer, an indication of a prolonged spell of dry weather could flag up to local authorities a need to keep a close eye on water reserves in their area.

So generally speaking, these long range forecasts are of limited interest and use to the general public, but with the exception of this time of year.

When the words "cold" and "winter" start to rub up against each other, you only need "Christmas" to be flung into the mix for a tabloid frenzy to ensue!

This is what the Met Office have actually said in their six to 30 day forecast:"There is an increasing signal that blocked patterns will become more dominant throughout this [November 23 to December 7] period.

"This will bring a greater chance of colder and drier conditions under the influence of high pressure.

"There will still be spells of rain and showers as frontal systems move across the UK, but these will be interspersed with drier, colder and brighter days.

"There is also an increasing risk of snow at times, more especially in the north.

"Temperatures will initially be near or slightly below normal, but with an increased chance of below normal temperatures later in the period, with an increased likelihood of overnight frosts."

A "blocked pattern" means a dominant area of high pressure that sits stubbornly out to the west of the country.

The position of this so-called "blocking high" creates a cold northerly airflow across the UK.

The weather in this sort of set up is often dry, bright and increasingly cold, despite that daytime brightness, with clear overnight skies allowing temperatures to quickly drop away overnight and not enough heat in a winter sun to warm it back up much by day, resulting in temperatures generally getting colder and colder.

Showers may blow in from the North Sea which would particularly affect North Sea coastal areas and they could turn wintry as they track over the cold land.

Likewise, any more general rain that does move across the UK would fall more readily as sleet and snow thanks to the depressed temperatures.

This was the set up during the long, cold winter of 2010 - and whilst it is still much too early to give more than an indication and a tentative heads up - at least now you'll know the real facts behind the headlines!