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Another cold winter ahead?

Paul Hudson | 17:14 UK time, Friday, 1 October 2010

It's the time of the year when there is always a lot of interest in winter forecasts, more so than in previous years, largely as a result of what happened last winter - which as we all know was the coldest for over 30 years.

And once again, a colder than average winter looks the most likely outcome based on the latest projections.

The latest American model is shown below.

The winter season is shown as colder than average - indicating that a 'blocked' weather pattern is on average likely to dominate with the jet stream further south than normal. The American model successfully predicted last winters' cold conditions.

The Met Office don't issue their seasonal forecast to the general public anymore, using them for internal research purposes only, but as I understand it, their forecast also suggests that the probability of a cold winter is higher than normal.

It would seem that one of the reasons for a higher probability of another cold winter is down to a forecast negative NAO (North Atlantic oscillation).

The NAO is a way of describing pressure patterns in the Atlantic.

Climatologically, a low-pressure system over Iceland and a high-pressure system over the Azores determine that westerly winds prevail across western Europe. The relative strength and position of these systems varies from year to year; this variation is known as the North Atlantic Oscillation (NAO).

A large difference in the pressure at the two stations (a high or positive NAO) leads to increased westerlies and mild and wet winters across the UK and Northwest Europe.

In contrast, if the index is low (negative NAO) westerlies are suppressed, and the UK and Europe suffer cold winters as the jet stream steers depressions through Spain and Portugal and into the Mediterranean.

The forecast NAO is calculated using a formula which incorporates sea surface temperature anomalies in Spring and early Summer.

But a negative NAO does not always mean a cold winter here in the UK.

For example, in the past, a negative NAO has been a successful indicator of a cold winter across Continent Europe - but with the UK right on the edge of milder air from the Atlantic, with a resulting battle between mild and wet weather from the west, and much colder weather further east.

Of course readers of this blog will by now know that the ongoing low solar activity, and the previous protracted solar cycle all suggest a higher than normal probability of a blocked winter weather pattern leading to colder than average weather conditions.

There are of course no guarantees with long range forecasts, but It could be another very interesting winter if latest projections prove to be correct.

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