Last winter saw below average temperatures and heavy snowfalls for a large part of December and January, before becoming slightly milder through February. But what are the indications for this winter? Will we see a return to the Atlantic driven, mild winters? Or will the cold and snowy weather continue?

The reason last winter was so cold was due to an exceptionally negative Arctic Oscillation (AO) this meant that high pressure developed across Greenland and something known as Northern Blocking developed, this allowed freezing air to flood across Europe and over to us here in the UK. This winter? The indications are looking promising for another cold and snowy one.

Firstly, we’ll take into account the Sea Surface Temperatures or SSTs, the chart below shows us both the warm and cold anomalies

Sea Surface Temperatures Anomaly

The oranges/reds on the image above show us where above average sea surface temperatures have developed. The blues/purples show us where below average temperatures. If you look at the Pacific Ocean, you can see a large area of dark blue and purples. This is known as a phenomena called La Nina, and it’s a cooling of the Pacific Ocean. Although not fully understood, it is believed to lower global temperatures and bring colder weather to certain areas and that usually includes the UK.

We also have areas of blue in the Mid Atlantic, this suggests that we’re likely to see high pressure developing and building up into the Atlantic, when this happens low pressure systems get stuck in the North Atlantic and subsequently, cooler air from the Arctic heads Southwards.

Another thing to take into account is the low sunspot activity in the last 2 years or so. Historic data show that during times of low sunspot activity, the UK experiances unusually cold winters, 1962 being one of them.

The Arctic Oscillation and North-Atlantic Oscillation are other things we should take into account, however because these are more variable and can change on a day to day basis, it’s much harder to use these to help forecast more than 5-10 days out, although we’ll take a look to see how they’re doing anyway.

North Atlantic Oscillation

The NAO is currently forecasted to shoot up into a weak positive before slowly moving back down into a weak negative. This sudden change suggests we’re likely to see the Atlantic becoming slightly more active in the next couple of days, we’re likely to continue this unsettled and cool theme for the next week or so at least. When we have a negative NAO, we’re more likely to see high pressure developing in the Atlantic, when we see a positive NAO, we’re more likely to see low pressures developing, and in turn, turning our weather more unsettled.

Arctic Oscillation

The AO is currently forecast to remain in a weak negative, perhaps becoming increasingly negative during the next 5-8 days. When we have a negative AO (as seen last winter) we have high pressure developing across the Arctic where we want it then if the correct conditions present themselves, that cold air will flood Southwards towards the UK. A positive AO means we see something called the Polar Vortex, which is a system of low pressure systems across the Arctic which make it far harder to lock ourselves into a cold pattern for more than a couple of days.

We also must take into account the fact that the Arctic has had one of the coldest summers on record, meaning we’re unlikely to have a problem with getting an embedded cold pool into place, ready to flood Southwards.

Taking all these factors, and more into account. I think we’re likely to see a generally damp and dull start to Autumn, before things begin to turn increasingly cold as we move through October and into November. I’m forecasting another cold, and snowy period of weather, particularly from the middle of November through until the middle of January, before we see the Atlantic kicking into gear again bringing some mild, stormy weather to end the second half of winter.

The Gulf Stream

I don’t think I could create this forecast without mentioning the current state of the Gulf Stream. It’s something that’s been a hot topic amongst weather forums in the last few weeks, and here’s one of the reasons why. This graphic shows what the Gulf Stream was like in September last year.

September 2009

And here’s what the Gulf Stream is looking like this year

September 2010

If you compare the two images above you can see there has definitely been a slowdown and slight decay of the Gulf Stream over the course of the last year, and this is what has gotten so many people interested. Whether this will have any effect on the weather in the coming months is yet to be seen, however.

Winter 2010/11 Overview

So just to summarize, I’m predicting a cold start to winter. I believe the first cold spell to take hold during the second part of September with frosts becoming more frequent across the United Kingdom and early snowfalls pushing down from the North and into Scotland and perhaps higher hills in Northern England.

I expect the first half of winter to be generally cold and snowy, as cold if not colder than last year. I think it’ll be generally cold and snowy through November to the middle of January before high pressure begins to break down and the Atlantic takes over for the second half of the winter bringing spells of wet, mild and windy weather, with brief cold snaps as low pressure systems push Eastwards into the North Sea.