Winter 2013/14 Forecast
UK Winter 2013/14 Weather Forecast – Moderate confidence of colder than average winter to come.
At this stage, we are only able to offer our very preliminary thoughts on the upcoming UK Winter 2013/14 Weather Forecast . We should be able to provide you with our more in depth preliminary UK Winter 2013/14 Weather Forecast during the Autumn, before releasing the final forecast some time in November.
At present, our expectations are of a cooler than average UK Winter 2013/14, carrying on the trend from the last few winters, with snowfall likely to be more frequent than was the case during the milder winters of the 90′s/00′s. December currently looks like it will finish slightly above average temperature wise, though rather unsettled conditions are likely to dominate.
January and February, as things stand, look likely to finish below average temperature wise, with some frequent colder spells particularly during February. The first of these cold spells at present would look likely to set in around the middle 2 weeks of January, with a more generally colder than average theme expected through the month of February as a whole.
UK Winter 2013/14 - How are our forecasts derived?
These very early indications are based upon the analogues used in our Summer 2013 forecast, and as such are very open to change. The biggest wildcard for the current forecast has been the recent uptick in solar activity (though this has now declined once again in the last couple of months), along with an unpredictable QBO state heading into the winter. Most of our clues for the winter tend to come through the month of October.
UK Winter 2013/14 - Snow Advance Index (SAI)
During October we look for a number of factors which help us to determine the possible general state of the atmosphere leading in to the winter months. One of these, called the Snow Advance Index, measures the rate of advance of the permanent snow cover for the winter across Russia and Asia. The faster the rate of this advance, the more likely it is that during the winter the UK will experience some spells of cold weather, owing to various feedback mechanisms within the Earth’s atmosphere, ultimately leading to an event known as a Sudden Stratospheric Warming.
UK Winter 2013/14 - El Niño or La Niña? – The ENSO index
Other determining factors include the state of the ENSO index – the index measuring whether the temperatures in the Eastern Pacific Ocean are conducive of El Nino, La Nina, or neutral conditions. We then combine these factors with various other global circulations to determine conditions through the winter season. The current forecast for the ENSO index, produced by the NOAA in America, is shown below and currently indicates a neutral state for the winter:
UK Winter 2013/14 - The Stratosphere – Explanation and Early Forecast
Please note: Some of the following information is very over-simplified. The result is that the information, whilst accurate, does not account for all variables involved in forecasting the likely state of the Stratosphere, but it should make it a little easier to understand overall.
The buzz word in recent years in the ‘industry’ surrounding winter forecasts has been stratosphere, and more prominently Sudden Stratospheric Warming (SSW for short).
What is this phenomenon and how does it affect the UK in winter? Well, as ever, there is a little bit of explaining to do, but stick with me!
So, as anybody who studied GCSE Geography will most likely know, the prevailing wind direction in the UK through the year generally, but especially through the Autumn and Winter months, is a West or South-Westerly wind. Why does this occur? Well during these months we normally have a set up of low pressure to the North of the UK, and high pressure to the South. The result of this pressure setup is for the winds to come in from the West of the UK and this usually proves to be a mild direction, as the gulf stream brings warmer waters up in to the mid-Atlantic ocean.
We mentioned the low pressure to the North of the UK, and this is key to winter forecasting in the UK. This low pressure, or should I say collection of low pressure systems, is known as the Polar Vortex. The Polar Vortex is an area of low pressure systems that encircle the Arctic usually during winter, and this has the effect of bottling up all of the cold air across the Arctic circle, preventing it from moving South down towards the UK.
So where does a Sudden Stratospheric Warming come in to all of this? Well lets start with the Stratosphere itself. The Stratosphere is the second layer up in our Atmosphere. The first layer of the atmosphere, the layer at the surface in which we reside, is called the Troposphere, and the Stratosphere is the layer above this, at approximately 8-12km up in the sky.
The Polar Vortex not only exists at the surface, but also rises up in to the sky all the way through the first layer of the atmosphere, the Troposphere, and up in to the Stratosphere. It is the intense cold air found here that causes the Polar Vortex to form at the surface.
For various reasons what we sometimes see though is the air in the Stratosphere above the Arctic circle start to warm significantly. This has the effect of weakening or even sometimes completely disintegrating the Polar Vortex in the Stratosphere, as the winds in the Stratosphere also change from a Westerly to an Easterly direction, and these effects can then filter down the atmosphere and all the way down to the surface, replacing the low pressure that usually encircles the Arctic circle with high pressure (known as Northern Blocking). This has two effects. First of all, it allows the cold air across the Arctic to flood South and secondly it has the effect of reversing the direction of the wind from a Westerly to a Northerly or Easterly – both cold directions usually for the UK in winter.
To be truly defined as a Sudden Stratospheric Warming (SSW), the wind direction in the Stratosphere above the Arctic has to completely reverse in direction, but often just a slightly warmer than usual Stratosphere can result in the Polar Vortex at the surface being weaker, and allowing the colder air to flood Southwards across the Northern Hemisphere.
So, it is for this reason that we often look to our various signals for the winter to see whether we think a Sudden Stratospheric Warming will take place – as most of the time such an event will result in colder than average conditions taking hold across the UK. (It should be noted that not every SSW will automatically result in cold weather for the UK, but most of the time it does).
Got that? …possibly not, its a very complicated system, and there are many other variables to take in to account. However, all you need to know is that this phenomenon is a good indicator for us of the potential for cold outbreaks as we enter the winter months.
So how about the prospects for an SSW event this year? Well, this is a VERY preliminary outlook, with our greatest clues to the likely state of the Stratosphere heading in to winter not coming until the end of October. However, at this very early stage we have compiled forecasts for the temperature of the Stratosphere through December, January and February, based around our chosen composite years from our Autumn forecast (again, these are very much subject to change for winter at this stage).
30mb (Stratosphere) Preliminary Forecast Temperature Anomaly – December 2013
30mb (Stratosphere) Preliminary Forecast Temperature Anomaly – January 2014
30mb (Stratosphere) Preliminary Forecast Temperature Anomaly – February 2014
If we look at those, we can see that over the Arctic, the month with the best chance of warming in the Stratosphere is January. This actually ties in very well with our early winter thoughts, with mid January currently showing in our research as having a good chance of being the first major cold spell of the winter for the UK, and for February to be a rather cold month overall.
It should be mentioned that not every cold spell the UK experienced is caused by a full SSW event. For example, the second coldest December in recorded history, December 2010, came about not due to a full Sudden Stratospheric Warming event, but instead because of the Stratosphere being slightly warmer than average going through November and in to December, preventing the polar vortex from fully forming until later in the season.
UK Winter 2013/14 - Solar Activity – Solar Cycle 24
And finally we also take in to account activity on the Sun, and how it may affect the winter season. We have found in previous years that a high sunspot count on the sun’s surface can over-ride any other signal that we use to forecast for the winter season. For example, in late 2011 we saw a surge in the number of sunspots visible on the sun. This impacted our winter significantly, and so whilst all other signals pointed towards a cold winter for the UK, it wasn’t until February, co-incidentally enough around the same time as the sunspot count had begun to decrease once again, that we saw the colder weather arrive on the shores of the UK. The current solar cycle, cycle 24, has been unusually quiet and many are linking this factor in to the reasoning for why Northern Hemisphere winters have been colder of late.
For this UK Winter 2013/14 -, well all eyes have been peeled after a spike in sunspot number through the middle of 2013. However, in recent months the sunspot number has begun to decrease once again, and so at present we are not anticipating any over-riding influence from solar activity. Obviously though with several months still to go before we reach the winter season this is still very much open to change. The recent sunspot count is shown below:
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Forecaster: Kris Surtees
UK Winter 2013/14 Prediction