Winter Forecast 2013/14 – Final thoughts
UK Winter Forecast 2013 /14 Weather Forecast – Slightly colder than average winter expected.
That’s right, we are going to delve straight in to the UK Winter Forecast for 2013/14. All of the literature supporting the case for the forecast is available to read further down the page and may prove an interesting read for some of you. However we know that for many of you the most important part will be the forecast itself, so here it goes:
The overall expectation from the forecast is for the winter to finish up slightly cooler than the long term average overall. For those of you specifically seeking the chance of colder weather the better news for you comes later in the forecast. Whilst through December and January there is the potential for some short cold snaps (the sort of thing a forecast such as this isn’t really designed to pick up on), the biggest potential for colder weather this year comes in February.
December 2013 Sea Level Pressure Anomaly Forecast
December 2013 Temperature Anomaly Forecast
December 2013 Precipitation Anomaly Forecast
Currently there is no overall strong signal for a dominant pressure type to affect the UK during December. This is likely to mean that we will see a rather mixed month overall, with bouts of high pressure then low pressure alternating through the month. With the weak high pressure anomaly signal placed just to the North of the UK there is the suggestion that at times, as in recent weeks, the flow will switch around to more of a North-Westerly direction, bringing some short cold snaps at times. Temperature wise it looks likely to finish close to the seasonal average or perhaps just slightly above, with precipitation looking close to average too.
January 2014 Sea Level Pressure Anomaly Forecast
January 2014 Temperature Anomaly Forecast
January 2014 Precipitation Anomaly Forecast
A tricky month to call. From looking at the pressure anomaly it looks as though the dominant force will be low pressure over to just to the South of the UK. The difficulty comes from the signal for high pressure just to the North-East of the UK. The composite years individually point towards the possibility of a cold spell during the middle 10 days of January (or perhaps just slightly before), but otherwise it looks likely to be a mostly unsettled but cool month. Temperatures are likely to finish close to or slightly below the seasonal average, with precipitation a little above average.
February 2014 Sea Level Pressure Anomaly Forecast
February 2014 Temperature Anomaly Forecast
February 2014 Precipitation Anomaly Forecast
February 2014 for some time now has been displaying signs of a cold month for Western Europe. We can see from the pressure anomaly above that the general expectation is for high pressure to be close to the North of the UK. Generally when such a setup occurs it leads to a flow from the East, which at this time of year tends to mean a colder setup is likely across the UK. So the general expectation is for a colder than average month with winds sourced from a Northerly or Easterly quadrant. Whilst the precipitation signal is for a much drier than average month any precipitation that does occur would likely be wintry in nature. However with the high pressure shown to be close to the UK a predominantly cold and dry theme seems most likely, especially in the North, with the greatest chance of precipitation across Southern and Eastern areas of the UK.
UK Winter 2013/14 - How are our forecasts derived?
These indications are based on a whole array of factors outlined below (plus a few more that we haven’t mentioned here due to their complexity!). Generally speaking for this winter there are a lot of conflicting signals and sow confidence in the forecast as a whole is low. You should also bear in mind when reading these predictions that weather forecasting out to 5 days can prove tricky, let alone 3 months, and so the usual caveats of long range forecasting apply to this forecast. It is still very much an area of early development and so the accuracy of such forecasts can not be guaranteed at such timeframes – part of the reason that we, unlike some others, would not charge for long range forecasts.
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.
This year the Snow Advance Index (SAI) was lower than in previous recent years, and so the net prediction from this particular variable is for the Arctic Oscillation (a measurement of pressure across the Arctic) to be positive. This leads to less chance of colder air affecting the mid latitudes, including the UK. However, it is a signal that applies to the winter as a whole and as such the signal for February in particular to see higher pressure to the North of the UK is still entirely possible. It just suggests that a winter with all three months containing raging cold conditions should not be expected, contrary to some other reports.
There is also a new index that has been developed by Italian researchers called the October Pattern Index (OPI). This is one again a research project in its very early days and is unfortunately at the moment written mostly in Italian (an English translation is said to be forthcoming) but the track record using historical case studies has proven to be rather interesting with a higher correlation between it and the overall Arctic Oscillation through the winter than found with the Snow Advance Index. The long and short of it for now is that they are also projecting a positive Arctic Oscillation for the winter for a whole, but interestingly the authors are suggesting that February could well differ slightly from this.
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?
30mb (Stratosphere) Preliminary Forecast Geopotential Height Anomaly – December 2013
30mb (Stratosphere) Preliminary Forecast Geopotential Height Anomaly - January 2014
30mb (Stratosphere) Preliminary Forecast Geopotential Height Anomaly - February 2014
The general expectation for the stratosphere is for things to really become more interesting later in the season. The plot shown above for December shows the polar vortex overall slightly displaced from its central axis, but still rather strong and organised. Crucially it looks likely to be based somewhere close to Greenland for much of the Month, and this tends to suggest that blocking to the North-West of the UK is less likely during December.
As we head in to January we start to see a change, with higher than normal heights appearing across the Eastern half of the Northern Hemisphere, helping to push the polar vortex further away to the West and slightly further away from the North-West of the UK. This gives the chance of something such as a scandinavian high pressure forming at times and giving the risk of something a little colder across Western Europe.
And then in to February and we see a stark change, with the polar vortex shown to be really very weak. This leads to an increased chance of higher pressure forming around the Arctic regions, and has the net effect of allowing colder air from the Arctic to flood Southwards in to the middle latitudes including, potentially, Western Europe.
So on the basis of the Stratospheric forecast the general idea, similar to the main forecast, is one of a backloaded winter, with colder conditions more likely through the second half of the season than the first. However, a full SSW by definition (reversal of the overall wind direction from Westerly to Easterly in the Stratosphere above the Arctic) does not look as likely to occur this year, with the highest chance of such an event being in Late January/Early February.
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. This year it looks more likely to be a case of the displacement or dissipation of the polar vortex being caused by a more minor warming event.
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 we have seen an increase once again in solar activity during the course of October after a very quiet month in September, but signs from NASA are now that the reversal of the magnetic poles on the sun which cause the variations in solar activity has now completed or is very close to completing at least. So whilst we did see an increase in activity similar in scope (but not quite as high) to the winter of 2011/12 things have now quietened down once again through November. If we were to take 2011/12 as a benchmark that year colder weather in the UK was restricted to February. If we were to follow such a pattern for this year the changes have taken place a month earlier than in 2011/12 and so the conclusion to draw would be that whilst the potential for anything significantly wintry is more restricted in December, as we progress through January and February the chances increase. Again, this sits fairly well in line with the overall forecast.
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Forecaster: Kris Surtees
UK Winter 2013/14 Prediction