Winter 2012/13

Due to the length of the previous page, I have decided to add the official forecast on a page of its own. To see the previous two winter outlooks to see what I’ve been expecting up until this point, click HERE

Disclaimer

Forecasting months in advance is something that is incredibly difficult to do in particularly so for the UK’s weather because there are so many different variables that affect our weather, the fact that we’re sandwiched between a massive landmass and an ocean only adds to the complexity and chaoticness of our weather. Long Range forecasts should never be taken too seriously and should be used as more of a “guide” to most probable weather, rather than an actual forecast.

If you’d like to skip all the technical details, scroll to the bottom and read the conclusion.

Winter Weather Outlook 2012/13 – Issue 3, Official Forecast

The previous two updates have pointed to a winter consisting of below average temperatures across the UK. Have things changed?

To find out, I’m going to start by once again taking a look at the current snow cover across Scandinavia, Russia and Eurasia. During October we saw a sudden expansion of snowfall across these regions allowing some strong, deeply embedded surface cold to become established. As we go into the winter months this can act as a brick wall against Atlantic weather systems.

Northern Hemisphere Snow Cover

Whilst Solar Activity typically cannot be used to forecast short-term, looking at activity over the last couple of years could give us some idea of where we sit in this area and what possible weather this could lead to. There has been increasing amount of research into solar activity and it’s effect on our weather and there is now evidence to suggest low solar activity leads to more blocked European winters.

Solar Activity Cycle

The above chart shows us where we currently stand with solar activity, the forecast and the previous solar cycle. We can see that despite approaching a solar maximum, activity compared to the last cycle is considerably lower. We also saw a period of prolonged low activity during 2008 through until 2010. This suggests to me that a more, blocked European pattern is possible however there are of course many other factors at play here. Current solar images show that there are currently very few sunspots on the solar disk.

Solar Disk

I mentioned the QBO in both the first issue and second issue however things remain the same on this front. We’re in a weakening negative phase and this helps to reduce mean zonal winds across the Arctic helping to reduce the strength and effect the Polar Vortex will have during the winter. We’ve already seen this happening with a relatively weak Polar Vortex compared to what we would usually be seeing at this time of year. For reference, I’ve copied and pasted information about the QBO below to explain exactly what it is.

The Quasi-biennial oscillation or QBO for short is the measurement of the equatorial zonal winds between Westerly and Easterly in the tropical Stratosphere, the alternating phases develop at the top of the stratosphere and propagate downwards over a period of months. Current forecasts suggest going into this winter we’ll be in an Easterly Phase and this basically speaking can lead to a higher risk of Sudden Stratospheric Warming which I’ll go into in just a moment.

Stratospheric Warming is something many people have started to pay close attention to over the last few winters. So much so that even the Met Office have started to use Stratosphere Temperature data in their seasonal model data. Current Stratosphere temperature predictions are very good if you’re looking for a cold and blocked winter as over the last week or so forecasts have been hinting at some Stratospheric Warming which looks likely to lead to a splitting of the Polar Vortex as we move into early December.

100hPa Polar Vortex Chart showing a very weak Vortex

If we take a look at the mean zonal winds, we can see that going into December zonal winds are decreasing which is simply an effect of a weakening or splitting Polar Vortex.

100hPa Polar Vortex Chart showing a very weak Vortex

Blues indicate weaker zonal winds or Easterly winds. Reds are zonal Westerlies and we can see from the chart that these are under attack

Current Stratosphere forecasts suggest an increase in Wave 2 activity as we go into early December, this puts pressure on the Polar Vortex and explains why it’s likely to weaken and split.

Pink colours and positive numbers indicate Wave 2 Activity in the Stratosphere

The chart below confirms that there is currently a fair amount of pressure being put on the Polar Vortex from both above it, and below it.

Cut section of the Polar Stratosphere. Blues depict cold reds and oranges depict warm

Based on the above, and should these forecasts come to fruition then I’d expect the first half of winter (December & January) to consist of periods of blocking conditions across the Arctic which is likely to bring below average temperatures across the United Kingdom during this period.

This leads me on quite nicely to the current ENSO conditions across the Equatorial Pacific. In the last update we were looking at the possibility of weak/neutral conditions and this remains the same.

ENSO Forecast

A weak/Neutral ENSO generally speaking means that other factors will come into play and ENSO conditions will not over ride these so we can’t really get too much of a signal for these conditions.

We’re going to take a look at Atlantic Sea Surface Temperatures now. In the previous updates SST anomalies were favouring a negative North Atlantic Oscillation (NAO) which when combined with a Negative Arctic Oscillation (AO) can bring colder conditions to the UK. Below I’ve annotated a chart to explain what we’re currently seeing. This is suggestive of a negative NAO

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That’s the technical stuff out of the day so why don’t we take a look at some of the seasonal weather forecast models to see what they suggest? We’ll start with the JAMSTEC Model. We’ll take a look at both October’s update and November’s, we’re looking for consistency within the output.

JAMSTEC October Update

JAMSTEC Temperature Chart November Update

The charts above show surface temperatures. The October update was keen to bring in very cold temperatures across Europe and the UK. The November update however brings in milder, above average temperatures across Europe but keeps the UK below average. That’s generally quite a large flip with little consistency away from the UK prediction.

Now we’ll take a look at the ECM Three Month update, unfortunately the ECM keeps much of its data behind close doors so I can only show you the temperature chart. It’s predicting above average temperatures across much of Europe, the UK is also falls under above average but the signal here is less strong. Interestingly the model is forecasting above average temperatures to the North and East of Iceland, perhaps indicative of some sort of blocking feature.

ECM Three Month Forecast

Moving on we’re going to take a look at the Russian Model. This model no longer covers December so the period we’ll be looking at here will be January through until March.

In the first issue, this model was predicting near normal/above average temperatures for the UK. In the second issue it had changed to below average temperatures and that’s a theme it’s stuck with in the latest update.

Russian Model Temperature Forecast

The precipitation charts are quite interesting too as they suggest below average rainfall across the UK and to our North, but above average rainfall to the South. This is indicative of a Southerly tracking Jet Stream and blocking to our North.

Russian Precipitation Forecast

Next we’re going to take a look at the Met Office Probability chart. This one can be a little difficult to understand so I’ll write out exactly what the model is showing.

Chance of Above Average:  0-40%

Chance of Average: 20-40%

Chance of Below Average: 40-60%

Met Office Temperature Probability

The signal for below average temperatures across the UK this winter is clearly there on the Met Office chart, although the signal perhaps not quite as strong as some of the previous updates suggested.

The chart below shows the ensemble mean sea level pressure for this winter. Despite the above signal for below average temperatures the signal from this chart suggests quite a mild, zonal winter with low pressure to our North and high pressure to the South. Clearly there’s a little bit of confusion over this as there are conflicting signals within the same model output.

Met Office Ensemble Mean Pressure

Moving on, we’re going to take a look at the Beijing Climate Center seasonal model. The below chart shows temperature anomaly predictions.

Beijing climate center temperature chart

The above chart doesn’t really give a strong signal either way and is more suggestive of temperatures being around, or slightly above the seasonal average. Interestingly though if we look to our North and West across the Atlantic and up into Greenland, temperatures are forecasted to be above average. Again perhaps an indication of some kind of blocking feature.

Indeed, the height chart from the Beijing Climate Center mirrors the above, high pressure very much over the Arctic with low pressure tracking South of the UK.

Beijing Climate Center Height Chart

Conclusion and Forecast 

There are three important key factors that I believe will play into how this winter shapes up/

• State of the Polar Stratosphere
• Sudden expansion of snowfall across Scandinavia, Russia and Europe
• Negative QBO Phase

Given the above, I’d be more inclined to suggest a below average winter with the coldest weather focussed during the first half of the winter. Through the first part of December I think we’ll see a mixture of colder weather interspersed with very brief milder periods before colder air really begins to establish itself leading to a below average December.

January I’m expecting further Stratospheric attacks on the Polar Vortex keeping it weakened and displaced, this again leads me to believe we’re going to see a below average January.

Confidence of course drops away towards the second half of the winter though I’d expect to see colder conditions beginning to weaken with more of an Atlantic influence effecting our weather perhaps bringing longer milder periods of weather.

• December & January – below average with milder periods being generally short-lived
• February – Around average with milder periods being more prolonged under the influence of the Atlantic.

Forecasting specifics such as snowfall simply isn’t possible this far out, however below average does of course heighten the risk of snowfalls across the UK. For specific details over which areas will see snow, please keep an eye on the front page and Snow Watch Page, when snow is in the forecast, I’ll be talking about it.