Winter Weather Outlook 2012/13 – Issue 2
Issue Date: 31st October
The general layout of this outlook will be similar to the first. I’ll start by taking a look at the various different drivers of our weather before taking a look at what various long range models are predicting before giving a brief summary/conclusion of what the most likely outcome for this winter is.
Firstly i’ll take a look at Arctic Sea Ice levels. We’ve seen some rapid ice growth over the past few weeks with a fairly large jump in ice extent, we have however just had one of the largest ice melts on record and this added solar absorption in the polar regions is thought to cause an increase in Northern Blocking, however at this stage it’s pure speculation.
Next I’m going to take a look at Eurotion, Russian and Scandinavian snow cover, whilst disputed by some, there is an idea that rapid snow cover during the course of October across Russia, Europe and in particular Scandinavia can lead to an increased chance of colder weather for us here in the UK. Taking a look at the charts and historic data, we currently have record breaking snowfall cover for this time of year. Over the last week we’ve seen rapid snowfall increases across the above mentioned areas.
Moving onto Solar Activity, something that has been largely popular with long range forecasters over the past couple of years. We’re still very much below where we should be this far into the cycle with the sun having remained very quiet since the first issue of the forecast. There’s now a lot of evidence which suggests low solar activity leads to an increased risk of blocked European winters.
Onto the QBO and there isn’t much I can say that I didn’t say in the first forecast issue because the QBO remains unchanged. Here’s what I said in the first forecast issue.
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 Statosphere and propagate downwards over a period of months. Current forecasts suggests 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.
Now onto perhaps one of the more important factors when it comes to long range forecasting, the Polar and Tropical Stratospheres. In my last update I mentioned the Polar Stratosphere but I couldn’t go into it very much due to how early in the season it was. We can now start to get a feel for how the Stratosphere is at the moment and how it may effect the first part of winter in particular.
The Polar Stratosphere has been calling as you’d expect moving into winter as the sun begins to set across the Polar Regions, we did see a period of below average temperatures but small warmings have recently brought us back up to around average, whilst not spectacular if you want a cold winter then being around average is far, far better than being below average. Current Polar Stratosphere forecasts, whilsts quite far out at the moment do suggest some warming around the middle of November across the Canadian side.
A warming of the Stratosphere helps to disrupt the Polar Vortex which can aid the development of blocking. Interestingly so far this Autumn the Polar Vortex has been very weak and very disrupted which is why we’re currently seeing high pressure over Greenland instead of a developing Polar Vortex. One of the reasons for this I think is down to the Tropical Stratosphere being well below average. This helps reduce the temperature differential between the Tropics and Polar regions which in turn, weakens both the Jet Stream & Polar Vortex.
With this in mind, I think the chances of a colder start to winter 2012/13 are increased.
Moving onto Sea Surface Temperatures things haven’t changed a great deal since the first issue. We still have that cold PDO across the Northern Pacific. We still have warmer waters, albeit slightly less warm than before in the Western Atlantic around Newfoundland, we still have warm Atlantic Tropic waters and we still have a cold pool of water to the North-East of Iceland. This very much favours a more negative NAO as we move into the first part of the winter season, we’re already seeing the effects with the NAO recently moving into the negative with little sign of going positive at the moment.
This takes us nicely into ENSO conditions across the Equatorial Pacific, we’ve seen some slight warming which perhaps hints towards an El Nino but it really is struggling to develop at the moment and it’s certainly a lot weaker than it was previously predicted to be. The latest predictions suggest we’re going to see a very weak El Nino or Neutral ENSO conditions as we move into the winter period, this looking increasingly likely to develop into a La Nina in the later and final stages of winter. With such a weak/neutral ENSO likely I think other factors already talked about above are going to be the real drivers, with ENSO taking more of back seat this winter.
Now that I’ve talked about some of the main drivers of our weather, I’m going to take a look at the longer range models to see what they’re showing and whether this correlates to the above.
Once again we’re going to start off with the JAMSTEC Model, I’m not going to be showing the Precipitation chart for all models because I find them to be fairly unreliable, I will instead be focussing on temperature predictions and pressure pattern predictions, I often find these to be more accurate. For the purpose of the forecast I’m going to be focussing on the period December – February.
Whilst the model was consistent through it’s August and September updates, the latest update is much colder than it was previously predicting, this does tend to correlate with the previous discussed drivers. When it comes to Long Range models I like to look for consistency, this model may be picking up on a new trend, I’d like to see the November update to see whether that continues the cold forecast before making too much of a prediction based on this one model alone.
Next we’re going to take a look at the Met Office Probability Maps, when I issued the first forecast these charts were predicting a high probability of below average temperatures across the UK. This model is fairly difficult to explain so I’ll talk about exactly what it’s showing.
The first chart shows temperature probability. I’ve written below the predicted percentage is for a particular outcome based on this model.
Above Average – 0-20%
Average – 0-40%
Below Average – 60-80%
There’s quite a strong signal from this model for below average temperatures across the UK this winter. This model has been remarkably consistent in going for below average temperatures over the last few updates.
Sticking with the Met Office charts we’re going to take a look at the Ensemble Mean Pressure chart. It suggests an increased risk of High Pressure to the North and East of the United Kingdom with quite an extensive blocking setup, this correlates very well to the strong signal for below average temperatures and also remains consistent with the previous updates.
We’re now going to take a look at the Russian Model prediction. The temperature prediction chart below shows something quite different to what we’ve seen in previous updates and that’s a massive flip towards colder conditions across the UK & Europe with above average temperatures in the Atlantic and up into Southern Greenland. This is indicative of some sort of blocking feature.
I mentioned above that I like to look for consistency in these long range forecast models and up until now, this model was consistently forecasting above average temperatures so I think we need to take this model, like all other seasonal models with a big pinch of salt. It’s interesting to see it coming into line with some of the drivers of our weather though, nonetheless. Taking a quick look at the Russian Precipitation model we can see a large below average Precipitation anomaly being predicted over and around the United Kingdom.
We’re going to take a look at a model that wasn’t featured in the first outlook now and one that might be a little confusing at first but fairly easy to get your head around once you know what you’re looking at, this is the Brazilian Model. The key thing to remember for this chart is the colours appear to be the opposite to most models. Blues indicate high pressure and reds indicate lower pressure.
The chart below is the projected height anomalies for December, January and February. This model is predicting low pressure to our South and high pressure to our North, indicative of quite cold conditions across the UK during the winter months.
This models temperature prediction doesn’t appear to follow the above charts prediction however and instead brings average temperatures across the UK, though I suspect it’s simply not picking up much of a signal either way.
The precipitation chart from this particular model however does go along with what the Height Anomaly chart shows and predicts below average precipitation to the North of the UK and above average precipitation to the South.
Now we’re going to take a look at the CFS charts. In the last update the CFS was predicting above average temperatures across Europe and Southern England, before I move onto those charts I want to take a look at the monthly 500MB pressure anomaly charts.
Sticking with the CFS model we’re going to take a look at the temperature anomaly charts. These charts continue on from the last update with above average or average temperatures predicted for the UK & Europe.
The CFS chart whilst consistent, is very much on its own with regards to predicting above average temperatures for the UK with a majority of models now going for a colder regime with blocking heights to the North. Even models which were previously predicting warmer conditions are now seemingly showing colder conditions for the UK this winter.
Conclusion and Thoughts
In the first issue I mentioned that the models predicting above average temperatures were causing me to be slightly more cautious than I usually would be in making a seasonal forecast. The drivers talked about during the first part of the forecast all point towards colder than average conditions for the UK this winter. If the Stratosphere does indeed warm like it’s predicted to during the middle half of November then we are likely to see a much colder December than we did last year with things becoming increasingly blocked. The atmospheric drivers are seemingly primed towards much colder conditions and now that we’re seeing many of the Long Range models also trending that way, I’m now more optimistic about the UK experiencing a cold and potentially snowy winter.
The Stratosphere needs to be watched over the coming weeks but at the moment I think the first part of winter could be cold or at times very cold. The second half of winter of course a little too far out to forecast but I’d imagine things would be slightly milder with more Atlantic interludes.
Winter Weather Outlook 2012/13 – Issue 1
Issue Date: 21st September
Firstly, I’d like to take a look at the current Arctic Sea Ice Levels, this year we broke the 2007 record for ice minima and there has been some speculation that lower sea ice levels lead to colder European/UK winters, whilst this is pure speculation at this stage with little scientific backing or research, it does seem to tie in with our colder winters since the previous record breaking 2007 Ice Minima, bar 2011/12, whether this is simply a coincidence or not is something that’s yet to be seen, but I thought it might be worth mentioning.
Next I’d like to take a look at Solar Outputs, despite nearing a Solar Maximum sunspot activity is still very quiet compared to the more recent solar cycles, there’s gaining evidence and agreement within the scientific community to suggest lower solar activity lead to a more sluggish Jet Stream during the winter months, this in turn can leads to increased blocking in the higher latitudes which leads to cooler European/UK Winters.
Next we’re going to take a look at something slightly more technical, but I’ll try and keep things simple. 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 Statosphere and propagate downwards over a period of months. Current forecasts suggests 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.
Westerly QBO = Positive Phase
Easterly QBO = Negative Phase
Above I mentioned Sudden Stratospheric Warming, a warmer stratosphere is usually indicative of High Pressure, the warmer air can disrupt the seasonal Polar Vortex allowing heights to build, Sudden Stratospheric Warmings are thought to bring colder weather to the UK a few weeks after occurring and at this time of year it’s hard to get an idea of what state the Polar Stratosphere will be in come winter. The Stratosphere temperatures are however incredibly important and can often over ride other global signals, SSW events often lead to a Negative Arctic Oscillation, the AO is the measurement of pressure over the Arctic.
Positive AO = Low Pressure, strong Polar Vortex
Negative AO = High Pressure, Weak Polar Vortex
Whilst all other factors could be pointing towards a bitterly cold winter, if the Stratosphere temperature is below average the risk of prolonged cold weather is significantly lower. Below is a chart showing how the temperatures at 30hPa look at the moment, the temperature in the Stratosphere is currently spot on average for the time of year.
Now we’re going to take a look at the Sea Surface Temperatures, these can have quite a large impact on where pressure patterns develop and they can actually help us determine whether or not a negative or a positive North-Atlantic Oscillation is likely, the NAO is the measurement of pressure in the Atlantic.
Positive NAO = Low Pressure, unsettled weather
Negative NAO = High Pressure, blocked, potentially cold weather
The current Sea Surface Temperatures are fairly impressive, the excess and record breaking Arctic Ice Melt has lead to a very cold pool of water to the North of the UK and East of Iceland. Northerly Winds in this region have helped push the cold Arctic water down towards the Atlantic. Cold Sea Surface temperatures around this area often hinder Low Pressure development and favour High Pressure development, High Pressure in this area would bring colder weather to the UK, thats not to say High Pressure WILL develop here, but there is an increased risk if this cold anomaly continues into the winter.
We also need to take a look towards the Western Atlantic, off of the coast of New Foundland and to the South of Greenland we have these very warm Sea Surface Temperatures, this area here is what we call the Atlantic multidecadal oscillation or AMO and this is currently in its warm phase, we also have another area of warmer weather further South to the West of Africa and when we see Sea Temperature Anomalies placed in this particular way a negative NAO is favoured, as we talked about earlier a negative NAO can lead to cooler, snowier weather during the winter months.
The above SST chart as well as showing what I’ve just talked about above, also shows the state of the Pacific at the moment and whether we’re in a La Nina or El Nino, at the moment things are fairly uncertain whether El Nino is going to start to develop, the NOAA chart shows that if it does develop it’ll be a very weak affair so I think a Neutral ENSO/Weak El Nino is likely as we go into the winter months and whist this in itself is unlikely to have a major effect on our weather, it will allow other signals to effect our weather more strongly than they would if the ENSO signal was stronger.
Another thing we can gleam from the chart above which is interesting is the Pacific decadal oscillation or PDO for short, this is shown by the area of blue, colder weathers in the center of the Northern Pacific, this goes through varying 30 year phrases of warm and cold, it’s currently in it’s cold phase and because El Nino is a measurement of warmer waters across the Pacific, having a cold PDO does tend to hinder El Nino development.
Now that we’ve spoken about a few of the more technical drivers to our weather, what they mean and what potential conditions they could bring, I’m going to talk about something slightly less technical and that’s the Long Range Model Forecasts. It’s important to remember that these models change fairly frequently and are subject to wild swings within their outputs, they are highly experimental and at this stage it’s really about spotting trends and consistency within the outputs rather than the what models themselves actually show, the drivers talked about above will probably offer more of an idea than the models below but I think it’s important to talk about what they show.
Firstly I’d like to talk about the JAMSTEC Model, to get a feel for this models consistency I’m going to post it’s July, August and September issue for the period December – February. All three are fairly consistent in what they show which is an average-slightly below average winter temperature wise
For the same December-February period I’m going to post the Precipitation predictions, what we see is fairly interesting. The model is forecasting average or below average rainfall across the UK with below average rainfall to the West of the United Kingdom so this is perhaps indicating High Pressure being out in the Atlantic more often than not during the winter months.
Whilst the model does change between the two issues slightly, the overall trend from the model is for average or below average rainfall, with average or slightly below average temperatures during the period December – February.
Next we’re going to take a look at the Met Office probability model, whilst this is fairly difficult to explain the chart itself I will lay out exactly what the model is showing to make it easier to anyone who’s only just getting into the realms of the weather models. Like above, we’re going to be looking at the period December – February.
This first model is for temperature, and what it’s showing is this;
Above Average: 0-20% so at this present time, there is around a 20% chance of an above average winter
Average: 20-40% so at this present time, there’s a slightly higher chance that we’ll see an average winter than we will an above average winter
Below Average: 60-80% which means, overall, the signal is for a below average winter, however we have to remember there is also a fairly good chance that winter will also be average, going by this chart alone.
Sticking with the Met Office charts, we’re going to take a look at Ensemble Predicted Pressure Patterns, the first graph below is for the period November through to January and as we can see the model predicts Higher Pressure to be located in the Atlantic stretching up to Southern Greenland with lower heights over Europe, this is generally indicative of a cold, North or North-Easterly flow.
This second graph is going to again look at pressure but instead will be for the period December through until February, again it is an Ensemble mean run as above – This chart shows a slightly similar pattern although with High Pressure in the Atlantic closer to the West of the UK and low heights across Europe not quite so pronounced, this generally indicates dry, settled and probably cold weather
I’m going to post the Met Office Precipitation charts now, although unfortunately they don’t really give us a clear signal at the moment, I often find the models aren’t very good at handing overall precipitation amounts. The chart below shows an equal statistical chance of Above Average, Average and Below average rainfall
I’m now going to take a look at and discuss the Beijing Climate Center long range computer model. The chart below covers the period December – February and shows us pressure anomalies at 500hPa, there’s quite an extensive high pressure system across Western Greenland and into Canada but lower pressure over the United Kingdom suggesting a very mixed weather pattern.
The temperature prediction from this website is going for warm being indicated by the oranges, this would generally make sense considering the 500hPa signal for low pressure.
The precipitation signal from this model, as you’d expect given the above to charts is going for quite a wet winter season
Next we’re going to take a look at the CFS Model, again like the above models this model is also subject to wild swings within it’s output and at this stage shouldn’t be taken too seriously.
We’ll start with the Pressure Anomaly chart for the period December, January and February. The model is forecasting higher pressure down over the Mediterranean and lower pressure to the North of the UK, this indicates a mild and wet Westerly airflow.
Below is the temperature prediction for the months December, January and February, at this moment in time the model is predicting above average temperatures in the South and average in the North
Below is the chart showing the precipitation signal, the model is forecasting above average rainfall for much of the country during the period December – January
Conclusion & Thoughts
If I were to base my thoughts on the atmospheric drivers talked about in the first section I’d be inclined to say that we’re heading for a below average winter with the potential for some cold, snowy outbreaks. The Weak ENSO, -NAO primed Sea Surface Temperatures combined with Low Solar Activity and a Easterly QBO really does point towards increased amounts of blocking across the Northern Hemisphere and a weaker Polar Vortex. However I also have to factor in the various Long Range Computer models.
At this current time I’m expecting winter temperatures to be slightly below average with more snowfall than we saw last winter. The models throwing up the milder patterns, despite being against current background signals do throw some caution into the mix which is why I’m holding back slightly on going for a significantly cold winter. I also have to think about the fact that we don’t really have an idea about how the Stratosphere will be this winter, as mentioned earlier the stratosphere can make or break a winter despite what seems like very good background signals.
My next update will be issued on October 31st, by then we’ll have a much clearer idea of the Stratosphere and I’ll hopefully be able to say with a lot more confidence, at least for the first part of winter, whether we can expect to see cold and snowy weather.