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Forecast for Winter 2006/7
Temperature
As in last month's forecast, probabilities weakly favour temperatures warmer than 1971-2000 averages over much of western and central Europe. However, there has been a return to a finely balanced situation over northern Europe where the influence of El Niño may encourage cold outbreaks during mid to late winter; at times such outbreaks can be expected to affect parts of the UK.

For the UK, December is expected to continue mild in most regions. Near average or warmer-than-average temperatures are the more likely outcomes for the winter period as a whole with the potential for lower temperatures (relative to average) in mid to late winter.


Precipitation
Signals for precipitation from our forecasting methods are mixed over Europe, and in most regions there is no clear indication for either above or below normal.

For the UK, above-average rainfall is likely to continue through much of December, and best estimates slightly favour average or above-average precipitation for the winter as a whole.

Winter 2006: Close to average
Temperature
Close to or slightly above average temperatures across much of the continent.
Precipitation
Close to or slightly above average.
December
1) Mild and wet during the first half of the month. The second half of the month may bring a trend towards colder weather with the possibility of some wintry conditions for a time.
2) Temperatures well above the average during the first half of the month. Turning colder later with snow.
3) Mild and unsettled during the first half of the month with above average levels of precipitation. Possibly colder with some snow during the second half of the month.
4) Often settled with close to average temperatures, however, there is expected to be an increased tendency towards cooler and less settled weather later.
January
1) Unsettled at times during the first half of the month. During the second half of the month there is a possibility of a cold and wintry spell of weather developing and persisting for some time, with snow at times.
2) Unsettled with sleet or snow at times during the first half of the month. Increasingly settled later.
3) A cold month is expected with frequent spells of sleet or snow.
4) A rather cool and unsettled month is expected with above average levels of precipitation.
February
1) Mixed at first, but probably becoming increasingly settled later on. Temperatures close to or above the seasonal average.
2) Turning unsettled at times with close to average temperatures. Precipitation levels close to the average.
3) Cold conditions at the start of the month are expected to give way to milder and less settled weather later.
4) A trend towards mild and increasingly dry weather is expected to develop throughout the month.
Country key
1)Northern and Central France, Benelux countries, Western Germany and Denmark
2)Norway, Finland, Sweden
3)Poland, Eastern Germany, Austria, Switzerland, Hungary, Czech Republic
4)Portugal, Spain, southern France, southern and central Italy

Winter forecast 2006/7
Winter colder than average

Once again we approach the winter months with a period of remarkable warmth just behind us. The May to October period was the warmest ever recorded.

Does this mean that the winter will be very mild ? In my view it does not and here I will explain my reasons for believing that the coming winter will be close to or below the long term average temperature wise.

The principal factors to consider for the winter forecast in my view are :

ENSO (El Nino Southern Oscillation)
NAO (North Atlantic Oscillation)
AO (Arctic Oscillation)
SSTAs (Sea Surface Temperature Anomalies)
Solar Activity
The other factors that influence my thoughts are recent trends both in terms of general synoptic patterns and of course the overall general warming.

The ENSO is a measurement of sea surface temperatures in the equatorial Pacific region. During the Summer SSTAs in this region rose by a significant enough amount to define a warm phase of the ENSO (El Nino). Such an event, and the strength of the El Nino has a downstream impact on weather conditions in Europe. A strong El Nino, such as the events observed in 1982 and 1997 mean that a mild winter, when considered along with other factors, is likely for the UK and Western Europe, as we see a strong jet stream throughout the winter months.

Indications at the time of writing suggest that this year’s El Nino will be classed as weak to moderate. This can be consistent with average or colder winters in Western Europe depending on other factors.

The NAO is crucial to the winter. A positive NAO is usually signified by High pressure in the Azores region and Low pressure in the Iceland area. This typically brings mild weather to Western Europe whilst the reverse position (Higher pressure around Greenland/Iceland, lower pressure in the Azores) usually mean colder conditions.

I believe that the overall NAO pattern of any winter period is essentially driven by cycles and SSTAs in the Atlantic. Much of the 1990s and the early part of this decade saw the NAO in a strongly positive phase. The last eighteen months in particular however have seen neutral or negative NAO patterns prevail for much of the period.

SSTAs in the Atlantic continue to see a persistent warm anomaly on the Western side of the ocean whilst there is cooler water around Newfoundland and cooler patches off Greenland. This can change over time of course, but these SSTAs are consistent with negative NAO synoptics.

The Arctic Oscillation is a measurement of height anomalies at the pole. Autumn thus far has seen a lot of high latitude blocking in this region and when this occurs, the pattern is often continued throughout the winter months. How the AO interacts with the NAO is not clearly understood (at least not by me! ) but we can use this information to further argue that high latitude blocking could be a factor in the winter ahead.

We are currently in a phase of low solar activity and there are grounds for arguing that such periods can have an impact on our weather patterns, possibly leading to a weakened jet stream and more blocking. I believe that this may have a particular influence in the second half of a winter period.

So how can we piece all these factors together and produce a winter forecast for the British Isles ? Well, I would anticipate that the El Nino together with SST anomalies in the Western Atlantic will drive a strong jet stream for much of the winter. For much of the winter last year we saw the Pacific jet driving the pattern. This meant lower pressure over Greenland and High pressure over Eastern Europe. It is inevitable in my view that the differing patterns expected this winter will mean periods when the jet barrels through to the North or close to the UK meaning that we will be on the mild side of the Polar Front. However, I would also expect much of the energy to channel into the Subtropical jet stream which will mean pressure build over Greenland, and later in the winter High pressure over Scandinavia is likely to bring cold continental air Westwards.

I am loathed to quote analogue years when all the factors that I have described are broadly similar to what we are looking at for the winter ahead as I believe such comparisons are less significant in the modern climate but 1968/69, 1977/78 and 2002/3 have similarities with the situation right now.

The key statistics and how I expect the synoptics to develop over the winter months are :

Precipitation slightly below normal
Temperatures slightly below normal. I would expect the overall CET to be around 3.5C making it the coldest winter since 1995/6
The mildest and wettest conditions to be in the second half of December and early January.
The possibility that February will be the coldest since 1991
There are, in the modern climate, a number of issues to consider particularly in relation to the UK and Ireland when outlining a general pattern.

One factor that I noticed last winter was the development of ‘shortwave’ low pressure features that inhibit the High latitude blocking that is needed to sustain cold air over the UK. This is particularly a factor when considering whether or not continental cold can reach the UK.

Another issue is that so many factors are needed to fall into place at the right time for persistent cold spells to affect the UK. Although hardly a scientific analyses, the old adage ‘if it can go wrong it will’ springs to mind.

Please note that the above are not caveats for the overall general pattern of the forecast being wrong – just possible reasons why there may not be prolonged colder spells.

However, with all factors that I believe relevant to be considered, the call is that the winter will be cold when compared to recent years and very interesting for all weather watchers.