In our February forecast of last week we mentioned the general stabilisation of the West-European weather, because of increasing influence of high pressure systems. Yes indeed, not one, but two.

Currently high pressure over Scandinavia dominates the weather and cold easterlies have already set in, bringing continental frost to Germany and the Low Countries, and even carrying a touch of that cold weather across the North Sea, to (the eastern) parts of England and Scotland – but no further to the west, not to Ireland, and that’s because of one oddity of this winter weather episode: a positive NAO*.

[*) Above image shows two-week NAO forecast by NOAA – and the dominant NAO phase pattern in previous months.]

The North Atlantic Oscillation (NAO)

The NAO, or North Atlantic Oscillation, is an air circulation pattern, which determines the route of the normal west circulation over the North Atlantic Ocean. When the NAO index is positive, depressions are dominant in the north of the North Atlantic (around Iceland) and high pressure is dominant in the south of the North Atlantic (around the Azores), when the NAO index is negative it is the other way around.

When it comes to West-European winter weather, no single factor is more important than this NAO index. A positive NAO brings a strong Atlantic influence, with mild and wet air. A negative NAO brings a continental winter, dry and cold.

Back in October several weather agencies predicted NAO would dip to a negative state in the first half of the winter, and that in West Europe especially December would therefore be very cold*.

In response we pointed to a connection between NAO and sunspot observations – which led us to expect during the winter of 2011-2012 a positive NAO would be more likely, as solar activity has clearly picked up and we are currently experiencing a new solar maximum.

[*) In case you are interested, here is a list of specialised climate forecasting companies who were wrong. If your own favorite day-to-day weather forecaster was your personal source for this flawed prediction – you can’t really blame them, as in general weather people really have little idea about climate trends (and of course the other way around) – so all they do is copy such predictions (and be sure to say something similar as their commercial competitors) – and who is to say that would seem a bit dumb is ignoring politically that is actually the safe market position, to outsource even your thinking.]

NAO is positive for almost four months on end

If we look back to November, December and January NAO has indeed been almost exclusively in the positive state – and West Europe has thus far experienced one of the mildest winters on record.

But as meteorological models show the picture will be quite different at the start of February, with a strong high pressure system over Scandinavia, which forces cold air over the northern parts of West Europe.

Oddly though (you won’t hear these cold winter forecasters about this – because the current temperatures conveniently mask the fact that they were wrong anyway) this frost period occurs without NAO turning to a negative state [which we think is an indirect influence of SST anomalies caused by the current La Niña].

That leads to an exceptional European weather chart with (when you include part of the Atlantic) low pressure in the northwest and high pressure in both the northeast and the southwest.

It is a weak position for true winter weather, as it leaves no safe escape route for the Atlantic depressions around Iceland. For persistent winter weather one would want them to slip down a southern route, underneath the Scandinavian high, bringing rain in the Mediterranean and snow in Central Europe. When this route is blocked by a bridge between the Scandinavian and Azores high, the depressions could create a northerly route, over northern Scandinavia. This would be detrimental for the cold winter scenario, as it would cut of the Arctic supply of cold air towards East Europe.

This is the scenario we thought most likely in our forecast last week.

As models now show however there is a third way: lingering. As long as the two highs stick to their positions and do not really connect neither route is more attractive for the depressions than the other – so they stay put. Meanwhile a big chunk of the air molecules in the troposphere that are lifted up over the ocean around Iceland are dumped to the east, feeding the high pressure system over Scandinavia – and therefore even strengthening the winter weather in parts of West Europe.

February forecast update: frostier

Therefore the frost outlook has changed. Last week different leading forecasting models showed clearly different scenarios for the first week of February, with for instance the European ECMWF model much more inclined to support the cold scenario and the American GFS much more inclined towards a milder outcome. (Besides, all models switched sides at some point.)

Now the models agree on the cold scenario, at least for this first week.

With a persistent positive NAO that is exceptional, but when compared to climate, that is indeed what weather always is.

But as we stated last week – it is still the likely thing to happen at some stage in early February that the Azores high will move in and end the frost. But also in that scenario West Europe can enjoy an esthetical and quiet weather type – relatively dry and with a good chance of enjoying winter sunshine.

© Rolf Schuttenhelm | www.bitsofscience.org