A negative phase of the North Atlantic Oscillation (NAO) index has been a common feature of summer months in the last decade. A negative NAO phase in summer is typified by unusual frequency of high pressure areas over Iceland and Greenland, low pressure areas over the UK and southern Scandinavia and persistent high pressure ridging over southern half of Europe.
A positive NAO phase in summer tends to favour high pressure areas more widespread over north-west Europe, central Europe and Scandinavia and shallow and slow moving areas of low pressure over southern and eastern Europe. MeteoGroup believe that a negative phase of the NAO will probably be most dominant during summer 2016, but confidence is only moderate for this outcome.
A consensus of latest guidance indicates that low pressure troughing associated with the negative NAO phase may be most focussed over Scandinavia, rather than both the UK and Scandinavia. Meanwhile, persistent high pressure areas are most likely over central and south-east Europe and also over the north-eastern Atlantic, close to the UK.
Looking at the summer overall, a strong north-south gradient in temperature anomalies is expected, with cool and rather wet conditions across Scandinavia and warm and dry anomalies most likely across Cent and southern 2/3rds of Europe, including much of France and Germany, and also Italy and Switzerland. Confidence is lower for north-western parts Europe, especially the UK, with a number of statistical indicators (based on previous, similar years) predicting drier/calmer than normal conditions overall through this summer due to high pressure ridging being more frequent than usual. However, most likely is for a couple of wet spells here too, during the course of the summer. There is potential for several heat waves over southern Europe, especially southern parts of central Europe, Italy, Balkans and eastern Europe.
Drought conditions appear possible over eastern Europe, Balkans and south-western Russia, due to persistent ridges of high pressure. A negative NAO pattern in summer often favours the jet stream to be further south than climatology over central and western Europe. This, combined with hot conditions over southern half of Europe, can bring the risk of severe thunderstorms on the northern edge of the heat – most often over France, Low Countries, north-western Alps and Central Europe.