Europe will be influenced by rather volatile changes in weather patterns over the continent during the autumn 2017 season, as predicted by both numerical model guidance and MeteoGroup’s own statistical analogue analysis. As a result, predicting the dominant seasonal anomalies is more challenging than normal, as it is more difficult to determine which weather patterns will be most persistent or most extreme during the three months.
One driver on the autumn pattern over Europe will be a neutral phase of the ENSO (El Nino Southern Oscillation), with the slight risk for weak La Nina conditions for a time (cooler than normal sea surface temperatures in the central and eastern tropical Pacific). This differs from expectations earlier in the year, when a consensus of forecast guidance indicated potential for weak to moderate El Nino conditions to develop during the autumn.
Lack of El Nino this autumn favours an increased number of Atlantic tropical storms and hurricanes, especially with above normal sea temperatures over much of the tropical and north Atlantic ocean. We have already seen 8 named tropical storms and 2 hurricanes so far in the Atlantic sector, even before reaching the most active part of the tropical cyclone season (September and October). Impacts on Europe will be the potential for a few episodes of strong winds and very heavy rainfall over northern and western areas, as remnants of hurricanes sometimes move towards Europe from the Caribbean. Additionally, an increased chance of a warmer than normal autumn over Spain, France and UK. An increased number of tropical storms over the Atlantic may also reduce predictability and skill in weather forecasts, as the model guidance still occasionally struggles to resolve the evolution of tropical systems passing into the mid-latitudes and their effect on jet stream.
The North Atlantic Oscillation (NAO) index is expected to swing from negative to positive several times this autumn, indicative of the volatile strength of the Atlantic jet stream and westerly wind flows into Europe. September will see the driest and warmest conditions over Italy and central and eastern Europe, with further hot spells at times here. The north-west of Europe and Scandinavia will be less warm, with very wet and windy conditions at times. Spain and the Alps are likely to see a few rounds of heavy thunderstorms and windy spells.
Our view is that October will once again feature above normal frequency of high pressure blocking patterns over northern Europe. This brings the threat of some chilly nights over central and eastern Europe, calm and dry weather in the north-west, while Italy and SE Europe will see swings in temperature from warm to cool and some very wet and windy conditions at times. November is expected to see wet/windy conditions return to the far north and west of Europe, while drier and more settled weather develops over the Alps, Italy and the Balkans. Eastern and south-eastern Europe could see some chilly spells at times in November, but mild in the north, west and central areas.