Turning colder next week

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  • ECMWF Surface Pressure map for 00z Friday, showing a large high pressure system over Scandinavia and a draw of cold air from the east. Areas of low pressure are displaced well to the south and west of normal over Iberia and the Mediterranean. Image: MeteoGroup.

    ECMWF Ensemble temperature plume for UK averaged temperatures over the next two weeks. The ensemble average is shown in dark blue, with range of possible scenarios represented by the shaded area. Image: MeteoGroup

    Recent Arctic Oscillation trends and forecast for the coming few weeks. A sharp dip into negative territory is forecast over the next few days. Source: Prescient Weather via www.worldclimateservice.com

  • Turning colder next week
    04.02.2017 17:36

     

    After a drier than normal January across much of the UK, the start of February has seen a return to more unsettled and windier conditions. This has been due to a weakening and eastward shift of the high pressure system that has dominated the pattern in recent weeks, with an active jet stream driving several deep low pressure systems across north-west Europe, south of climatology. Indeed, while parts of southern Britain were battered by some wet and windy weather on Saturday, a more intense secondary low slammed into western France overnight. Gusts of over 90mph were recorded on the Aquitaine coast, leaving 250,000 homes without power.

    Over the next few days, however, a major change in the pattern will take place as an intense area of high pressure builds over Scandinavia, extending its influence westwards across Europe. Not only will this act to block the low pressure track, turning things drier and calmer across the UK, but easterly winds will start to draw in some cold continental air by Wednesday. Temperatures are therefore likely to widely below normal from mid-week with a return to some widespread overnight frosts and the threat of snow showers across eastern areas. As is often the case, the cold will be much more pronounced across Central and Eastern Europe with overnight minima below -25C across Belarus, Ukraine and western Russia. Meanwhile, areas of low pressure will be diverted south-eastwards around the high into the Mediterranean. This will bring wetter than normal conditions to Spain, Italy and southern France with some heavy rain and mountain snow, perhaps exacerbating the poor growing conditions.

    A climate index useful for quantifying the upcoming change in pattern is the Arctic Oscillation (AO). The positive phase of the AO is characterised by lower than normal pressure over the Arctic and higher than normal pressure over mid-latitudes. This gives rise to a well organised polar vortex and vigorous westerly jet stream, bringing milder and wetter than normal conditions to northern and western Europe. In contrast, a negative phase is associated with a weaker polar vortex and a more amplified ridge-trough pattern. Cold air outbreaks can result where blocking areas of high pressure develop at high latitudes. The next five days will see the AO drop sharply into negative territory as a result of high pressure development over both Scandinavia and Siberia, with late November the last time the index fell negative.

    There remains some uncertainty in how far west the cold, continental air will progress, and in how long it will remain in place. A slight eastward shift in the positioning of the high pressure system could mean the cold air remains over the continent, with the UK seeing more influence from milder Atlantic systems. Conversely, there are additional colder risks should the high shift further west, resulting in a more persistent and deeper feed of continental air. At present, it looks as though temperatures will be at their lowest Thursday to Saturday before a gradual moderation takes place, but stay tuned to our website and social media for further updates.

    By: Billy Payne