BBC monthly outlook
Summary
Mostly mild but often wet and windy in the north
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Wednesday 1 January – Sunday 5 January
Generally mild with rain, mostly in the north
New Year's Day will be rather cloudy for many, but some sunny spells are likely for parts of central and eastern Britain. Some patchy drizzle is likely in places, mainly in the west, but most areas will stay dry. Overnight and into Thursday morning, a weather system will move in from the west and northwest, bringing some bands of rain to Scotland and Northern Ireland. By Thursday afternoon some rain may reach into north Wales and northern England but further south and east, things will stay dry. Turning quite breezy in Scotland, with some strong wind gusts for northern areas.
By Friday, the front will clear away to the east with a ridge of high pressure expected to build in behind it. This will bring temperatures down to nearer the seasonal average. A weak front will linger over northern areas of Scotland and the Northern Isles, but elsewhere things will be dry with plenty of sunshine. Through the weekend, the weak front will remain in northern Scotland with dry weather elsewhere, but by Sunday the next weather front will begin to push in from the west bringing a wet and windy end to the weekend across Northern Ireland and Scotland. A mostly mild and dry start to 2020.
Monday 6 January – Sunday 12 January
A north-south split in the weather but mild
The first full week of January will see much more active weather systems move into the UK from the North Atlantic, often in rapid succession. However, this will be finely balanced, thanks to a ridge of high pressure that is expected to build into central Europe. The UK will lie on the edge of the influence of this high pressure zone, leading to a distinct north-south split in the expected weather. Southern areas of the UK will tend to feel the presence of the high pressure centre more often, and this will keep things mainly dry and fine. Occasional weather fronts may reach into the area, but they will tend to be weak and bring only patches of rain with some moderate to fresh winds. Southwesterly winds will continue to feed in tropical air from the Atlantic resulting in a mild outlook for the time of year.
This will be contrasted quite strongly with northern areas, which will see frequent fronts move through with some very strong winds, making for a stormy outlook. Rain may be heavy and persistent at times, especially for northern Scotland, with some gales or even severe gales along the coasts. The big question mark for this period is how far south the rain and winds will extend into the UK. There is still some uncertainty on the strength of the high, and there is a chance that is may be a little more dominant than forecast. This would tend to push fronts off to the northwest, resulting in drier and calmer weather for northern area as well as the south.
Monday 13 January – Sunday 26 January
Temperatures nearer normal, perhaps stormy
For the middle and latter weeks of January, things become a bit trickier to forecast. This is because the UK will sit on the edge of high pressure and low pressure, so very minor adjustments in the strength and position of these features will either see the UK remain mostly dry or become quite wet and windy. One thing is more certain; there are currently no strong signals for any prolonged cold outbreak throughout January, and temperatures are expected to be near or above average for most of the month. We do gradually expect high pressure to become more dominant as the month goes on. Things will turn slowly drier and calmer as weather fronts are pushed further northwest and closer to Iceland. This will be a very gradual transition, so mid-January will likely still be quite unsettled and possibly stormy at times, in parts of Scotland especially. Occasional weak fronts will likely reach into southern and eastern areas as well, but increasingly infrequently.
By late-month, high pressure should shift closer overhead with things drying out for most places. The only exception being the extreme north of Scotland and the Northern Isles, which may continue to see weak fronts bringing rain and some stronger winds. The high pressure will cut off our access to the milder tropical air, so while things will turn drier and calmer, the temperatures will tend to dip nearer to average. There is a risk that strong, more active weather systems could overpower the high pressure from Europe, keeping things unsettled and stormy for the UK. Therefore, confidence is medium to low towards the end of the month, and some of the details are likely to change in future updates.
Further ahead
We will try and pin down where exactly high pressure will reside through January and how much of an influence on our weather it will have.
https://www.bbc.co.uk/weather/outlook