Changeable with occasional colder interludes
Since the previous forecast update last Friday, we have seen a few changes in the forecast emphasis for December. A huge and persistent low pressure circulation over the northernmost Pacific ocean will be a key global weather anomaly in next few weeks. Downstream of this feature, across the USA and Canada, the north Atlantic and Europe, the jet stream will take on a much wavier pattern than recently. As we predicted last week, low pressure areas will often be near to the UK in December, but they will dip south to Spain and Italy quite frequently. This will take the main thrust of wind and rain away from the UK at times, leaving drier, less windy and occasionally colder interludes.
Turning drier and calmer. Patchy fog and frost.
The first half of this week has seen mild air across the UK, with winds from the south-west. While the northern and western half of the UK has seen the majority of the heavy rainfall, a weakening cold front will move slowly eastwards across the UK during Wednesday 25th. This promises a dull, damp and grey day in the southern and eastern half of the UK. The north and west will brighten up and it will feel less mild than of late, especially by evening when temperatures will dip sharply.
The rest of the week will see a large area of high pressure shifting east across the county, influencing the UK's weather. Winds will much lighter than during the past couple of weeks, especially over northern and western areas, where strong south-westerly winds have been very sustained.
Overnight frost and locally dense patches of fog will become the primary hazards through the rest of the week, with Thursday night into Friday especially prone to both of these hazards. A little more breeze from the south-east on Friday and Saturday will reduce the threat of overnight frost and fog, but Sunday night is looking frosty again, particularly in the south. A lot of dry weather to come from Thursday onwards, but occasional rain in the far north-west this weekend.
A settled but chilly start. More unsettled later.
A rather complex and changeable week of weather. The computer forecast models have been struggling for consistency on the details, especially the timing of a significant mid-week pattern change. It is recommended to stay up to date with the very latest forecast information each day, as the exact timings of this forecast could change. Despite this uncertainty, the overall trend to see a shift from a mostly dry, calm, chilly pattern early in the week to a more unsettled regime later in the week seems likely.
High pressure ridging will still be extensive over England and Wales on Monday and Tuesday, even as some weak fronts edge southwards into the far north of Scotland. Chilly evenings, nights and mornings with some frost and patchy fog are expected, with locally sharp frosts over sheltered inland areas. There will be a lot of dry weather and a few bright or sunny spells will break through from time to time.
The most likely outcome for the second half of the week is for Atlantic low pressure areas to encroach from the north-west. A milder, windier and wetter couple of days around Thursday and Friday is most plausible, although don't be surprised if the high pressure and chilly, dry weather hangs on for a day or two longer. By the end of the week, there is a reasonable chance of colder air returning from the north, with some night frosts and scattered wintry showers.
Changeable conditions. A few cold snaps.
Compared with our previous updates last week, the main change to the mid-December outlook is that areas of low pressure are more likely to track southwards across the UK and down into France, Spain and Italy. This will offer the UK a break from a relentless conveyor belt of Atlantic low pressure areas and allow temporary high pressure ridges to build across the country.
The reason for the forecast change appears to be a huge low pressure circulation across the far north Pacific ocean, which looks like it will go nowhere for a few weeks. This will help to set up a wavy jet stream, with large north to south loops, across the north Atlantic and Europe, encouraging low pressure areas to track southwards over western Europe at times.
All of this suggests that a changeable weather pattern is on the way for mid-December, with mild, wet and windy spells alternating with a few colder and less wet interludes. A couple of very wet and windy days are still on the cards, as deep low pressure areas roll across, but the prospect of a succession of deep winter storms and frequent days of widespread heavy rain is less likely. Wintry showers and perhaps even some lowland snowfall are now more likely in the north. High pressure will occasionally offer some drier, calmer and sunnier days, with night widespread night frosts and some more fog patches.
December promises some rather changeable and challenging weather conditions to predict! Keeping up with the latest trends in the guidance and identifying the times when cold or stormy spells are more likely will be our main priority as we look towards Christmas.