John Hammond’s Month Ahead – UPDATE! – Cold easterlies on their way

John gives us his latest thoughts on likely weather trends over the next few weeks. To ensure you are reading the very latest weekly blog, check out Wow and Why

*UPDATE IN BOLD BELOW – Cold easterlies now much more likely

December has brought us a curious and entertaining mixture. The jet stream fluctuated in strength and position, and in concert with this, we’ve seen an array of weather – from mild and damp, through cold and frosty and, occasionally and unexpectedly for some, deep, crisp and even.

But our cold snaps have paled into insignificance compared with the bitter weather afflicting parts of North America in recent days. Is this a portent of what might be coming our way? Well no, not really. If only it was that simple. As I’ve written here, American weather rarely reaches us in the same guise as it left the States.

Instead, the intense cold air across the ‘Pond’ will help to strengthen the jet stream. A strong jet stream helps generate low pressure systems and these look set to propel spells of wet and windy weather in our direction.


Wild West Winds

As I look at the charts in front of me, they suggest that every day will bring a different complexion to our weather. A series of low pressure systems will be hurtling towards us across the Atlantic, each bringing a spell of wet and windy weather. In between, there will be some drier and brighter interludes.

Very small changes in the strength and position of the jet stream will have a big impact on exactly where and when these low pressure systems will spin up – something the computer forecast models often struggle with. So their timing will be very difficult to nail down until we can see the whites of their eyes.

But some of these systems could be powerful enough to bring some significant impacts from both wind and rain – perhaps even another named Storm?

With westerly winds dominating, temperatures shouldn’t fluctuate too far from normal. Southern areas of the UK will tend to be mildest. Further north, brief injections of colder air may bring some transient snowfall, especially to the hills.

However, there are hints that by the end of the week the jet stream will develop more of a ‘kink’. Warm air sent north to Greenland may open the door for a burst of arctic air to surge down across the UK. I expect a colder weekend, with frost and wintry showers. The question is “What happens next”?

… will next weekend’s cold snap be just a brief encounter?


To block or not to block?

Over the last few days the the majority of computer forecasts have drifted towards a much more ‘blocked’ pattern in the atmosphere. As the jet stream becomes increasingly contorted, a cold easterly will have developed by the start of the week.

The air, though not severely cold, will be chilly enough for wintry showers, especially across central and eastern areas of the UK.

This cold, blocked pattern, looks like persisting well into the week. As Atlantic fronts run into the cold air and try to dislodge the block, so some disruptive snow is possible. But there is still huge uncertainty about the timing of this – if indeed it happens at all. 



Top to bottom westerlies?

Most longer term computer output currently indicates a  ‘syncing in’ of strong stratospheric westerly winds around the northern hemisphere with the jet stream further down the atmosphere. A strong westerly jet stream would in turn deliver weather systems from the Atlantic, turning us milder, but often quite wet and windy.

But will next weekend’s cold snap be just a brief encounter? Or will an atmospheric ‘block’ change the dynamics yet again.

As I tweeted a few days ago, we’ve seen some twists and turns in December, which the computer models really didn’t ‘know about’ that well in advance. So their ‘confidence’ through January should be treated with a degree of caution.  It’s proving to be an intriguing winter.