John Hammond’s Month Ahead – Signs of early Spring?

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

We’re over halfway through Winter 2017/18 and in broad-brush terms it’s been truly unremarkable. Taking the UK as a whole, temperatures, snowfall, rainfall and sunshine haven’t been too far off the normal. Yet, that disguises a picture of huge variety from one place to another, and from one day to the next. This constant ‘chop and change’ has been symptomatic of a fickle jet stream. From the stratosphere above to the oceans below, a combination of known and unknown factors have led the jet stream to behave, well, erratically, to say the least.

And the ability of forecasters to predict its next move has been similarly unreliable through early winter. In recent weeks I’ve advised caution against the output of long-term forecasts which, time and again, have wrongly predicted the strength of the jet stream.

Yet as February approaches, I’m happier with the latest output of such forecasts. As I mentioned last week, the stratospheric westerly winds that circle the Arctic are set to weaken quite substantially. In time, this slowing will impose a braking effect on winds further down through the atmosphere, stalling the jet stream and bringing more settled, drier weather to our shores. What it means for temperatures is another matter…

The drip…drip… drip… of a thaw won’t take long to do its work!



A brief burst of warmth

At the moment the jet stream is anything but stalled. As it surges up across the UK through the early part of the week, we can expect waves of rain then warmth to replace the recent wintry weather from the southwest. How warm? Well before midweek, where the cloud breaks, some spots could reach the mid-teens Celsius. The drip…drip… drip… of a thaw won’t take long to do its work!

A couple more ‘ripples’ in the weather pattern will follow. Around midweek a  cold front will edge down across the UK from the northwest, bringing a period of wet and windy weather, followed by chillier air. Then a warm front will follow northeastwards again by the weekend, introducing milder, damper weather again.

But each ripple will be weaker than the last. On the large scale, the jet stream will be starting to meander slowly away northwards. This will allow higher pressure to build from the south. Trailing fronts may still bring some damp, breezy weather to the far northwest of the UK, but as we start the next week, for many: ‘High’ means ‘Dry’.



High and Dry

Large meanders in the jet stream tend to move more slowly. Once high pressure has got a foothold across the UK, it may prove stubborn.

The drier signal is the easy bit. However, the position of the high pressure will be crucial to the brand of dry weather we experience. Initially, it’s southern areas which will be driest and calmest. Some fog and lingering frost may be a feature of the weather, with daytime temperatures on the chilly side as the air stagnates.

Furthest north, we will continue to receive a feed of moist and mild southwesterly winds for a time. So here it may well be cloudier with a little dampness, but with temperatures higher than further south.

This broad pattern may persist for much of the week, although there will be day-to-day changes in detail as the high pressure subtly shifts around.

… compared with the ‘frantic’ weather of recent weeks, February looks very different



Dry and cold or Dry and mild?

I’ve had correspondence from some readers from southern England who feel short-changed! Sheltered from all the action, they have been ‘starved’ of snow. “Is it time to give up on the winter?”. Of course further north, there’s been too much of the white stuff at times and, for some, Spring can’t come soon enough.

Well whether you like it or not, with high pressure wafting up from the south, snowfall seems unlikely to be an issue anywhere in the short term. The mostly dry theme continues.

Whilst there is a consistent signal from computer models about this aspect, what they agree less about is where the high pressure will end up. If it is centred to the south of the UK, enabling a gentle feed of air from the Atlantic, then mild weather will dominate. However, should the high pressure centre drift towards Scandinavia, much colder easterlies would creep in, perhaps finally raising hopes for southern snow-lovers.

There are no clear indications which way this one will go. What is more obvious to me is that compared with the ‘frantic’ weather of recent weeks, February looks very different.