John Hammond’s Month Ahead – A twist in Winter’s tail?

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

The elements pay no respect to our arbitrary divisions in the calendar. But occasionally, quite by accident, the mood of the weather can change in sync with the month. Through December and January, mild westerlies and colder northerlies have often jostled for supremacy in quick succession. It’s been a winter of constant change, with each flick of the jet stream bringing a blend of mild, wild then wintry. But February looks different.

There are compelling signs, (both from computer forecasts, and from our knowledge of how the oceans and atmosphere interact), that the path of the jet stream will soon meander away from our shores. In its place, a ‘blocking high pressure’ looks like bringing a period of quieter and drier weather as Winter reaches its conclusion. But the position of that block will be crucial to whether we end the season with bitter winds or a feeling of early Spring.

MONDAY 29TH JANUARY – SUNDAY 4TH FEBRUARY

Constant change

For the time being, it’s a case of ‘as you were’. The jet stream will continue to ripple weather systems across the Atlantic towards us, bringing a week of variety. It’ll be hard to keep up!

After a very mild weekend,  a cold front will spread rain, then chillier weather down from the northwest. Frosty weather will follow, but by midweek the next area of rain and blustery winds will cross us. Then another burst of colder northerly winds will bring wintry showers later in the week. As we’ve seen so much this winter, it’s the north of the UK that will bear the brunt of any significant snowfall.

By the weekend, rain will be edging in from the west as yet another low pressure system arrives. But its progress eastwards looks less certain. The jet stream will begin to slow and become more meandering.

… beware the hyped-up headlines

MONDAY 5TH FEBRUARY – SUNDAY 11TH FEBRUARY

Running out of puff

As I mentioned last week, large meanders in the jet stream move more slowly. The transition to a drier spell looks like being a messy one and may take a few days. Given these uncertainties, details of day-to-day weather can’t be attempted at this range.

However, I expect high pressure to build across the UK through the week. As it does so, the weather will become drier than for some considerable time. The winds will become lighter, but whilst the westerlies will run out of puff, the direction that replaces them will be important for temperatures. There is an increasing chance that winds from between north and east will arrive, leading to a downward slide in the mercury.

“Does that mean snow?”, I hear you ask. Well, beware the hyped-up headlines as interest builds in this change in the weather. Whilst wintry showers are a distinct possibility, it’s foolhardy to speculate on the extent of any disruptive snowfall at this range. What’s much more likely is that overnight frost will become quite widespread, and possibly severe.

 

MONDAY 12TH FEBRUARY – SUNDAY 25TH FEBRUARY

As the days get longer, does the cold grow stronger?

Mid-February looks cold. Available evidence suggests that the atmosphere will remain quite blocked, with a trend for easterlies to fend off the jet stream’s normal supply of Atlantic weather systems.

It’s worth stressing again that at this range, any prescriptive detail is impossible. But in these types of atmospheric set-ups, snowfall potential will come from two possible sources. Circulating around the ‘blocking’ high pressure, small vortices of atmospheric instability can approach from the east. These tend to enable snow showers to build as they pick up moisture across the North Sea, with northern and eastern coasts most prone.

With time, the other source of snow potential may come as Atlantic fronts attempt to displace the cold block and introduce milder air from the west. Such fronts can become very slow-moving and produce heavy snowfall, before the milder air eventually wins out and turns it back to rain.

Alternatively, the high pressure may continue to bring dry, cold and frosty weather across the UK.

As we all know, nothing in weather forecasting is straightforward. But it looks like a fascinating period ahead as February takes us in a different direction.