Since March got underway, there’s been a lot of wet and windy weather affecting the UK.
Early today, things stepped up another gear in my locality, with my own back garden suffering some of the worst damage…
I’m sure many in the UK are wondering what’s up with all this wet and wild weather, and how much longer we’re going to have to put up with so much of it.
ANORAKS & VORTEXES…
Culprit A: Strong Polar Vortex
Have you ever wondered why it tends to be wet and windy so often between October and March? Well, it’s the polar vortex to blame.
“It may sound like a cheesy band from Lapland but no, it’s something to be taken much more seriously…”
That may sound like a cheesy band from Lapland but no, it’s much more serious than that – a vast circulation of cold air and low pressure systems (a.k.a. depressions or winter storms). It develops mid-late autumn, becomes strongest in the depths of winter and then dissipates early-mid spring. Both hemispheres have their own polar vortexes, the northern one rotating anticlockwise and the southern one clockwise.
This past month, the polar vortex has been particularly strong. When raging away like this, the broad circulation drives a succession of low pressure systems from west to east across the North Atlantic. These often pass near or over the UK, delivering the spells of strong winds and umbrella-defying rain that have become widely associated with the British climate.
On this occasion, however, the polar vortex hasn’t been acting alone…
Culprit B: A Strong Azores High
This accomplice to the crime is usually the bringer of drier, warmer than usual conditions to the UK. When the polar vortex is strong, however, it turns from friend to foe.
“This accomplice to the crime is usually the bringer of drier, warmer than usual conditions to the UK. When the polar vortex is strong, however, it turns from friend to foe…”
This is down to something known as the pressure gradient force (PGF). In a nutshell, the more rapidly the air pressure changes between two points, the stronger the wind blows.
When an area of high pressure strengthens, the surface pressure beneath it increases. This means that anywhere peripheral to that high will see an increase in wind speeds. Similarly, when low pressure systems intensify, they lower the surface pressure beneath them, leading to stronger winds around them.
Oct-Mar, the UK typically spends a lot of time situated between high pressure focused to the southwest (the Azores High) and low pressure systems passing near or over the nation. That’s why it’s normal to experience at least a few gales during these months.
Change of Fortune
In recent weeks, and in fact for most of the winter, the Azores High has been stronger than usual.
The UK was lucky enough to avoid serious consequences of this Dec-Feb, thanks to the polar vortex being largely near or below normal intensity. The anomalously strong Azores High was able to extend across the UK and sometimes much of Europe too, bringing some long runs of dry and mild weather.
In late Feb, this pattern went into overdrive and brought the highest winter month (Dec-Feb) temperatures on record to each of England, Wales and Scotland.
The strengthening of the polar vortex for early March has, however, meant that the Azores High hasn’t been able to extend across the UK despite being stronger than usual. As a result – as illustrated in Figure 1. – it’s worked to further increase the pressure gradient across the UK beyond what the low pressure systems would have achieved alone.
This is why it’s been so windy of late.
Figure 1: Diagram showing the anomalous weather patterns that have predominated during the first 10 days of March (and counting), and how this has affected wind speeds.
How Much Longer Do We Have to Put Up with This?
Please don’t shoot the messenger – there’s not a lot of change being indicated by the guidance tools for at least the next 6 days, with further spells of rain and strong winds expected.
For example, Monday night into Tuesday morning looks to see a band of heavy rain and winds gusting to near 50 mph move northwest to southeast across the UK. Then, after only a brief lull, a vast swathe of strong winds (gusting to 50-60 mph) are expected to affect all but the northern half of Scotland Tuesday night through Wednesday morning. More showers with that too, for good measure.
Spring Will Come…
After this point, though, there are signs that longer spells of drier, calmer conditions will start to feature in the UK’s weather, initially across the far south, then gradually extending further north.
In fact, I’d not be surprised to see another prolonged dry and warm spell take shape by the final few days of the month, though I wouldn’t go betting on it just yet!
James Peacock MSc
Head Meteorologist at MetSwift