Part 2 of 2
As explained in part one, some disparate & distant weather systems came together to create unusual events and drenched UK citizens wistfully awaiting summer.
Strong warming of the atmosphere well above our heads, combined with powerful Indian Ocean thunderstorms, joined forces. These placed a slow-moving low pressure system across the UK & Northwest Europe; bringing exceptional rainfall totals.
However, something else has been in the mix…
Such extraordinary weather requires more than just an area of low pressure being on the scene.
There has to be an influx of atmospheric moisture (a combination of tiny water droplets and water vapour – water in gas form, which condenses into droplets when sufficiently cooled!) to ‘feed’ the rain-bearing clouds.
The bigger the influx, the more the clouds are able to develop, and the more rain they produce.
As visualised in Figure 1, a large-scale northward movement of air has been set in motion by a combination of low and high pressure systems.
This airflow transported a lot of hot air northward from Africa, into which the spin of the low injected a lot of moisture from the Mediterranean.
“This airflow transported a lot of hot air northward from Africa, into which the spin of the low injected a lot of moisture from the Mediterranean.”
…Prompting thunderstorm outbreaks to occur across central parts of Europe in recent days. Though despite all their furious conversion of atmospheric moisture to rainfall, a large of has been left untapped.
This is because thunderstorms operate on a relatively localised scale to turn the atmospheric moisture into rainfall by circulating the air up and down (another topic, for another blog piece in the works!).
The Droplet Machine
The most effective way to produce a lot of rainfall from some very warm or hot air with high moisture content (i.e. high humidity) is to lower its temperature on a large scale.
By drawing sections of the hot, humid air mass across the chilly North Sea and mixing it with relatively cool air from the North Atlantic, the spin of the low is lowering its temperature. By the time it reaches England, it’s cooled by close to ten degrees Celsius! In this way, massive transformations of water vapour to rainfall are taking place over England and Wales (Figure 1 provides a detailed visualisation of all this).
In fact, I believe this low could hardly be better positioned for achieving this result!
Figure 1: An illustration of the pattern of low and high pressure during 10th-12th June 2019 (the low then moves north across the UK on 13th), the most important movements of air – with the moisture it contains – that this setup is causing to occur, and the resulting pattern of rainfall and thunderstorms.
Locating Risks & Local Flooding…
The stratospheric and tropical events enabled a low to locate somewhere across Northwest Europe. The exact position is decided by countless small-scale variations in the atmosphere. These are so chaotic that even the most advanced supercomputers can only provide a range of possibilities until just a day or two in advance!
“The exact position is decided by countless small-scale variations in the atmosphere.”
So, from that perspective, the low being in a position to produce about as much rainfall as possible across the UK was a matter of chance.
What we can say from the perspective of risk prediction, is that the odds of such bad luck were higher as a result of the stratospheric warming in late April and the development of strong thunderstorms over the Indian Ocean during the past week.
Summers’ on Hold & Still Stormy…
For How Much Longer?
There’s both good and bad news here.
And again, tropical thunderstorm activity is affecting your dry / damp chances.
I’ll start with the good news. Spells of drier, more settled, warmer weather are expected to develop across the UK as a result of tropical thunderstorm activity weakening in the Indian Ocean and becoming stronger in the Pacific Ocean.
Now the bad news – the above is likely to be a slow process, with showers or longer spells of rain continuing to feature at times for at least another week, especially for western parts.
(The Storm Before the Calm)
For example, next Mon-Tue (17th-18th) looks mostly dry for the far south of the UK, but a few showers are likely elsewhere, and then there’s a chance that an area of heavy showers or thunderstorms will affect many areas on Wed or Thu as a low moves close to or over the UK.
As of writing this, it looks like it will be at least 21st June before any widespread dry and warm weather becomes established that has a chance of lasting more than a couple of days.
The chance of such fine weather keeps increasing all the way to the end of June, though, so there’s still plenty of reason to be hopeful if that’s what you’re after!
James Peacock MSc
Head Meteorologist at MetSwift