Part 1 of 2
We look at how 2019 is bending the rules in weather pressure patterning, and why Indian Ocean thunderstorms mean you need to remember your umbrella.
For the UK, the story of June 2019: Exceptionally High Rainfall.
As if writing this, a very wet four days are expected for the southern half of the UK. By the end, rain totals are likely to be 30-80 mm widely. The most unfortunate areas may see over 100 mm (nearly twice what’s usually seen during the whole of June!).
This will lead to some risk of flooding, especially from surface water (rainwater accumulating; not river overspill). This is detailed in the official flood risk forecast provided by the government.
“Rain totals are likely to be 30-80 mm widely. The most unfortunate areas may see 100 mm – nearly twice what’s usually seen during the whole of June.”
Strangeness in the Stratosphere –
Southern shifting low pressure
Every year, as part of the transition from winter to spring, there is a large atmospheric warming approximately 5 and 31 miles above the ground. This is known as the Final Warming (for reasons beyond the scope of this blog entry).
This occurs within a layer is known as the stratosphere. It’s able to strongly influence weather patterns within atmosphere immediately beneath it (the troposphere; the layer in which we live and breathe).
For a number of weeks following the final warming, weather patterns typically feature an increased amount of high pressure with dry weather across polar latitudes (e.g. Alaska and the northern halves of Canada & Russia).
Areas of low pressure (lows), which usually favour polar latitudes i.e. hang out around them, are forced to travel further south, potentially affecting anywhere from Southeast Europe to Northern England (see Figure 1).
These lows bring showers or longer spells of rain.
Hence, spring showers.
Figure 1: Diagram illustrating the typical weather patterns relevant to Europe (A) while the stratosphere is cool and (B) for a number of weeks following a large springtime warming of it.
The Final Warming
Not as apocalyptic as it sounds…
And bending rules in 2019.
The final warming usually occurs sometime between early March and early April. That’s why the overall seasonal increase in temperatures and reduction in rainy days across the southern half of Europe tends to be interrupted by a cooler, wetter period of weather.
2019, however, decided to bend the rules a bit – the final warming occurred on April 25th; one of the latest on record. Not only that, but it was unusually strong, increasing the size and duration of the effect on the weather patterns below it.
This led to an unusual number of lows affecting the southeast quarter of Europe in May 2019, where it was unusually wet. For example, many central parts of Italy saw more than 250 mm of rainfall, close to twice the usual amount.
This year, the final warming occurred on April 25th; one of the latest on record. Not only that, but it was unusually strong, increasing the size and duration of the effect on the weather patterns below it.
“2019, however, has decided to bend the rules a bit:
‘The final warming’ occurred on April 25th; one of the latest on record. Not only that, but it was unusually strong, increasing the size and duration of the effect on the weather patterns below it.”
Things have been different in June, though – it’s been the UK and France being subjected to more lows than usual.
Believe it or not, this change can be traced back to the tropical Indian Ocean.
Trouble with the Tropics
“Believe it or not, this change can be traced back to the tropical Indian Ocean.”
Within the tropics, between the Western Indian Ocean and the Eastern Pacific Ocean, powerful thunderstorms release huge amounts of energy into the upper levels of the troposphere.
This energy is then transported away from the equator (i.e. poleward). In this way, it can influence the behaviour of weather patterns across most of the Earth.
“Weather, globally, everywhere.”
For reasons that will soon be explained in a dedicated blog post, when these thunderstorms are strongest in the Indian Ocean, their influence causes areas of low pressure to move from the North Atlantic to Northwest Europe, usually passing close to or over the UK along the way (see Figure 2 A).
Figure 2: Diagram illustrating the typical weather patterns relevant to Europe (A) when there is enhanced thunderstorm strength in the tropical Indian Ocean and (B) when there’s also been a final warming event.
Combine that with the effect of the final warming, and you get areas of low pressure dragging their heels as they wander around the UK & Northwest Europe (see Figure 2 B).
This equates to an unusually wet run of weather for the UK. But, this week is set to be even wetter than would be expected based on this information alone. There’s another key ingredient in the mix, which will shortly be detailed in a follow-up to this blog entry. Yes, we’re delving deep into the roots of this exceptional event!
More to Come… Try to Stay Dry.
Part II out now!
James Peacock MSc
Head Meteorologist at MetSwift