Warnings issued as heavy rain brings flood risk
Following a fine and warm weekend for most of us, the weather has turned more unsettled this week, with the prospect of some intense rain in the next 48 hours.
The jet stream has dipped southwards just to west of the UK and is drawing up a plume of warmth and humidity ahead of it.
As a result, a south to south easterly wind is going to deliver batches of heavy, thundery rain to most places during Thursday and Friday, with a risk of localised flooding.
In these kinds of situations, it is always difficult to pinpoint which locations will see the greatest amounts of rainfall, but the weather computer models are now gaining consensus that western areas will see the wettest weather.
Low pressure’s influence
At the moment, there is an area of high pressure to the east of the UK and low pressure to the west.
Given the way that the wind flows around each, this set up acts as a conveyor belt for very moist air to be pushed over us.
However, as low pressure inches ever closer, this allows moist air to rise more readily and form clouds, giving rain.
As well as this, there are two other factors that will help to concentrate the heaviest rainfall in particular areas – convergence of winds and orographic enhancement.
Convergence of winds
When the wind blows, its direction can be subtly influenced by the surface over which it flows.
As the sea is smooth and generally uniform, the wind encounters negligible friction and flows freely. So, a southerly wind will continue to be a southerly wind.
However, when the wind flows over land it encounters friction. This not only slows it down a little, but alters its direction – causing it to back.
Therefore, what was a southerly wind over the sea can become more of a south easterly wind over the land.
It’s the clashing (convergence) of these winds from different directions that enhances the rate at which air rises, resulting in more moisture in the clouds and thus greater amounts of rainfall – something that will happen in western areas during Thursday.
Orographic enhancement
A brisk southerly wind will mean that the very moist air travelling northwards across the UK will encounter hills and mountains.
This will cause the moist air to be lifted even higher up in the atmosphere than would be the case if it was moving over flat terrain.
The result will be for clouds containing greater amounts of moisture to form over the hills and mountains in the west, enhancing the intensity of the rain that falls – a process known as orographic enhancement.
How much rain is expected?
Scotland, Northern Ireland, Wales and the western side of England could see 20-30mm falling widely in the next 48 hours. Elsewhere, only 5-15mm of rain is expected.
The worst affected areas – Cumbria, eastern parts of Northern Ireland, western Scotland and the mountains of south Wales – could see 50-80mm.
Weather warnings have been issued by the Met Office, with localised flooding highlighted as a potential hazard.
Don’t forget, you can get the latest forecast on the Channel 4 Weather website. I also post regular updates on Twitter – @liamdutton