Thunderstorms bring flash flood risk this weekend
Following the recent heatwave, the weather has now flipped into a more unsettled mode – something that many of us will get a taste of this weekend.
A plume of heat and humidity will drift towards us from northern France and as this potential energy is destabilised in the atmosphere, intense thunderstorms will form.
The weather computer models have been hinting at this general trend for a few days now, but what they’ve been struggling with is the detail of where the greatest amounts of rain will fall.
You may ask why this is important. Well, given the amount and intensity of the rainfall that these thunderstorms are capable of producing, there is a risk of some flash flooding – hence the desire to know where this is most likely to happen.
Why are the computer models struggling?
Thunderstorms are complex in the way that they form and how they develop over time, in addition to being quite small features for weather computer models to track.
Weather computer models work by dividing the earth’s atmosphere into boxes – both horizontally and vertically around the planet.
In each of these boxes, the model does calculations based on temperature, pressure and humidity, which after some fancy atmospheric equation solving leads to a forecast being produced.
The problem with thunderstorms is that they are often not big enough to fill one of these boxes. This means that the model may know that there’s good chance of a thunderstorm forming inside the box, but not know precisely where.
In effect, it’s sort of like pinning a tail on a donkey whilst wearing a blindfold. You know that there’s a donkey in front of you, but given a lack of sight, you’re not sure exactly where the tail is located.
Where will the wettest weather be?
Whilst subject to change, the latest forecast takes the heaviest rain across central, southern and eastern England during Saturday afternoon, before it pushes northwards into northern England and Scotland overnight into Sunday.
These areas are expected to have 10-30mm of rain, with 30-50mm falling locally, leading to a number of weather warnings being issued by the Met Office.
This may not sound like a lot of rain, but I think problems may arise from the fact that some of it will fall in a short space of time, giving a risk of flash flooding – something that happened in the Midlands on Tuesday evening.
Don’t forget, you can stay up to date with the latest forecast on the Channel 4 Weather website. I’ll also be posting updates on Twitter – @liamdutton