Extreme Weather

Despite the level of probability still involved in weather forecasting, sometimes there are weather events like severe flooding that we need to prepare for. To find out how we make sure that we see these events coming, Chris spoke to Nick Graham from the MET Office and started out by asking, just what can be called 'extreme weather'.

Nick - Well, the MET Office define extreme weather as heavy rain, severe gales, heavy snow, ice and also fog. All of those things can cause an impact on the UK.

Chris - How do you go about trying to predict them?

Nick - We have forecast models that we run here from the MET Office. We also see information from other forecasting centres and the team of key forecasters bring all that information together, and if we see something that's developing a few days in advance then initially, we will take some action of beginning to inform people that there's a risk that there could be some extreme weather on the way. And then as we get near the time, say, sort of 1 or 2 days in advance, we tend to get stronger signals. That means then we can sort of up the tempo if you like and then we can begin to warn, we can begin to put things out to the media and it sort of builds up from there, and we have a National Severe Weather Warning Service which basically comes up with a colour, and that is a way of depicting severity of the weather we're expecting.

Chris - How do you know though when you're looking at the information presented to you by a whole raft of measurements obviously? How do you pick out from that, the danger signals?

Nick - Take wind for instance. To get very strong winds across the UK, you've got to have quite a major low pressure system coming across the Atlantic. These systems almost start from nothing that you begin to see the clouds are sort of thickening up in the western side of the Atlantic, and when you get a major low pressure developing and pushing across the UK, you can almost estimate the amount of wind you're going to get as it hits the UK, and then that's when you begin to sort of nail down your information that goes into the warning service.

Chris - So, where are you getting your information from? Obviously, you're saying cloud cover, so you must be looking at satellite images, but what else?

Nick - Well, yeah, as I say satellite images is quite a key aspect these days. We're also getting radar, so when we're forecasting heavy rain, our radar network across the UK is very good indicator.

Chris - Sorry to interrupt you, but is that because basically, radio waves get soaked up by water and so, you're looking at how much does or doesn't get bounced back?

Nick - Well, it's a bouncing back of raindrops if you like, so when you send a pulse of radar up into the atmosphere then it bounces back off the raindrops so the sort of bigger the bounce, the greater the amount of rain that's there. So, that's one way of seeing it. And as I mentioned before that the forecast models from the computer these days are pretty accurate. I mean, over the years, we've seen big improvements in forecast and in fact, our 4-day forecast these days is as good as 1-day forecast whilst 30 years ago - and we see all these information and it's then sort of bringing all that together distilling it and say, "Okay, what does that mean for the public?" and that's when we distill it into a warning on the Met Office website.

Chris - But obviously, you've got quite a lot of power there because in your hands, is warning people to deploy various resources and potentially spend a lot of money defending against something that hasn't even happened yet. So, you're in quite a stressful position, aren't you?

Nick - Well, what we do is share the information, the sort of weather information to other teams. So, if we take rain for instance, ourselves and the environment agency work together. In fact, in the MET Office, we have what's called a Flood Forecast Centre and we've got environment agency people and MET Office people working together. Now, it's a little bit like putting a jigsaw altogether because the impact of rain could be different on certain occasions. If you take a certain amount of rain that's expected to fall, so that'll be the MET Office saying like, "We expect say, 50 millimetres of rain to fall in that area tomorrow." So, that's the first piece of the jigsaw. Now, if it's been very wet prior to that, then 50 millimetres of rain can cause huge impacts, I mean we saw through the course of last year, we had the rain events on and on and on, that we needed less rain to cause major issues.

Chris - Because the ground is already saturated.

Nick - That's right, yeah.

Chris - Now, the predictions that we read from climate modellers is that if global warming and climate changes plays out to the way they are anticipating, we can expect to see more severe weather in the future. In fact, some people are saying, "Look, hurricanes are already intensifying if you look at what's been happening in America in the last few years." So, is the MET investing actively in more people doing your sorts of jobs and looking for patterns in order to meet this threat head-on?

Nick - The simple answer is yes, because with the general temperature of the air across the globe heating up, it means it can hold a bit more moisture, I think we've certainly got to cater for the fact that the risk of extreme rainfall and the impact that occurs to be like flash flooding as we've seen maybe in the last few summers, we certainly got to cater for that as happening more regularly than we've seen in the past. And that's the kind of information that is being fed to local authorities. We got a set of scientists here working in place with the Hadley Centre and they're looking at possible scenarios in the future based on warming trend. People that are then forecasting on a day to day like myself and my team, we're also aware that those kind of challenges are going to be faced. And we may have to change our definitions of extreme based on what we're seeing. I mean, authorities may need to think about how future drainage systems, town layouts, concrete, etc., may need to take into account some of the changes that we're expecting over the next say, 50 years.