The weather across the UK could be split this week with heavy downpours in southern England and a heatwave in western Scotland, with the expected rainfall prompting a yellow warning from the Met Office. Southern England will have “a spell of unsettled weather” that could include thunderstorms, lightning, strong winds and hail, while the worst affected areas may experience more than 50mm of rainfall within two hours. The Met Office has issued a yellow warning for heavy rain from 9pm on Wednesday to 9am on Thursday in all of south-east England, as far west as Taunton in Somerset, and south-east Wales. Peterborough, Norwich and parts of Birmingham are also within the warning zone. Buildings and roads could experience sudden flooding or lightning strikes that could lead to road closures and disrupt public transport. In Scotland, conditions are expected to be more settled, with temperatures of up to 26C (78.8F) in western areas on Friday and Saturday. Oli Claydon, a Met Office spokesperson, said: “What we’re seeing is a development of a bit of a split in the weather between the north and south. We could even be fringing on the possibility of nudging into heatwave thresholds in parts of western Scotland by the end of the week.” The criteria for a heatwave is a period of three consecutive days where the maximum temperature meets or exceeds the heatwave temperature threshold for that particular area. The threshold for Scotland is 25C, according to the Met Office. Claydon said: “As we move into September, we start to see the overnight temperatures falling a little. So some chilly nights are still around, and even when they’ve got the higher daytime temperatures, we could still see lows of 7C in parts of rural Scotland on Friday night.” South-east England is forecast to have the highest temperature this week of 27C on Friday, but this may be accompanied by cloud and rain, Claydon said. Current temperatures are “around average” for this time of year, he added.