Thunderstorms set to sweep across the UK this weekend could cause flash flooding, forecasters have warned. Parts of the UK could face train and bus delays and some communities may be cut off by flooding as warm, humid air causes storms to develop, the Met Office said. Some areas could be battered with up to 30mm of rainfall in an hour on Sunday and the Met Office has issued a yellow thunderstorm warning across most of England and Wales for the whole day. The heat is set to continue with temperatures reaching the high 20s. The forecast comes after NHS leaders said the hot weather had contributed to “one of the busiest days on record” in hospital accident and emergency departments on Thursday. The charity Asthma + Lung UK said the humidity had caused an “unseasonal spike” in asthma-related A&E visits and warned that thunderstorms over the weekend could cause “life-threatening” asthma attacks. Hosepipe bans have been introduced in Kent and Sussex due to “record levels” of demand for drinking water. Met Office spokesperson Grahame Madge said: “There are risks of flash flooding associated with these forecast thunderstorms. Thunderstorms can unleash very intense rainfall and even hail. “The impacts from this sudden release of precipitation can create local difficulties, especially in urban areas, as intense rainfall can lead flood waters to rise on roads and underpasses. “This far out, it isn’t possible to forecast exactly where these systems will form but we are confident that some will experience intense conditions. “We advise the public to follow the forecast and warnings so they can be prepared should thunderstorms develop at short notice.”