Storms expected after temperatures peak early in week 21 July 2013 Last updated at 04:03 The UK heatwave will hit its highest temperatures at the start of next week, before giving way to thunderstorms and potential flooding, forecasters say. BBC Weather's John Hammond said temperatures will reach 30C (86F) again on Sunday and as high as 34C (93F) on Monday in some parts of the country. But from Monday evening through to Wednesday increased humidity will cause thunderstorms and risks of flooding. The Met Office has issued a 24-hour rain warning for Tuesday. The yellow alert covers all of Wales and all but the far north of England, and warns people to be aware that there may be disruptive storms. A Met Office spokeswoman said large amounts of rainfall coming down in short periods on to dry ground could lead to localised flooding. Rain expected The UK is in its first prolonged heatwave since 2006 - though Saturday was the first day in seven when temperatures did not go above above 30C anywhere in the country. While the warm weather was welcomed initially following an indifferent start to the summer, it has had negative impacts, including grass fires in Scotland and Epping Forest in east London. And research by the London School of Hygiene and Tropical Medicine has estimated there might have been between 540 and 760 extra deaths in England, and 60 to 100 in Wales, due to the hot weather. The current heatwave has not matched the summer of 1976, when temperatures above 32C (89.6) were recorded on 15 consecutive says. Indeed, Mr Hammond said that by the latter part of the week the heatwave will be over and rain was expected. Met Office heatwave alerts are triggered when threshold temperatures - which vary by English region - have been reached for one day and the following night, and the level depends on the likelihood of those temperatures being reached again the next day. In the four years since the current heatwave alerts system was introduced the Met Office has never used its top level-four category - "national emergency." A Met Office spokeswoman said this would only be used after a long period of extreme hot weather leading to issues such as water shortages, infrastructure damage and deaths among "fit and healthy" people.