Heavy showers, which could still involve hail, are likely to push across the southern parts of England as the night progresses, starting with Dorset from around 6am.They are then predicted to move eastwards, hitting London between 8am and 10am, and clearing East Anglia by lunchtime.Further showers are also likely in Lincolnshire, where the weather warning is in place until 5pm. The wet weather is likely to make conditions muddier than many will have expected at Glastonbury Festival. Want the best of The Telegraph direct to your email and WhatsApp? Sign up to our free twice-daily Front Page newsletter and new audio briefings. A 16-year-old boy died at a reservoir in Rochdale, Greater Manchester, on Monday, while a 15-year-old boy died after going into a lake with friends in the Pelsall area of the Black Country, in the West Midlands, on Tuesday evening.UK weather forecast: Thursday, May 22 The sweltering temperatures placed “unprecedented demand” on ambulance services in London, with people fainting, collapsing and becoming unconscious in the heat.On Monday, London Ambulance Service call handlers answered 6,613 emergency calls, compared with 4,695 the week before – a 41 per cent increase – and the service warned this was expected to continue while the heatwave lasted.Youngsters were urged not to go swimming in lakes, rivers and reservoirs during the hot weather, following the deaths of two teenagers in separate incidents. Met Office meteorologist Alex Burkhill said: “Once these showers clear away it’s not looking too bad.”It should be a fairly fine day, cloudier than of late but there will still be some sunshine.” Most areas can expect mid to high 20s (77-82.4F) – nowhere near the sweltering 34C reached on Wednesday, the UK’s hottest June day since 1976.Heathrow in west London had recorded temperatures of 34.5C (94.1F) by 4pm, the highest for June since the 35.6C (96F) recorded in Southampton on June 28 1976.The 40-year high – which is the hottest summer solstice on record – was recorded as parts of Britain sweltered for a fifth consecutive day with temperatures above 30C (86F). Sunseekers enjoy the hot weather in Brighton on WednesdayCredit:Gareth Fuller/PA He added that Thursday will see a “marked drop” in temperatures, with conditions feeling fresher into the afternoon.In the far south-east corner of the UK, Kent is expected to hold on to the heat for the longest, with maximum temperatures of 27-28C (80.6- 82.4F) predicted. With the potential for severe #thunderstorms and #hail be prepared for sudden downpours today and stay #weatheraware pic.twitter.com/XwOBxVS6ZZ— Met Office (@metoffice) June 21, 2017 A band of thunderstorms is expected to sweep across the UK and put an end to its record-breaking heatwave.Severe weather warnings are in place throughout Thursday, with a cold front set to bring heavy rain and thunderstorms to parts of southern Scotland, northern England, north Wales and the Midlands.The Met Office warned of the potential for torrential downpours, frequent lightning, very large hailstones and strong gusts of wind, which could lead to localised flooding and temporary disruption of power supplies overnight until early on Thursday.