By Stephen Adams Published: 8:14PM BST 01 Jul 2009
Raising the Heat Health Watch alert to ‘level three’ for London and south east England, denoting a heatwave, forecasters said “dangerous” conditions were likely to persist until Friday.
The Met Office raised the alert level to one below the maximum after persistently high temperatures over the preceding day and two nights.
On Wednesday 87F (30.3C) was recorded at Heathrow airport, the third day in a row that temperatures have exceeded 30C (86F).
But the north east of England was hit by a torrential thunderstorm and forecasters warned that south Wales and south west England should be braced for heavy downpours with a risk of flash flooding on Thursday.
An Environment Agency spokesman added: “The Environment Agency and Met Office are warning homeowners, businesses and drivers in south west England and South Wales that heavy, thundery rain could lead to flash flooding, particularly from surface water drains and small watercourses.”
On Tuesday, the hottest day of the year to date, the temperature hit 89F (31.8C) in Wisley, Surrey, while on Monday St James’s Park in London registered 87F (30.4C).
Night time temperatures in London have not dropped below 63F (17C) since Saturday night, while on Tuesday night the minimum in Manchester was 67F (19.2C). Average night time minimum temperatures in June are just below 50F (10C).
Helen Chivers, a Met Office forecaster, said: “For London and the south east heatwave conditions have been met.”
She warned Thursday could be the hottest day of the heatwave.
“It might touch 33C on Thursday somewhere in the Greater London area,” she said.
The temperature might nudge higher than previous days and the humidity increase as easterly winds die due to an approaching front from the Atlantic, she noted.
While that will bring thunderstorms and then cooler weather to the south east by Friday, it will be uncomfortable in the meantime.
Public health officials have been particularly worried about high night time temperatures, which make it difficult for elderly people to recover the stress their bodies have suffered during the day. During the 2003 heatwave 2,000 more people died than normal, in part due to the high nocturnal temperatures,
London Ambulance Service said calls were 20 per cent higher on Tuesday compared with a week beforehand.
The Met Office stressed there was no chance of the alert reaching level four, when it is “so severe and/or prolonged that its effects extend outside the health and social care system”