The county baking in temperatures clear of 30C (86F) today, while parts of the South East could see temperatures climb as high as 35C (95F) - close to the record for July of 36.7C (98.06F).
Meanwhile there is a yellow warning of rain stretching this evening and into Wednesday evening, with up to 2in (50mm) expected in some parts, bringing a risk of flash flooding.
Public Health England (PHE) is urging people to look out for elderly relatives and friends who may be susceptible to the high temperatures - and to take extra care of young children, babies and those with heath issues.
With the warm weather expected to last until the end of Wednesday, PHE is advising a common-sense approach to dealing with the heatwave.
People are urged to drink plenty of water and avoid dehydrating alcoholic and caffeinated drinks, close curtains to keeps rooms cool, open windows and avoid people and animals being left in closed, parked vehicles which can get extremely hot.
Dr Angie Bone, PHE’s head of extreme events, said: “For some people - such as older people, those with underlying health conditions and those with young children - summer heat can bring real health risks.
“This summer we’re urging people to keep an eye on those at risk and, if you’re able, offer help to stay cool and hydrated.”
Pollen levels will be high over the country for the next few days, causing havoc for hay fever sufferers - and there may be another restless night on Tuesday as temperatures reach 22C (71.6F) overnight.
The hot weather has been caused by a warm plume of air that has worked its way northwards from Spain, the Met Office said.
Wednesday will see highs reaching the low 30Cs, but by Thursday fresher conditions from the Atlantic will bring temperatures down to the more comfortable mid-20Cs, reaching around 22C (71.6F) to 24C (75.2F) by Friday and Saturday.
Met Office forecaster Sophie Yeomans said: “We are looking at probably the hottest day of the year today with quite a few places reaching 34C or even 35C in the South East.
“With all the warm and humid air it does mean we are going to have a thundery breakdown later on this evening. The only part to escape will be the very south of England, with lightning, hail, gusts of wind and 30mm of rain falling in the hour, but this won’t be persistent. There’s potential for nearly a month’s rain in some areas during the time we have the warning.”
Tonight’s forecast for Yorkshire
After a clear start cloud will spread across with a risk of a thundery shower, mainly later in the night. Remaining very warm with winds easing inland.
Hot and humid with intense thunderstorms affecting some areas, mainly at first in the morning and again during the afternoon, with possible gusty winds and large hail.
Thursday to Saturday
Feeling cooler with temperatures returning closer to average values for the time of year. A lot of dry, bright weather with some showers, these most likely during Friday afternoon.