Wales is on track for the so far with highs of 32°C before
The Met Office has said a three-day heatwave is expected across the UK with highs of 36°C in some parts of the UK by Thursday.
The sweltering heat is the result of an 'African plume' sweeping in from the south and there will be high temperatures across Europe too.
Fog has reduced visibility along part of the M4 on Tuesday morning, but Welsh forecaster Derek Brockway said that "low cloud will clear during the morning."
He added: "Temperatures will soar. Over 30°C in places making it the hottest day of the year so far in Wales. Heavy showers and thunderstorms tonight!"
The Met Office forecast is showing that it will be 19°C by 9am in Wales, by 10am any cloud will have lifted and temperatures will have risen to 20°C, and to 28°C by 2pm. It will be around 30°C by 4pm.
The temperatures will remain high overnight into Wednesday, making it uncomfortable to sleep with overnight temperatures of 24C (75.2F).
Marco Petagna, Met Office forecaster, said there is a "60% chance" Thursday's weather might surpass the current July record of 36.7C (98.06F) set at Heathrow in 2015 and a "30% chance" of going over the all time UK temperature record of 38.5C (101.3F) which was recorded in Faversham in August 2003.
Before then, thundery showers are expected to hit parts of Wales in the late hours of Tuesday evening and the early hours of Wednesday morning.
Despite the moody and cloudy atmosphere, temperatures are still expected to be fairly high during the day with a potential 25C (77.6F).
Met Office forecast for Tuesday to Saturday
Today
Early low cloud and fog will soon clear to plenty of sunshine through the day. There will be unbroken blue skies into the afternoon and evening and in the light winds it will feel hot. Maximum temperature 32 °C.
Tonight
After a fine start, thunderstorms will arrive from the south. There will be the risk of hail and some bursts of heavy rain, but it will become drier by dawn. Minimum temperature 16 °C.
Wednesday
There will be some early low cloud and showers, but warm hazy sunshine will develop for most. Winds will be light and it will be a little fresher than Tuesday. Maximum temperature 24 °C.
Outlook for Thursday to Saturday
Thursday will see hot sunshine. There will be thunderstorms overnight, then it will turn much fresher on Friday. Sunny spells and showers will then continue on Friday and Saturday.
