It said Wednesday would bring the highest temperatures so far this year.
Warnings are in place for Central, Tayside & Fife, Dumfries, Galloway, Lothian & Borders, East Midlands, East of England, Highlands & Eilean Siar, London & South East England, North East England, North West England, Northern Ireland, South West England, Strathclyde, Wales, West Midlands, Yorkshire & Humber and are in place from until 9pm.
The storms come before warm air floods in from the Continent pushing thermometers to 28C (82F) over the next few days.
Britain saw the hottest day of the year so far at the weekend when thermometers topped 27.8C (82F) at Porthmadog, Wales, on Sunday.
However sun-lovers were braced for a shock to the system with the heavens opening.
The Met Office warned much of the country will be affected with only the far southwest and parts of the east coast escaping the onslaught.