"Slow moving heavy showers will affect parts of Northern Ireland on Saturday morning, easing later on Saturday afternoon", the spokesperson said.
A Met Office yellow warning for thunderstorms is in place in the East and West Midlands, the East of England, London and the South East, and Yorkshire until 21:00 BST on Thursday.
But parts of England are experiencing heavy rainfall and thunderstorm warnings are in place.
The British record is 38.5C set in August 2003 near Faversham in Kent, southeast England, while the highest July temperature was 36.7C in 2015 at Heathrow airport.
"Large hail and strong, gusty winds are also likely and combined could lead to hard driving conditions as a result of spray and sudden flooding", Mr Ramsdale said.
The Met Office forecast for the weekend is for much cooler conditions - more than 10degC lower than most of this week.
She said the high temperatures were due to warm air coming up from France combined with high pressure across the country.
Mr Deakin said: "The reason there's some uncertainty is because of the thunderstorms, how many we see and where they get going. torrential rain, the risk of local flooding, large hailstones potentially damaging, lots of flashes of lighting and rumbles of thunder".
"Roads like the M11, M18, M1 and A1 won't be very pleasant, a lot of spray and surface water around".
The weather service introduced the new advisory for thunderstorms last month.
Temperatures on Thursday reached 35.1 degrees Celsius at Wisley, in Surrey, southern England, officially making it the hottest day of the year so far.
There is now a 90 percent chance of heatwave conditions between Monday and Friday this week.
The level 3 warning, one behind a national emergency, has been issued with temperatures set to be in the high 20s this week.
Meanwhile, London's fire chief has called for barbecue bans to be imposed in the capital's parks after a record-breaking number of grass fires.