The heatwave could break in a stormy way, forecasters are predicting.
Temperatures are now predicted to reach 34°C tomorrow in York and North Yorkshire.
But rain, hail and lightning could be on their way, a Met Office spokesperson has said.
A yellow thunderstorm weather warning has been issued covering the county, running from 3pm on Saturday until 4am on Sunday.
The Met Office warned the most intense storms could produce “frequent lightning, large hail and gusty winds”, along with a chance of flooding.
It comes as a number of areas around the country could reach heatwave thresholds on Friday.
Met Office spokeswoman Nicola Maxey said: “There are scattered thunderstorms coming through.
“Some of the rain could be quite intense, and frequent lightning, hail, gusty winds and some heavy downpours, which we haven’t seen for a while.
“Some of the ground is quite hard at the moment, and when you get heavy rain hitting hard ground, it can cause surface water issues.
“You might find surface water on the roads, drains finding it difficult to cope and a small chance of homes being flooded.”
Ms Maxey said a number of areas around the country could hit heatwave thresholds on Friday before the storm hits.
An official heatwave is recorded when areas reach a certain temperature for three consecutive days. In Yorkshrie, the threshold is 25°C.
“We have a number of sites which have reached two days, so we’re waiting to see what happens,” she said.
“Today is going to be another warm day, so the possibility is there.”
An amber heat-health alert, issued by the UK Health Security Agency (UKHSA) for the first time since September 2023, is in force until 9am on Monday.
It warns “significant impacts are likely” across health and social care services because of high temperatures, including a rise in deaths, particularly among those aged 65 and over or people with health conditions.