"The best conditions will be if the rainfall is moderate", he said.
"RNLI lifeboats and lifeguards have had four times as many rescues of inflatables so far this summer than they did previous year".
Police forces have been called following concerns for people's welfare at rivers and a lake.
Warwickshire Police officers remain at the Bishops Bowl Lakes area in Bishops Itchington following a report of a 17-year-old boy going into the water on Thursday evening.
There is now a 90 percent chance of heatwave conditions between Monday and Friday this week.
One boy was rescued by a lifeboat crew, checked over by medical teams and reunited with his family, but the second remains unaccounted for. The company also cancelled thousands of day trip tickets to ease congestion.
On Friday, the heat was seriously disrupting travel across the UK. It will be much cooler in the west, due to the rain sweeping through the area.More news: Pakistan Election 2018: 53,000 observers will monitor polls
The yellow thunderstorm warning is in place from 2pm until 9pm today (July 26) as Northamptonshire could be subject to heavy rain for a short period of time.
With potentially record-breaking high temperatures in the coming days, the Government is being warned it must do more to protect vulnerable people in extreme heat.
NHS Providers - the trade body which represents NHS services - said some hospitals were back to "winter conditions" after reporting record numbers of A&E admissions.
The thunderstorms will clear away after midnight, perhaps lingering in the far north east of England into the early hours of Saturday.
The hot weather has also caused medics and patients to fall ill in hospitals, it has emerged.
"But across the east, one more day of exceptional heat, widely from Lincolnshire, across East Anglia, to the southeast, over 30 celsius".