Thunderstorms and torrential rain are set to hit parts of the UK on Thursday afternoon.
Flooding and lightning strikes could cause delays to trains and buses, forecasters warned.
The Met Office has issued a yellow weather warning for a stretch of southern England from 12pm to 7pm.
Up to 30mm of rain could fall in an hour and there is also a risk of hail.
The Met Office said: "Heavy showers and thunderstorms are likely to break out this afternoon before fading away by early evening.
"20-30mm of rain in an hour is possible in a few places, leading to surface water flooding, whilst there is a chance of one or two impacts from lightning and hail.
"Where flooding or lightning strikes occur, there is a chance of delays and some cancellations to train and bus services.
"There is a small chance of fast-flowing or deep flood water causing danger to life."
It is expected to be cooler on Friday before getting warmer again over the weekend.
Met Office spokesman Grahame Madge said: "We have got a cold front for part of the UK that will bring a dip in temperatures and make conditions more fresh.
"Over the weekend a ridge is building from the South West that will set fine conditions and increase temperatures for a few days or another week."
Temperatures in the South are expected to reach 32C on Sunday and Monday.
"We will see a long spell of fine, dry, sunny weather," Mr Madge added.
He said the heat is being caused by an area of high pressure, adding: "That has created an area of clear air above the UK, cloud is suppressed and winds have been low and that has allowed maximum heat to come through from the sun."
"It's homegrown warmth."
The hot conditions are expected to continue into next week.