Drivers in south London and Surrey are being warned amid heavy rain and fears of localised flooding.
A Met Office forecaster warned of flooding and "difficult driving conditions" as heavy rain continues to fall.
He said: "Heavy rain, locally very heavy, affecting many places this morning, leading to difficult driving conditions and a risk of localised flooding.
"Later, scattered heavy thunderstorms may develop where the rain clears, these thunderstorms most likely across Kent, Surrey and Sussex."
Temperatures would not go above 15C, he added, before thunderstorms struggled to pass this evening.
"Thunderstorms, most likely across the southeast of the area and also London, will only slowly fade away," he said.
"Elsewhere the rain will gradually become lighter and more patchy."
Tomorrow would see more rain, he said, before sunny intervals develop in the afternoon, mixed with scattered heavy showers.
He predicted a maximum temperature of 17C.