Hot and humid air is expected to be pushed into northern Europe this weekend, including Britain, creating a "Spanish Plume", forecasters said.
As the wind becomes southerly today and tomorrow, a brief surge of heat from the continent will affect central and eastern parts of England.
However, heavy rain is likely to become widespread through the weekend, especially in the west and north. Severe thunderstorms may affect some areas, MeteoGroup said.
The areas most likely to see fairly hot weather include London, which may see temperatures of around 30C on Saturday, Norwich and Cambridge (28C), Bath, Birmingham and Lincoln (25 or 26C).
However, it looks like a slow-moving frontal system will affect western parts of Britain, with heavy rain developing through the course of tomorrow.
In addition, the heat and humidity in central and eastern areas may trigger some severe thunderstorms, especially for the Midlands and northern England. Hail and strong wind gusts are possible.
The rain and areas of thunderstorms then look like spreading into Scotland overnight and into Sunday, with further rain or showers still possible further south. Northern Ireland may escape much of this activity though. It looks like continuing to be unsettled into the start of next week too, and turning rather cooler again.
:: A Spanish Plume is a colloquial description of a weather situation in which a large southwards dip in the high altitude jet stream develops to the west of Europe encouraging a deep southerly wind flow.
This pushes hot and humid air from Iberia north and north-east into northern Europe, including the British Isles. The proximity of active weather systems moving along with the jet stream along with heating from the summer sunshine can encourage thunderstorms to develop. The strong winds from the jet stream help to organise the thunderstorms and can aid in their severity.