The Met Office forecasts temperatures up to 28C in south-east England on Friday. Cooler weather is expected across the UK over the weekend.
High pressure over continental Europe is drawing warm air towards Britain. Early fog patches in southern areas should clear before temperatures climb.
Met Office deputy chief forecaster Dan Holley said: “Friday will be exceptionally warm for late September.”
A weakening cold front should move south-east overnight into Saturday, bringing temperatures closer to the seasonal average. Much of Saturday looks dry and fine.
The UK is set for a grey and damp start in some areas on September 25, with mist and fog possible ahead of rain, particularly across the southwest.
Conditions are expected to brighten from the northwest, although a few showers may persist. Meanwhile, southeastern parts of the country are likely to hold on to warmer and sunnier spells. By Sunday, rain and showers become more likely, particularly in western areas.
Forecasters are monitoring a potentially deeper low-pressure system that could pass close to Britain during the first half of next week. Its track and effects remain uncertain, Holley cautioned.
Read: Super El Niño Pushes Pacific Temperatures to Record Levels
Wet and windy conditions could develop on September 29 and continue into September 30. High spring tides may combine with the unsettled weather to affect coastal areas in the west and south-west.
The Met Office expects to refine that outlook as the system develops.
