Temperatures are not set to rise into the 20s until the weekend, the Met Office forecasts.
The weather experts predict today (June 9) will see 19C highs with clouds and sunny spells in Bristol.
Every day from Wednesday to Sunday is likely to have a mix of rain and sunshine, except Thursday which is expected to be rainy and overcast.
The Met Office forecasts 16C highs on both Wednesday and Thursday, 18C on Friday, and 20C on both Saturday and Sunday.
In its South West forecast for Tuesday, the Met Office says: "A bright start to the day with sunny spells, further cloud will develop into the afternoon.
"Lighter winds will make it feel slightly warmer than of late. Maximum temperature 18C."
The forecaster adds: "Outlook for Wednesday to Friday - cloudy with occasional showers on Wednesday leading to a much wetter and windier day on Thursday, rain slowly clearing away on Friday."
The Met Office has also given a longer-range UK forecast for June 13 to 22.
It says: "Over the weekend northern areas of the UK should see the driest and brightest weather, particularly western Scotland but with showers or longer spells of rain possible in the east.
"Elsewhere, conditions will be rather changeable and breezy for many with some sunshine but also areas of rain or showers.
"Thunderstorms are also likely in places. Many parts will become warm, perhaps very warm, particularly northwestern areas.
"Temperatures will probably continue to be above average generally."
