It is forecast to be the hottest day for most of England, Wales and Eastern Scotland, but it is not all wall-to-wall sunshine for Wales as a band of rain will bring heavy showers. Temperatures are expected to be around nine degrees above the average.
The Met Office says that highs of 23°C could be recorded in parts of Wales by the end of the day, and as high as 26°C in London. But during the afternoon heavy rain will increasingly affect much of west and south Wales.
The Met Office forecast for Wales on Tuesday (May 17) says: "Any early low cloud will lift to leave a fine morning. East Wales will hang on to warm and bright, albeit increasingly cloudy conditions until late afternoon. Meanwhile heavy rain will increasingly affect much of west and south Wales. Maximum temperature 23 °C.
"Heavy rain will cross all parts through the early evening, but should be well to the east before dusk. A few showers following on into western areas, otherwise dry overnight. Minimum temperature 7 °C."
The outlook for Wales shows that Wednesday and Thursday will be cooler but will be "drier and sunnier". More rain and showers are forecast for Friday before it becomes "mostly dry" again on Saturday.
Forecasting for Tuesday, BBC forecaster Simon King said: "We've jumped to summer with a warm/very warm day for most of England, Wales and E Scotland. Temperatures around 5-9 degrees above average. Exception being NI after yesterday's warm weather...back to average today."
The Met Office long range forecast for the weekend and next week says there will be "plenty of fine and dry weather", with a risk of some thundery showers in the south west at times. The BBC long range forecast says: "In late May we expect the southern half of the country to have a more settled pattern with slightly above average temperatures. Scotland could still see a few wet and windy days. In June the pattern is on the drier side in England and Wales, more seasonal elsewhere. Temperatures look a little above for early summer."
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