Saturday was the warmest day of the year so far, but there was also more than 3,000 lightning strikes - and forecasters are warning of even more today. The Met Office says that high temperatures on Sunday will trigger showers and thunderstorms across parts of Wales with up to 40mm of rain in an hour.
The thunderstorms are expected to develop through the afternoon, with parts of south Wales likely to see some through the night.
BBC weather forecaster Simon King said: "Lovely start to Sunday morning with just some mist around coastal areas but it was quite a sticky night. It is going to be another hot day and there will be a greater chance of picking up a thunderstorm today.
"There were 3,000 lightning strikes on Saturday and there could be more today. They are likely to be more into the afternoon and they will be torrential if you get caught in one. There will be very heavy rainfall, gusty winds, potentially some hail and really some quite loud thunder and lightning.
"Into the evening and the thunderstorms will continue into Wales. South Wales will see a band of particularly heavy rain and thunderstorms moving west. It will be the same on Monday as we see thunderstorms develop into the afternoon. The afternoon commute could be difficult."
There is a Met Office yellow warning in place across the whole of Wales. It says: "Some of the thunderstorms are likely to be torrential , perhaps bringing a few places 30-40 mm in an hour. The odd place, affected by multiple showers, could see more than this, perhaps in excess of 60 mm in a few hours, most likely across Wales and central England.
"As well as rain, additional hazards will be hail, strong gusts of wind, and lightning. Despite this, some places in the warning area will remain dry, or see only small amounts of rain, due to the scattered nature of the heaviest showers."
Here are the Met Office maps for Sunday.
This is how it will look at 2pm
Some more dark clouds around at 4pm
The map for 7pm and it looks brighter but humid
By 9pm there are some showers moving in from the west
More of a chance of seeing a thundery shower by 10pm
Temperatures are expected to remain high into next week with a heatwave thresholds likely to be met with highs 20s in Wales throughout the week.
The Met Office outlook for Wales says: "Monday will be another warm day with light winds. Further heavy showers developing in the afternoon with possible thundery downpours. Low cloud around coasts which should gradually clear through the morning. Maximum temperature 25 °C.
"For Tuesday to Thursday, a high pressure builds, many will see lots of warm weather, with plenty of sunshine and a very low risk of the odd shower. Staying humid, perhaps feeling uncomfortable overnight."
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