Mark Davies was one of the eyewitnesses, as water cascaded over the wall of a car park after debris blocked a culvert.
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Plenty of fine dry weather to come today and prolonged spells of sunshine. A cool easterly breeze will take the edge off the temperatures.
The three-day event in the Welsh capital is expected to attract more than 20,000 visitors.
Steve Ashton's picture of a jacket potato and corned beef served to his 81-year-old mother has been shared 130,000 times on Facebook.