The warning from the weather agency will run from 9am on Saturday, September 20 until 6am the following morning. The Met Office says there is a small chance that heavy rainfall may cause some transport disruption and flooding. "From mid-Saturday onwards, increasingly strong gusty winds and perhaps some thunder will also accompany the rainfall, further increasing the risk of disruption. "By the early hours of Sunday, persistent heavy rain will have likely cleared from Wales, with this rain easing for Northern England and Scotland by dawn on Sunday morning. "Showers then follow and winds remain strong through Sunday."