The Met Office said heavy outbreaks of rain are expected, with some places seeing 40mm of rainfall, and has warned of surface water flooding where the heaviest showers hit. A spokesman said: A frontal zone stalling over much of the area on Saturday will produce spells of showery rain. Some of this rain will be heavy, perhaps with thunder, and locally torrential. Where the heaviest rain occurs is very uncertain, largely due to the complex fashion in which this frontal zone develops. Current thinking suggests northern and western parts of the region covered by the warning may improve later on Saturday while further south and east improvement is considered likely to be slower.