Forecasters are warning that where there are thunderstorms, there will be torrential downpours, frequent lightning, large hail and strong gusts of wind. A spokesman said: Several areas of heavy, thundery showers will develop over England and Wales from Friday night before moving northwards during Saturday. Not everywhere within the warning area will see thunderstorms, and indeed some spells of warm sunshine are expected at times, but where thunderstorms do form, some torrential downpours are likely with frequent lightning, large hail and locally strong gusts. Significant flooding is possible where these do occur from surface water as well as from small, fast-responding watercourses. The Met Offices chief forecaster added: The public should be prepared for the risk of disruption from any of these elements. The very warm and humid airmass across the UK will become increasingly unstable on Friday night and through Saturday. This will lead to the development of areas of intense thunderstorms, these moving northwards across the UK and affecting different areas at various times through the day. In this situation, large amounts of energy are available in the atmosphere and this, coupled with high moisture content, can lead to torrential downpours along with frequent lightning activity, large hail and locally strong gusts. As is usually the case with thunderstorms, broad areas can be identified as at risk some time in advance but detail will remain very uncertain until very close to the event. Some areas will miss the worst of the storms whilst nearby spots experience severe downpours with some localised flooding, so the public are advised stay in touch with the latest forecasts and warnings through Saturday. There is also a yellow warning of rain for Sunday.