Heavy showers and thunderstorms could break out this afternoon (Friday) the Met Office has warned. A yellow warning for rain is currently in place until 9pm tonight - but many places will miss the heaviest showers. Those hit are likely to have prolonged rainfall and there could be a risk of flooding. A chief forecaster said: An unstable airmass over the south west of England will allow heavy showers to develop this afternoon, both enhanced and focussed by convergence of the wind at low levels. This convergence will cause the showers to become organised and slow moving, leading to some localised and prolonged torrential, thundery downpours, which could give 15-20 mm in a short space of time, and very locally in excess of 50 mm in two to three hours. As is often the case in heavy shower situations, many places will miss the heaviest downpours altogether, whilst the precise location of the largest totals is still uncertain.