The Met Office has issued a yellow warning for thunderstorms in the East of England from 15:00 BST until 22:00.
It covers Bedfordshire, Buckinghamshire, Hertfordshire and Northamptonshire, as well as parts of Cambridgeshire, Essex, and Suffolk.
Up to 3cm (1.2in) of rain could fall in less than an hour, with some disruption possible.
It warned there was a slight chance of power cuts, flooding, difficult driving conditions and delays, with some cancellations to bus and train services.
