The Met Office has warned of heavy rain today and tomorrow across south west Hertfordshire.
The yellow warning was issued today and warns of the possibility of localised flooding and disruption to transport.
The yellow alert is the lowest level of severe weather warning, and asks members of the public to "be prepared for adverse weather".
The Met Office’s chief forecaster said: "An active complex frontal zone is expected to move northwards into southeast England this evening bringing the threat of some heavy rain along with thunderstorms and possibly gusty winds.
"Whilst some spots could get 20 mm in less than an hour and over 30 mm in two or three hours, other places could well miss the worst."
The warning is valid from 6pm tonight until 9pm tomorrow.