The Met Office has warned of heavy rain on Saturday 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: "Warm, humid air is likely to be pulled northwards from France late on Friday and during Saturday. Within this air mass, there is an increasing likelihood that heavy, thundery showers will break out. It is likely that many places within the current broad warning area will miss the worst of the storms; over the next few days it may be possible to focus on the areas most at risk."
The warning is in place from 12.15am on Saturday until 11.45pm.