The Met Office has warned of heavy rain across south west Hertfordshire this afternoon.
The yellow warning was issued today and warns of scattered heavy showers and thunderstorms.
The public are also asked be aware of risk of localised surface water flooding and the possibility of difficult driving conditions.
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: "Showers could become aligned into slow-moving bands giving some places 10-15 mm of rain in less than an hour with a low risk of 30 mm or more in 3 hours in the worst affected spots."
The warning is in place until 8pm.