The Met Office has warned of heavy rain across south west Hertfordshire this afternoon.
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: "Scattered but potentially intense thundery downpours are likely to develop from the middle of the day onwards across this region.
"The public should be aware that where these occur, they could bring a risk of surface water flooding, with hail and gusty winds posing additional hazards locally."
The warning is in place from 12pm until 8pm.