The Met Office has warned of heavy rain on Friday 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: "A low pressure is expected to develop somewhere to the southwest of the country on Thursday but its subsequent track may take it safely to the south (across Biscay and France), or curve it into the south of the UK overnight and into Friday.
"In the case of the latter outcome, heavy rain falling so soon after the previous batch would be expected to exacerbate problems with flooding, (especially as further heavy rain seems very likely early on Saturday)."
The warning is in place from 12.05am on Friday until 12pm.