A rare Met Office amber weather warning for heavy rain has been issued throughout Monday, which includes Monmouthshire just across the Gloucestershire border.
The 24-hour alert for South Wales runs from 12am to 11.59pm and disruption and flooding is likely.
The Met Office is forecasting heavy rainfall starting on Sunday night and carrying on into Monday, with as much as 120mm of precipitation expected in some areas.
Due to the expected deluge on Monday, fast flowing or deep floodwater is possible, which could cause a danger to life, the Met Office warned. Strong winds are expected to accompany the heavy rain, particularly in coastal areas.
The wild weather could lead to train and bus cancellations and power cuts. Driving conditions could become dangerous, with road closures possible.
Monday's weather warning has been designated "medium likelihood and a medium impact".
There is always a risk in Gloucestershire that heavy rain in Wales leads to flooding on the River Severn and other waterways in following days.
Flood alerts are already in place on rivers in the Forest of Dean and on the River Wye, both near the border with Wales.
According to the Met Office, when an Amber warning is in place, "You should think about changing your plans and taking action to protect yourself and your property. You may want to consider the impact of the weather on your family and your community and whether there is anything you need to do ahead of the severe weather to minimise the impact."
An Amber alert is the second highest alert level issued by the Met Office, behind a Red warning - which is only issued when it is very likely there will be a risk to life.