The Met Office is warning that thundery down pours could hit the city between 2pm and 11.59pm on Friday (July 20).
Forecasters say that most places will be dry but there is the potential for slow-moving thundery downpours in places.
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A Met Office spokeswoman said: '˜High temperatures may set off some heavy showers and thunderstorms, which in some places could be slow-moving and persistent, bringing a risk of flooding.
'˜Should they develop, in the heaviest showers some places could see 30mm of rain in an hour, which would likely bring flooding impacts particularly should this fall on an urban area.
'˜There would also be a smaller chance of impacts from lightning strikes.
'˜As is the nature of showers, many places will miss them altogether, and it is not possible to say precisely where they will occur, but the warning area highlights the region at greatest risk.'Â
The forecasters are also warning that there is a small chance that homes and businesses could be flooded quickly, with damage to some buildings from floodwater.
The Met Office added: '˜Where flooding or lightning strikes occur, there is a chance of delays and some cancellations to train, bus and tube services, particularly during the evening rush hour.
'˜Spray and sudden flooding could lead to difficult driving conditions with the possibility of road closures
'˜There is a slight chance that power cuts could occur and other services to some homes and businesses could be lost.'
This is the third thunderstorm weather warning to be issued for the London and South East region, which includes Portsmouth and Hampshire, in recent weeks '“Â however our area has escaped any significant downpours or thunder.Â
Currently according to the Met Office's forecast for Friday, which is when the warning has been issued for, Portsmouth is expected to be cloudy throughout the day with highs of 23 degrees being predicted.Â