The Met Office has implemented a yellow weather warning over the city and across parts of Hampshire.
It is in place between 10am and 10pm.
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Thunderstorms yellow weather warning issued over Waterlooville, Fareham and parts of Hampshire by Met Office with chance of 'heavy rain'
A statement from the forecaster said: ‘A few places could see slow-moving heavy showers and thunderstorms.
‘Spray and sudden flooding could lead to difficult driving conditions and some road closures.
‘Where flooding or lightning strikes occur, there is a chance of delays and some cancellations to train and bus services.
‘There is a slight chance that power cuts could occur and other services to some homes and businesses could be lost.’
Currently, rain is forecast for midday in Portsmouth.
There is a chance it will set in beforehand.
It is due to get heavier at 1pm, though these intense showers are due to dissipate after 2pm, according to the Met Office.
Cloudy skies are set to be in place throughout the rest of the day, with a small chance of rain.
Intense showers are forecast for Fareham from 1pm until 4pm.
Rain is also predicted for that time period in Gosport.
This is due earlier in the day in Havant at 10pm, and is forecast to be scattered until 2pm.
There will be a high chance of thunder in Waterlooville at 3pm.
Heavy rain is due to be in place from 1pm and 4pm, with cloudy weather forecast for the evening.