Thunderstorms could bring flooding, travel disruption and power cuts across Gwent on Sunday.
Residents in Newport, Caerphilly, Torfaen, Monmouthshire and Blaenau Gwent have been urged to prepare for possible disruption.
The Met Office has issued a yellow warning covering all five local authority areas. It will be in force from 11am until 11.59pm on Sunday, August 30.
Forecasters expect heavy showers and thunderstorms to move into Wales late in the morning before becoming more widespread during the afternoon. Rain, lightning, hail and strong winds are possible.
Some places could receive 10mm to 15mm of rain in less than an hour. Isolated locations may see between 30mm and 40mm over several hours.
Heavy downpours could leave roads with spray, standing water and reduced visibility. Drivers across Gwent have been warned that conditions may become hazardous.
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The Met Office said there is a "good chance" driving conditions will be affected, with longer journeys possible for motorists and bus passengers.
Train passengers could also face delays as storms move across the region during the afternoon and evening.
Lightning strikes could damage a small number of buildings and structures. Short-term power cuts and interruptions to other services are also considered likely.
Some homes and businesses could be affected by localised flooding. The warning is classed as yellow, which means impacts are expected to be relatively limited.
However, the Met Office said thunderstorms can cause highly localised but significant disruption where the heaviest showers develop. Not every area will experience the worst conditions, but weather can deteriorate quickly where storms form overhead.
The warning was issued at 8.47am on Sunday and covers a large area of Wales, including all five Gwent local authority areas.
People planning to spend time outdoors have been advised to monitor the forecast throughout the day and prepare for changing conditions.
Anyone who hears thunder should seek shelter inside a substantial building or vehicle. People have been advised not to shelter beneath trees or other tall structures, which could be struck by lightning.
Residents in places known to be vulnerable to flash flooding are being encouraged to make preparations. These include assembling basic emergency supplies and making a plan in case flooding occurs.
Those travelling should check road conditions and public transport services before setting out. They have also been advised to allow extra time for journeys.
The heaviest showers and thunderstorms are expected to become more isolated later on Sunday evening. The warning is due to expire just before midnight.
The Met Office said the warning carries a medium likelihood of impacts but a relatively low overall severity. Disruption is not expected everywhere, although some communities could experience problems if thunderstorms develop nearby.
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