Travel disruptions are expected in parts of England and Wales where a yellow thunderstorm warning is in effect today.
Train services are likely to be delayed, and driving conditions can be dangerous, according to the Japan Meteorological Agency.
Up to 80mm of rain can fall in some areas in 3 hours, threatening to damage buildings.
The warning covers parts of England stretching north from Devon to Stoke-on-Trent and straddles most of Wales, including Cardiff.
It will last from 2pm today until 2am on Tuesday.
Get the 5-day weather forecast for your current location
The Bureau of Meteorology warns:
• Driving conditions are likely to be affected by splashes, standing water, hail, and gusts of wind, which can lengthen travel times by car or bus
• Some homes and businesses may be flooded, causing damage to buildings and structures
• Delays may occur on some train services
• Lightning strikes and gusts of wind may have damaged some buildings and structures.
• Possible temporary loss of power and other services
Oli Claydon, a spokesperson for the Japan Meteorological Agency, said the situation should improve by the end of the week, but said it could remain unstable for several days beyond the thunderstorm warning timescale.
He explained: “The main factor guiding our weather over the next few days, and indeed throughout the week, is the area of low pressure coming west of the UK.
“And it’s been there all week and is moving east very slowly.
“From that region of low pressure we get several fronts that are spinning off, as well as thunderstorms pushing up from the south.
“There’s also a cold front moving east from the low, bringing more rain.”
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