The Met Office has issued a yellow thunderstorm warning for much of England, including London, and parts of Wales, with fears of localized flooding due to intense rainfall.
The warning is set to be active for 19 hours, from 3 a.m. to 10 p.m. tomorrow. Forecasters anticipate that some areas could receive up to 30mm (1.2 inches) of rain in less than an hour, and potentially up to 50mm (2 inches) in two to three hours. This rainfall is expected to fall on ground that has become parched after an exceptionally hot and dry summer.
Potential impacts include rapid flooding of homes and businesses, damage from lightning strikes or hail, and possible disruptions to rail and bus services. Difficult driving conditions, road closures, and power outages are also risks.
The Met Office has advised those in areas potentially affected by flash flooding to prepare a flood plan and an emergency kit. Individuals caught outside during a storm are advised to seek shelter in a car or a safe enclosed space, and to avoid sheltering under or near trees.
This weather system follows a period of significant drought in the UK. Nearly three-quarters of England and all of Wales have been declared drought-stricken, leading to widespread hosepipe bans affecting 28 million people. Some regions have experienced extended dry spells, with Wisley in Surrey recording 62 consecutive dry days and London Heathrow having 56 dry days until recently. July was the driest on record for England, with some areas like East Anglia receiving minimal rainfall. The forecast rainfall of 50mm for tomorrow is comparable to the total rainfall observed across much of England over the past two months combined.
The Environment Agency has reported that seven reservoirs in England have "exceptionally low" storage levels. Looking ahead, forecasters are also warning of increased storm activity and rain this autumn and winter, potentially influenced by an "unprecedented" El Niño event. This El Niño is predicted to be one of the strongest since the 19th century, with sea temperatures in the tropical eastern Pacific expected to rise significantly above average.
Met Office deputy chief forecaster Mark Sidaway noted that while the start of the week would be dry, warmer and more humid air moving north across the UK increases the likelihood of heavy rain and thunderstorms from Wednesday onwards. He added that while some places might see 20 to 30mm of rain in a few hours, higher totals are possible if thunderstorms repeatedly hit the same locations.