Watford and the rest of Hertfordshire are forecast to get “heavy showers and thunderstorms” today.
The Met Office has issued a yellow weather warning for between 12pm and 8pm and said there could be flooding and travel disruption.
It comes after September’s record-breaking heatwave finished this week.
'Possible localised flooding'
The Met Office said: "Whilst many places will miss these, heavy showers and thunderstorms are likely to break out across parts of east and southeast England on Tuesday afternoon.
"20-30 mm rain could fall in less than an hour, with one or two places perhaps catching 40 to 50 mm within two hours, leading to some localised flooding.
"Lightning and hail may be additional hazards. Showers and thunderstorms will slowly die out during the evening."
What to expect
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Spray and sudden flooding could lead to difficult driving conditions and some road closures
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There is a small chance that some communities become cut off by flooded roads
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There is a slight chance that power cuts could occur and other services to some homes and businesses could be lost
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Where flooding or lightning strikes occur, there is a chance of delays and some cancellations to train and bus services
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There is a small chance that homes and businesses could be flooded quickly, with damage to some buildings from floodwater, lightning strikes or strong winds
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