Yellow weather warnings have been issued by the Met Office as heavy rain and thunderstorms are forecast to break out over parts of the UK later this week.
The warnings span Wednesday and Thursday and include parts of east Wales, the Midlands, northern England, East Anglia and southern England – including London.
It is likely to be the one of the most widespread outbreaks of heavy rain across southern and central parts of the UK this summer.
While good news for regions plagued by drought, significant accumulations could lead to flash flooding where the ground is too dry and hard to absorb the downpours. Lightning, large hail and gusty winds are also possible.
The storms, which will drift out of France and move northwards, will be sparked as warm and humid air sets in from the south.
Initially, a few showers are likely to reach Cornwall and the West Country during Tuesday evening, but heavier downpours bringing thunder and lightning are likely to hit later in the night.
The first band of heavy showers may reach the southeast coast of England around midnight, and then cross Greater London before moving northwards into the home counties and East Anglia.
The first yellow weather warning is set to come into force at 03:00 BST on Wednesday and last until 22:00 BST.
Though not all locations in the warning area will experience thunderstorms, flooding is possible in those that do, with 20-30mm of rain possible in less than an hour.
The hardest hit locations could see up to 40-50mm in two to three hours.
Flash flooding is possible in areas where the ground is dry and very hard, making it difficult for rainfall to be absorbed. The most intense downpours could also bring large hail with gusty winds.
During the morning on Wednesday, the storms may spread further north into the Midlands and the Peak District but the forecast becomes less certain in terms of location and intensity.
The storms and downpours which will have brought flash flooding and hail are forecast to continue into Thursday.
Some may become slow moving across the Midlands, Wales and Northern England leading to more flash flooding.
Many areas could see 20-30mm of rain in just three hours, with up to 50mm possible in some places.
According to the Met Office, there is also a small chance of considerably higher totals, particularly over east-facing higher ground, where more than 75mm could fall within 12 hours.
Lightning may be frequent and hail of around 1-2cm could fall.
Thunderstorms are notoriously difficult to predict because they can develop very quickly and on a relatively small scale.
Tiny differences in temperature, humidity, wind and atmospheric instability can determine whether a storm forms at all or where it moves.
Forecast models can often identify the risk of thunderstorms across a wider area, but pinpointing precisely where and when an individual storm will strike is very hard.
Even a few kilometres can make the difference between a torrential downpour with thunder and lightning, and somewhere staying completely dry.
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