Almost three weeks of rain fell on Brighton and Hove and surrounding areas this morning, according to the Met Office.
And forecasters say this could be the lower end of the reading due to hail thrown into the mix.
The Met Office said that because of the volume of hail, and the time it takes to melt, rain gauges might not even have picked up the true amount of water that fell.
Mark Wilson, a meteorologist for the Met Office, said: "The issue we have is that the rain fell in such a short period of time."
It says the July average is 53.1mm of rain. In the space of one hour this morning, a measuring station in Isfield recorded 37mm of rain. Another measuring station in Wych Cross recorded 30mm between 9am this morning and 3pm.
Brighton Station and Hove Station both experienced heavy delays after being hit by lightning this morning. At Hove lightning reportedly hit a rail sub station.
Trains were delayed by up to 30mins between Portslade and Hove due to electricity supply problems.
Parts of Patcham, Hove Park and Mile Oak have been affected by flooding.
The 1 and 1A Brighton and Hove bus services were unable to serve Mile Oak due to flooding, stopping at Boundary Road.
Elm Road in Portslade was the scene of a flooded house.
Simon Mander, 42, of Trafalgar Road in Portslade saw a fire crew from Hove pumping out the house.
The crew had come from a lightning strike in Hangleton.
"It was pretty intense," he said.
Peter Freeman, 41, lives in the house with his wife, a lodger and two dogs.
He said: "We are devastated. You have got to be British and keep a stiff upper lip, though."
South Street in Portslade was also the scene of flash flooding but residents waded into the road to lift manhole covers before water breached people's homes.
Bouts of hail are behind the flash flooding. Pea-sized hail stones fell across Brighton and Hove.
Harry Garth, 20, of Hove Villas in Hove was sweeping them away from the door of his basement flat this morning at 7am.
Other parts of Hove, including Wilbury Road, Clarendon Villas and Goldstone Villas, were hit by road flooding.
The Martlets Hospice reported that its warehouse was severely flooded this morning with more than 100 pieces of donated furniture damaged.
A heavy torrent of water was filmed near Sussex County Cricket Club in Hove.
Brighton Fire Station reported two fire engines out on multiple calls, including three lightning strikes, with Shoreham fire engines also jumping from one call to another since 5.50am.
A crew from Seaford was standing by in Newhaven after its crew was called out.
East Sussex Fire and Rescue Service responded to numerous calls this morning, many involving water affecting house electrics. Where water is effecting the mains switch and it is not safe to switch it off, ESFRS is advising people to call 999.
Duty Officer, Richard Fowler, said: "If water gets into light fittings and sockets, turning off your electrical supply will reduce the risk of fire. However, if water reaches the main electrical supply switch, then you should not touch it. In this instance, call the fire service."
There was also flooding at Worthing Station, where a partly submerged underpass meant passengers were stranded on platforms. Worthing fire crews were on the scene.
Sussex Police is warning drivers to be careful as weather forecasters warn of more to come.
Road policing officer PC Andy Huggett said: "Heavy rain after a long warm spell invariably leads to slippery roads and drivers need to immediately adjust to the new conditions.
"Along with the surface conditions, there are the added hazards of very heavy rain, flash flooding and poor visibility and it is essential that drivers make allowances. Use your lights, keep your distance and ensure that you can see clearly.
"If you do encounter flooding, slow down and don't go into the water unless you are absolutely sure that it is not too deep for your vehicle."
People flooded out of their homes were being looked after at Hove Town Hall.
Brighton and Hove City Council says essential services are being prioritised and maintained.
The Met Office said the rest of the week will be generally sunny in Brighton and Hove, with some showers.
On the next 48 hours, Mr Wilson of the Met Office added: "There is a continued risk of some showers and rain. There could be a bit more organised rain fall coming through."
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