Dramatic videos and photos of stranded cars and submerged streets have been pouring in from Chronicle & Echo readers.
Northampton is at the edge of an orange weather warning for rain tonight (May 27) as the downpour moves across the country.
The Met Office is warning the thunderstorm could lead to "deep floodwater" and power cuts.
Readers have described some of the scenes in Northampton as a result of the flash flooding.
Michelle said on Twitter via @_michpix: "Crazy. My dad had to practically wade through a river to get to his car in Far Cotton."
Joe Atkin , via @JoeAFC_, said: "Wooldale Road alongside Caroline Chisholm school is really flooded. Cars driving along the path. Wouldn’t advise driving down there tonight!"
Martin Emmerson @Martycricket said: "Well that was some afternoon. The apocalypse hit Northampton as I drove away from the ground. Roads flooded. Hail stones so big I thought they were going to smash the windscreen. Frightening."
Some people were blaming blocked drains for the flooding:
KevJedi, via @KevJedi, said: "Neglected blocked drain on public highway resulting in needing to move my car. Lucky I was home to do so and that there was somewhere else to park which isn't always possible."
The flash flooding has caused issues on the roads.
Highways England posted a tweet at 9pm: "2 lanes now closed on the #M1 southbound within J15 due to heavy rainfall which has caused a considerable amount of standing water in the carriageway. We have a crew on scene. @HighwaysEMIDS"
And later at 9.45pm, they tweeted that the A45 was closed: #Northamptonshire. #A45 westbound between the #A508 (near #Wooton) is closed due to heavy rainfall."