Exeter Airport was forced to close on Sunday because of flash flooding - as severe weather warnings were issued for large parts of the UK.
In Somerset, the Met Office said as much as 12cm of rain could have fallen in an area surrounding Taunton and Bridgwater - more than the average rainfall for the whole of September.
Video of Exeter Airport on social media showed people standing in shallow water on the terminal floor.
A spokesman for the airport in Devon said: "Following Sunday afternoon's flash flooding which caused the closure of the airport, our teams are working through the night cleaning up and we expect to be open (on Monday morning).
"Passengers are advised to check with their airline for the very latest information about their flight, and please bear with us while we do our very best to return all airport operations to normal."
At least five incoming flights, arriving from locations including Newcastle and Zante in Greece, were listed as cancelled on the airport's website on Sunday afternoon.
An amber weather warning for thunderstorms was issued across parts of Devon and Somerset until Sunday afternoon.
Heavy rain brought "torrential downpours" across the southwest of England, with localised flooding in south Devon.
Further east, the Met Office has a yellow warning for thunderstorms until 6am on Monday, covering an area including Portsmouth, Brighton, London, Oxford, Peterborough and Norwich.
There is a "chance that homes and businesses could be flooded quickly", the Met Office said, with damage to buildings possible.
Bus and train services could be delayed or cancelled.
The latest Sky News weather forecast
Forecasters are predicting a wet and windy end to the UK's short-lived September heatwave, with a spell of dreary weather expected for much of the UK next week.
There will be further heavy and thundery showers on Monday, although the weather will turn drier and fresher from the west into the evening, forecasters said.
It will also be increasingly blustery, particularly in and around any heavy showers or thunderstorms.
Later in the week, the forecast remains "unsettled", according to the Met Office, with "strong winds and outbreaks of rain on Tuesday and Wednesday".
Widespread showers are predicted on Thursday with a risk of hail and thunder.
Earlier this month, a new record was set after the seventh consecutive day of temperatures above 30C in the UK - the longest run ever recorded in the month of September.