Heavy rain and winds of up to 50mph have begun to clear the summer away from much of the UK.
Heavy rain may cause localised flooding and debris on the roads
Southern England will bear the brunt of a deep depression sweeping up from the Spanish coast.
The north of England and Scotland will also feel the cold. Forecasters predict low temperatures, winds and rain will cover the south west by midday.
Travellers have been warned to expect heavy rain, localised flooding and fallen debris on roads.
Localised flooding, with up to 7cm (2.8in) of rain falling in some areas, could cause problems on road and rail.
A Met Office spokesman said: "Although not a record breaker, this weather is certainly unseasonal for July, with the combination of strong winds, heavy rain and depressed temperatures making it feel more like November."
A spokesman for AA Roadwatch urged drivers to look out for debris on the roads and to slow down if spray reduced visibility.
AA patrolman Steve Neath advised motorists not to speed through standing flood water.
"It can take just an egg-cupful of water in the combustion chamber to wreck an engine," he said.
A drilling platform anchored off the Kent coast broke its moorings and washed ashore as the weather front hit southern England.
A spokeswoman for Dover Coastguard said poor weather caused the 10m x 12m jack-up barge anchored off Foreness Point to break up at about 0700 BST.
No one was injured in the incident and no fuel was on board so there was no risk of pollution, she said.
The barge, which was carrying out survey work for Southern Water, became beached at Cliftonville, near Margate.
Separately, two sailors had to be rescued by coastguards after becoming stranded on their yacht off the coast of Norfolk.