Showers will sweep across the country for the next few days while strong gales are set to batter the south west on Thursday.
Northerners should also not be surprised to find the first frost of the season on their windscreens on Thursday morning, forecasters said.
It comes after huge downpours Wednesday morning brought September's stint of Indian summer sunshine to an abrupt end.
UK weather - In pictures
Met Office spokesman Tom Morgan said that most of the UK "is now seeing a marked change in the weather".
He said temperatures are set to drop from the mid-20s to around 11-12C this week - below-average temperature for this time of year.
Mr Morgan said: "The next few days will be dominated by low pressure, with gales in the south west and rain sweeping across most parts. of the UK."
"Today has been more like autumn with temperatures in north western areas dropping to 11C," he continued.
"There has also been a lot of rain and showers in the north, which will move south east tomorrow along with cooler air covering most of the UK by the end of the week."
After a cold night, the first frost of the season is set to arrive tomorrow morning in the north, Mr Morgan added.
Thursday will see "strong winds and gales in south Wales and south west England," he said.
"There could be severe gales with the potential for trees to come down."
On Friday, colder air will funnel southwards to all parts of the country.
Temperatures will drop to 14C in the south - below average for this time of year by a few degrees, Mr Morgan said.
"It will feel particularly cold in the wind," he added.
Mr Morgan said the weekend will still feel chilly with eastern coastal counties having the biggest chance of rain.
While temperatures will drop to around 11-12C on Saturday, they will be slightly warmer on Sunday at 14-15C.
