"Washout Wednesday" floods will hit the UK on Wednesday as ex- Hurricane Laura boosts a 1,200 mile-wide storm starting autumn with up to three inches' rain and 60mph gales.
The Met Office said Laura, which had 150mph winds and killed six in Louisiana in the US, is strengthening an Atlantic low pressure system bringing heavy rain sweeping the country from west to east as shown on weather maps.
Up to three inches of rain is due in the north and the Environment Agency has warned of floods.
Meanwhile an inch of rain threatens the south on Wednesday evening and overnight.
Strong 60mph gusts are expected in the north-west, with 40mph even in the south.
Thursday will see more rain from Laura's low pressure system, with showers following.
"Ex-Hurricane Laura's bundle of energy is transitioning to a large area of low pressure, with wind and rain likely," Met Office forecaster Aidan McGivern said.
Met Office forecaster Marco Petagna added: "Wednesday's low pressure contains lots of air from former Hurricane Laura.
"But following the warm air, a push of colder air follows."
The Environment Agency said: "Local surface water and river flooding is possible on Wednesday and Thursday in the north of Wales and England.
"Land, roads and some properties may flood and there may be travel disruption."
Hopes of a warm spell ahead have been scrapped as forecasters said 21C highs will be followed by cold air and 19C from Friday for the weekend.
August was one of the wettest for 254 years and the summer overall one of the wettest for 11 years.
Last month was 61% wetter than average at 124mm in England and Wales, as the month bordered on the top 20 wettest Augusts since records began 254 years ago in 1766, Met Office figures showed.
Summer was around 25% wetter than normal, with UK summer rainfall bordering on topping 325mm, which would make it the wettest summer since 2009 bar 2012's 378mm.
Wet summers always follow the hottest Aprils, records indicate.
April was one of the top six hottest on record, all of which were followed by wetter-than average-summers, hitting 2020, 2014, 2011, 2009, 2007 and 1943, according to Met Office records.
"Summer was hit by the curse of hot spring weather, which often sees wet summers follow," the Weather Outlook forecaster Brian Gaze said.
"It's weather's way of evening things out. It's been a disappointing summer."