According to the rain warning issued last night, parts of the western coast, particularly in Scotland, face a shock 100mm of rainfall over the course of today.
The Met Office said the rain will hit homes and businesses by the likely flooding, with transport chaos also expected.
This predicted rainfall is greater than the monthly average of October in the UK, which generally sees 92mm throughout the whole month.
Over less exposed areas, rainfall “in excess of 40mm fairly widely in the warning area”.
The weather warning over Scotland will stay in place until the end of Tuesday.
The dramatic turn in the weather forecast comes as a violent weather front arrives from the Atlantic, brought on by Hurricane Leslie.
Met Office metereologist Aiden McGivern said: “Wind and rain is on the way.”
Meteorologist Mark Wilson added that “the rain will often be heavy”.
Alongside the surge in rain, Wilson also warns that “there will be widespread frost” today.
Temperatures will plummet across the country, which most Britons facing between 3C to 5C – however, this plunges in more rural areas.
Hurricane Leslie is edging closer to the UK, unleashing “two large waves of low pressure” by Wednesday and Thursday this week.
Bookmaker Coral has slashed the odds on this October being the wettest on record.
Coral spokesman Harry Aitkenhead said: “Storm Leslie is set to hit America over the weekend and forecasters believe it’s headed for Europe.
“If true, we can be sure of some horrible weather and it’s left us with no choice but to halve our odds on this month entering the record books as the wettest October.”