A farmer pulled me up on the lane approaching my village last week. We always stop to talk about the weather. I've written a book about it and he has a long memory. "Fearful night," he said. "Felt like I was sleeping in the Severn tunnel, and the train just kept on coming. This boy's a bad un. When's it going to end?"

By "this boy" he meant the low-pressure system or depression that's been barrelling off the Atlantic and into western Britain for days, bringing rain, south-westerly gales and now flooding in Cumbria. I can't remember when "this boy" began. Forecasters don't know when it's going to end. Welcome to the new Wild West.

I live in the Black Mountains, south Wales. From just behind the house, on a clear day – don't laugh – you can see beyond Offa's Dyke footpath and into England. That ancient border is increasingly irrelevant. There's a new divide in Britain. It may prove to be more significant than any ethnic, economic or social division that has previously concerned our governments. It's a meteorological divide.

A glance at any weather advisory map of Britain this weekend will show you where it lies. Anything marked red and "high risk" is the Wild West. The divide runs from the Isle of Purbeck on the Dorset coast north to Berwick-upon-Tweed, roughly following the line of two degrees longitude, give or take the odd fell and raging river.

The divide is most pronounced in autumn, when stalling low-pressure systems mean rain falls in the west for days; the heavens demonstrate their full armoury of precipitation, from mournful drizzle to the sort of squalls that presage the death of fishermen; when the wind drones on and on and on until I begin to think the devil is in the birch trees outside my house. I know it's a "bad un" when the lights are on at midday and my young spaniel doesn't sit by the door; when it rains in my dreams and I start shouting at the weather forecasters on television: "Wet and windy! My soul is being ground to dust like cumin in a pestle and mortar and you call it 'wet and windy'!'"

Actually, I try not to watch national weather forecasts. I look at regional ones on the web. It's less distressing. When you're suffering from a fit of what my wife calls the "manic depressions (south-westerly)", to learn that it's 14C and sunny in Brighton can be the final straw.

November has always been the month we endure. "Continuous rain for the last three days… novel progressing well," Evelyn Waugh noted on 1 November, 1939; "I really begin to doubt whether England is a beautiful country," George Bernard Shaw wrote on 2 November, 1896; "Misling rain all day," the Rev Gilbert White recorded on 3 November, 1770; and on 5 November, 1685, the diarist John Evelyn moaned: "Extraordinary wett morning, & I indisposed by a very greate rheume."

The iron age inhabitants of Britain brought their livestock down from the hill pastures on 31 October, their New Year's Eve and the beginning of what they called the "dark half" of the year.

I grew up on a cliff overlooking the Irish Sea. I rode a bicycle around the world. I live in Wales. I like weather. But it's changing. Our winters are getting wetter. Rainfall patterns are shifting. It's most notable in the west.

This week, Cumbria and Dumfries and Galloway have been worst affected. In 2006, it was Swansea; in 2005, Carlisle. We've just had the wettest November day since records began in England and Wales in 1766 – 243 years ago. The problem is, the record's being rewritten so quickly.

When a breathless television reporter says it's "a once-in-millennia event", that is meaningless. We're in new meteorological territory. The record could be broken right here, in south Wales, this weekend.

The Wrong Kind of Snow by Antony Woodward and Robert Penn is published by Hodder & Stoughton