By the time you’re reading this, it should be raining in Cornwall. If the UK’s Met Office’s predictions are correct, this will not be ordinary rain — that, after all, is no great revelation for this country in deepest, darkest November. No, this will be unusual rain, menacing rain, rain that falls not in dribs or drabs or pitters and patters or in the fashion of your typically sodden English winter but sheets and needles, thrown onto the ground and into the skin by gales running at cyclonic speeds. The rain will fall and the gutters will fill and the rivers running through Cornish granite valleys will burst their banks, carrying sewage and tree branches and the unmoored belongings of thousands of people past sandbags and sea walls along a new and uncertain course. This vision of Storm Ciarán has already appeared in the Met Office’s forecasting models, powered by some of the world’s most powerful supercomputers. But what these models cannot always predict are the details.