Having been caught unawares by the torrential rain this summer, the Met Office was somewhat diffident in issuing its first forecast for next winter.

It will be warmer and wetter than usual, but colder and drier than last year, and but for global warming it would be colder than usual too.

Got that? In other words, the forecast is for an unexceptional winter. Perhaps.

But just as amid last winter’s unusually mild weather there were cold snaps with ice and snow, so this coming winter could have short spells of extreme weather. A winter forecast given in the middle of summer is based on the temperature of the Atlantic in May, but stretches meteorology to its limits, and the predictions – particularly for rainfall – are deliberately vague. This explains why the long-range forecasts for this summer gave no hint of the downpours.

The Atlantic is crucial for the calculations because in winter it releases vast quantities of heat, equivalent to a million power stations, helping to insulate Britain from the savagery of a Siberian winter. The month of May is important because of an odd feature in Atlantic sea temperatures. At this time, if the seas are warmer than normal in parts of the sub-tropics and off the northeast coast of North America, but cooler than expected off southern Greenland, then Britain will tend to get a wet, mild winter. If this sea pattern is reversed, we are more likely to get a harsher, drier winter.

This makes for surprisingly good forecasts because May’s sea pattern can predict a seesaw in the atmosphere: the North Atlantic Oscillation (NAO). Two huge blocks of air pressure are stationed fairly regularly over Iceland and the Azores. When the Icelandic pressure rises and the Azores pressure dips, Britain catches bitterly cold air sweeping off northern Europe. But if the seesaw swings towards lower pressure in Iceland and higher in the Azores, then mild, wet air drives our way off the Atlantic.

Another forecast suggests that the rest of this summer could be a washout. Tomorrow is St Swithin’s Day, which predicts 40 days of rain if the weather is wet that day. Similar sayings are found through Western Europe, and there is a grain of scientific truth to them.

In early to mid-July the weather tends to settle into a pattern as the jet stream, a river of wind a few miles high, often fixes its track across the Atlantic – and where the jet stream goes, the wet weather tends to follow. For this St Swithin’s Day, the Met Office is predicting particularly heavy downpours for England and Wales.