This weekend is a time for thick pullovers and woolly hats as a taste of winter spreads over much of Britain. Raw winds sweeping down from far inside the Arctic Circle, even from the North Pole, will send a collective shiver through the country today, with heavy snowfalls in Scotland and parts of East Anglia.

The Met Office says that conditions will deteriorate further tonight as a mass of sub-zero air advancing from the north collides with warmer, heavy rains from the west. That clash will send widespread snow showers across Scotland and from the Welsh Borders eastwards.

An even bigger threat tonight is a rare and dangerous phenomenon known as an ice storm. This could lay down a treacherous covering of ice on the roads and anything else it touches. As cold Arctic air slices under warm Atlantic air, it will pour with rain that will not have time to turn to snow because the cold air is shallow. It could remain as freezing rain that turns to ice when it touches frozen objects such as roads and trees. Ice storms in Canada and the US are at times so savage that trees and pylons snap under the weight of the ice.

The menace that we are more likely to face is black ice - an invisible coating that can turn roads into skating rinks. Fortunately, most of the snow and ice will melt during tomorrow morning but there is more bad news on the horizon. The cold spell looks likely to persist next week and possibly well into December because of a blocking weather system.

Britain is caught in a titanic struggle between a big block of high pressure anchored in the mid-Atlantic and low pressure lying over Scandinavia. Vigorous jet-stream winds a few miles overhead have developed a kink in their tracks that has trapped the high-pressure system in a vice-like grip.

So are we heading for a bitterly cold freeze this winter? Records show that a cold November is no guarantee of a freezing winter. Three years ago a bitterly cold second half of November had no ill effects on the following winter and there have been many similar cases. Simlarly, some warm Novembers have been followed by bitter winters, as happened 80 years ago. The latest Met Office seasonal forecast still predicts a milder than average winter, although not quite as mild as last winter. There could be more cold snaps but there will be bouts of milder weather too.

Forecasters will be focused on a see-saw in Atlantic pressure systems between lows over Iceland and highs over the sub-tropical Azores. When these pressure systems become more extreme, Britain gets milder and wetter winters. When the see-saw eases off, it brings big freezes.