A revised version of the earlier "Winter Snow Setups/Non-Snow Setups" topics, this goes through the range of winter setups we can get.
As in summer, the main determining factors in what sort of winter weather we get are the positioning and strength of the jet stream. A strong jet stream means that depressions will frequently move from west to east, giving a "zonal" pattern over the UK. Because the Atlantic is relatively warm and moist, assisted by the warm North Atlantic Drift, zonal types often tend to be mild- but not always.
A weak jet means lows track less frequently from west to east and blocking highs can form more readily. Whether we get cold wintry weather depends on the positioning of the high.
Zonal, northerly tracking jet
When low pressure systems track well to the north of Scotland, we usually end up with high pressure close by to the south, and a mild moist tropical maritime airmass covering the north. It is usually dry and mild in the south with a fair amount of sunshine, but cloudier and wetter in the north and west.
An extension of the northerly tracking jet scenario, this setup sees the Azores High displaced over to Europe, keeping Britain in a persistent tropical maritime south-westerly regime. This setup brings Britain's warmest winter temperatures. This setup is often associated with large rainfall totals in the Scottish Highlands. Broadly speaking it tends to be dry and sunny wherever the high covers, northern and western Scotland tend to be dull and wet, and intervening areas often end up fairly dry but cloudy.
Zonal, jet tracking over and to the north of Scotland
This is the most common "zonal" winter pattern with low pressure systems regularly moving from west to east, bands of rain moving east at intervals, with brighter showery polar maritime air in between the rain belts. If the lows track from SW to NE then southern and eastern areas often spend a lot of time in "warm sector" mild moist tropical maritime air. It tends to be wet everywhere, sunny in the east and dull in the west.
If the lows track more from NW to SE much of Britain spends a lot of time in polar maritime air, giving sunshine and showers. It tends to be wet but sunny in most regions, especially sunny in the east and especially wet in the west. On rare occasions, if there is an influx of Arctic air or cold pools from Canada/Alaska into the mid-Atlantic, we get so-called "cold zonality" with widespread snowfalls, especially in northern and western regions. An extreme case of this occurred in January 1984.
Zonal, jet tracking right over Britain
This pattern tends to be very wet as the lows track straight over the British Isles. Temperatures tend to be close to normal but with a bias towards milder conditions in the south, and cold polar incursions often reaching the north, giving snow for Scotland and northern England.
Zonal, southerly tracking jet
This pattern is not very common but when it does happen it can usher in prolonged spells of cold snowy weather for the British Isles. Low pressure stays to the south, sometimes bringing fronts into southern areas which can bring snow as the milder air meets cold polar air to the north. Otherwise, high pressure oscillates between Greenland and Scandinavia bringing repeated bursts of northerly and easterly winds.
Blocked, high pressure over Britain
When a winter anticyclone settles over Britain the weather tends to be dry, but the weak winter sun is ineffective at burning away low cloud. Thus sunshine amounts can vary considerably depending on how much cloud is trapped within the high- in general an input of moist tropical maritime air, or an easterly drift from the moist North Sea, may result in days on end of "anticyclonic gloom" with low cloud and mist and no chance of any sunshine. Alternatively, a clear anticyclone may bring frosty foggy nights and sunny days, as happened in December 2001 (below).
Blocked, high pressure to the east
When high pressure is well out to the east, this allows Atlantic lows to come towards the British Isles but they stall to the west, which tends to give rise to mild, rather cloudy southerly regimes.
The equivalent of the summertime "tropical continental" southerly type which dominated the month of July 2006 rarely occurs in winter, but it does crop up occasionally. Depending on the amount of cloud circulating around the high's western periphery, a prolonged spell of anticyclonic gloom may ensue (as happened in February 1993 and early December 2004), or it may be warm and sunny by day but with cool nights, as happened in February 2008 (below).
Unfortunately, with highs both over and to the east of Britain there is no synoptic way of determining how cloudy the highs will be- satellite imagery, atmospheric profiles etc. are your best bet for guidance.
Blocked, strong Azores or mid-Atlantic high
This kind of blocked pattern results in mostly mild weather as north-westerly winds suck up mild air from the Azores and around the high's periphery to the UK. Sometimes as a low moves out into Scandinavia it may introduce a brief burst of cold northerly or north-westerly winds with some snow showers, but these blasts usually tend to be short-lived.
Blocked, Scandinavian high
The Scandinavian High is often regarded as the "holy grail" by many cold/snow lovers, because it directs cold continental air across from the east. However, the Scandinavian High is really more of a building block towards an easterly- if the high is kept too far east the continental air may well stay away to the east.
If an easterly does reach Britain then it will pick up moisture over the North Sea, and the resulting weather is largely dependent on the upper air temperatures, and the 850hPa temperature is often used as a guide. If the upper air is relatively mild (typically above -5C), the air will be stable and the moisture will give rise to layers of stratocumulus and persistent dull dry weather.
However, if the upper air is cold (typically below -5C, preferably -10C or below) then the air will be unstable, and will give rise to heavy, often prolonged showers, especially but not exclusively for eastern areas. This setup brings much of England and Wales its coldest weather, and can produce significant snowfalls as happened in February 1991 (below).
Northerlies are another major source of snow events, brought about by high pressure to the west, and low pressure over Scandinavia or the North Sea. However, northerlies too have a major "stumbling block" if it's widespread snow you're after. Unless there is a southerly tracking jet stream, or a strong anticyclone over Greenland (preferably both), we tend to get brief "topplers" with just 36-48 hours of northerly winds, a few wintry showers for exposed coasts, and then milder weather pushes in.
However, if a block can hold to our north-west for long enough for the northerly to sustain for upwards of a few days, then we will often see troughs form in the airflow bringing snow showers well inland. A large area of high pressure over Greenland, extending towards Iceland, will usually keep the British Isles affected by repeated bursts of polar air from the north.
The "polar low", a low that forms in cold northerly airstreams and tracks south, is a particularly prominent source of snowfalls in a northerly regime.
Although it is usually northern and eastern areas that see the most snow in a northerly regime, western areas can see the largest amount when pressure is low to the north, resulting in the Arctic air being sent south through the east Atlantic and around to Britain from the west or north-west (similar to the "cold zonality" described earlier, but via a northerly regime). Continuing the Christmas theme, this brought many western areas a white Christmas in 2004.
Finally, when pressure is high to the north or east bringing cold polar and/or continental air towards Britain, and this cold air meets Atlantic systems coming in from the south-west, causing the systems to stall, this can lead to prolonged outbreaks of snow. For example many western areas were heavily hit during early February 1996 from this kind of setup.