Jump to content

Archived

This topic is now archived and is closed to further replies.

Thundery wintry showers

Winter Snow Setups

Recommended Posts

Here is an overview of what setups generally do, and don't, bring snow during the winter months, which should hopefully answer a lot of the frequently asked questions.

"Easterly" snowfalls.

Generally the most well known of snow setups, this setup most often arises when high pressure comes out of Siberia into Scandinavia, and then ridges westwards giving the British Isles a cold easterly airflow.

east1.jpg

Continental air from Russia is fundamentally very cold and dry, but as this air passes over the North Sea, it warms up and picks up moisture. In most cases, this gives rise to layers of grey cloud and mostly dull dry weather, with a little drizzle or fine powdery snow for eastern areas. However, when the high pressure to our north isn't particularly strong, or the airmass has a long track over the North Sea, or there is a strong contrast between surface and upper-air temperatures, there may be enough instability in the atmosphere to produce cumulonimbus clouds, giving rise to heavy prolonged snow showers for eastern Britain, and the odd snow flurry for western Britain.

Sometimes, Atlantic systems can push against this easterly flow, and grind to a halt as they encounter the high pressure centred over Scandinavia. When this happens, there may be major snowfalls on the frontal zone between the mild Atlantic and cold continental air:

east2.jpg

However, it is over-simplistic to see a Scandinavian High and assume it is going to snow. Not only is there the issue of easterlies often bringing dull dry weather rather than significant snowfalls, if the high is too far south and/or east, the coldest air may remain out in the Continent while we end up in a dull, drizzly and fairly mild southerly regime.

east3.jpg

In easterly setups, there is often a northwest-southeast split in snowfall, with the south and east of England seeing the most snow, and western Scotland and north-west England tending to miss most of it. Over England and Wales, wintertime easterlies bring the coldest weather.

"Northerly" snowfalls

Northerlies are a much maligned source of snow these days, because of the high frequency of 36-hour topplers (see below), but when pressure is high over Greenland and low over Scandinavia or the North Sea, individual northerly blasts can last for 3-5 days or so, with a succession of troughs and polar lows moving southwards across the country. Northerly blasts can also occur one after another, and it has certainly been known for months to be largely dominated by northerlies, via several individual blasts from the Arctic.

north1.jpg

Northerlies can also give rise to frontal snowfalls when Atlantic lows, diverted to the south by the high pressure around Greenland, push against the cold Arctic air and generate big snowfalls at the frontal zone.

north2.jpg

However, if the high over Greenland is not particularly strong and there is a tendency for low pressure to form around Iceland, the Greenland High may be "toppled" by Atlantic weather systems as the northerlies reach our shores. The high is displaced south-eastwards into Europe, the northerlies are restricted to a 36-hour event (generally insufficient time for the coldest air to reach us, or for polar lows and troughs to affect wide areas), and away from northeast Scotland and the east coast of England, the weather is dry and sunny. Then mild south-westerlies follow in behind:

north3.jpg

Northerlies bring a northeast-southwest split in snowfall, with the north and east generally seeing the most, and the south and west generally seeing the least, although when the winds veer north-westerly the west tends to pick up a fair amount of snow.

Returning polar maritime snowfalls

Generally, westerly winds are associated with mild wet weather, but there are exceptions:

rpm1.jpg

When high pressure around Greenland ridges into the central Atlantic, there is much pooling of cold Arctic air in the eastern Atlantic. This cold air may then reach the British Isles via a westerly or north-westerly airflow, and if the air is of sufficiently cold origin and has a short enough track over the ocean, conditions may be cold enough for snow. There have been examples of months dominated by this kind of setup, and while they are rarely particularly cold, they can be remarkably snowy.

However, if pressure is generally low around Iceland, the coldest air often affects only the north and west of Scotland, while other areas pick up a warmer north-westerly airflow with a long track over the ocean, and not originating from a very cold source. These north-westerlies are usually temporary, as the low pressure around Iceland will enable Atlantic systems to bring in mild south-westerly air within a couple of days. England and Wales may have a mix of sunshine and rain, hail and sleet showers, with any significant snowfalls confined to Scotland.

rpm2.jpg

Generally, returning polar maritime incursions are more likely to be snowy the further north and west you are.

The omega blocking setup

generalsnow.jpg

The main setup for prolonged cold and snowy weather arises when high pressure is centred around Greenland, but has a tendency to ridge eastwards over to Scandinavia at times. When the high extends over to Scandinavia we end up with easterlies, while when the high retreats back to Greenland, pressure falls over Scandinavia and we end up with northerlies. This way, snowfalls occur via both "northerly" and "easterly" snow setups, and there is always a chance of Atlantic systems pushing against the northern high pressure and stalling, bringing big snowfalls at the frontal zone.

Snowfalls occur generally across the country in this setup, although sheltered parts of western Scotland and north-west England may see rather less snow than most other parts of the country.

Share this post


Link to post
Share on other sites

  • Recently Browsing   0 members

    No registered users viewing this page.

×