It’s 10:30 in the morning and the air temperature has just risen to freezing, from a low of -3.7C at dawn. That’s unusually cold for our location, but it’s not even the lowest temperature this winter (that was -4.7C on 8th February). We saw snow falling several times yesterday, but there is nothing on the ground and the sun is shining right now. Despite this, the media is full of stories about “The Beast from the East”. This has brought significant snowfall to Eastern counties, immediately causing the disruption and difficulty which even a small snowfalll always brings to England (mainly because our governments and businesses - local and national - are unwilling to spend money on infrastructure). There is a risk of much heavier snow here towards the end of the week, but, as so often, the media hype is out of step with the reality.
The easiest way to explain what is happening is to look at today’s pressure map (from the Met Office):
There is very high pressure settled over Northern Scandinavia. Here the air is descending and at the surface is flowing away from the high. This is associated with very cold air indeed (cold air is denser and sinks). The rotation of the Earth (the coriolis effect) means that winds want to sprial outwards clockwise from a high pressure, along the isobars (lines showing places with the same air pressure). Across the Mediterranean, there is low pressure. Here (warmer) air is rising, so air flows in towards a low pressure and the Coroilis effect makes it spiral in anti-clockwise as it does so. The flows around the highs and lows reinforce each other to create a strong easterly wind near the surface, all the way from Siberia (where the air is extremely cold at this time of year) to Western Europe and the British Isles. I’ve added blue arrows to show this.
When this air crosses the North Sea and English Channel, it is warmed from below as the Sea is relatively warm. Air over relatively warm sea is humid, containing water vapour. Warm, moist air rising into cold air is the classic set-up to create showers. The air keeps rising (as the air around it is colder than it is), the moisture condenses and we have rain, or snow because the air temperatures are low. This is known as “lake effect” snow, after a similar phenomenon when cold air from Canada crosses the Great Lakes and causes heavy snow in the USA.
Today’s radar image is speckled with snow showers. These are not randomly distributed. Showers and shower clouds have a tendency to form “streets” along the line of the wind. There is a very obvious “streamer” running from the Thames Estuary (and across Kent to Brighton) and then down the Channel. Places underneath this streamer will be receiving a lot of snowfall, while people even a short distance to either side will receive none. Everything depends, of course, on the exact course of the wind and where the clouds form. Current forecast are for this area of snow to move onshore during Thursday and Friday, with the possibility of even more snow as warmer air from the Atlantic begins to move in to the very cold air over the Southern UK.
The highly localised snowfall in this situation makes forecasting very difficult. A town or village may have no snowfall at all, and bright sunshine, while somewhere a few miles away may be in blizzard conditions. It is best to “nowcast” by looking at the radar as the winds and showers develop.