Snow situations at lowland locations are often marginal in maritime NW Europe. Why is this so?
(with thanks to Rodney Blackall for advice & suggestions with this and the following entry.)
And why do forecasters find it so difficult to get it right?
Whether snow penetrates to the surface as snow, or melts to rain or sleet on the way down depends upon the height of the 0 degC level (ZDL) above local terrain. It should be easy over relatively flat ground: forecast yes/no for precipitation and use a good forecast model (or dense network of boundary-layer radio-sonde ascents) to find the ZDL. If you are above this level, then expect snow, if below expect rain or sleet.
The 'air-mass' zero degree level is relatively straightforward to forecast. The problem is that snow situations in our part of the world often occur with surface temperatures 'around zero', and minor deviations from the air-mass (or synoptic-scale) ZDL are important, but difficult to predict. There are several factors that must be taken into account when assessing these potential variations in the ZDL. Among these are modification of the temperature profile in the lowest layers of the troposphere due to passage over warm or cold surfaces; cooling due to evaporation of the precipitation elements as they fall through the air and cooling due to latent heat exchanges when snow begins to melt in situations that are 'marginal'. These, and other modifying effects, are discussed this FAQ entry, but they will alter, sometimes dramatically, what type of precipitation actually reaches the surface.
In many of the countries of 'maritime' NW Europe, the major conurbations and the principal highways lie below the 200 m (circa 650 ft) contour. Variations in the low-level temperature structure, often involving changes in intensity from 'light' to 'moderate' or heavier precipitation can cause chaos, yet be difficult to predict and protect against except with very vague generalisations within forecasts. They are also difficult for road, rail & airport authorities, as it can be raining quite happily for several hours (when no precautionary measures can be taken), then all of a sudden, several cm of snow will accumulate as the precipitation intensity changes - or perhaps freezing rain is the result with obvious consequences. A ground height change of more than 30 m (around 100 ft) is quite normal within a town, so it is not uncommon for sleet to fall in one part of the town causing few problems, but snow in another spot nearby.