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Much colder Arctic air is on its way by the end of the weekend. Before that, there will be a good deal of fair weather. However next week sees more widespread frost, cold by day and snow for some. Read the full update here

Dear fellow Weather Observers In middle latidude islands and some continental areas in various parts of the World hundreds of miles of warm seas with average temperatures well above 0C in the coldest months separate the region from any source regions from which extremely cold airmasses (with a mean temperature well below 0C at sea-level) may originate. Such areas of the World include the UK, but also Ireland, various Mediterranean islands, southern Japan, Bermuda, New Zealand and Tasmania. Also included are southern Argentina/ Chile as well as southern Australia and South Africa because the continents to which they are attached are only large in latitudes too low to support serious continental cooling and so Antarctica is the only real source region for frigid air- meaning advection across the Southern Ocean is required for this air to reach these continental localtions- so for the purposes of the dynamics of very cold airstreams the southern tips of the Southern Hemisphere continents are included. Quite apart from the modifying influence of much warmer sea-surfaces in warming very cold airmasses passing over them: An airmass of 3 km depth in the lower atmosphere is warmed by roughly 10C in 24 hours if the temperature differential between it and the ocean surface is 20C to begin with because a temperatre differential of 15C leads to a 300 Watts per square metre net heating of the air by the warmer ocean (and the warming will be more than that to begin with), thouh in winter this is offset (to a rather smaller extent) by net radiative heating of the lower atmosphere. In addition, there will be a significant amount of heat (latent heat of condensation) as the water vapour pressure from the warm sea surface greatly exceeds that from the cold atmosphere above- with the result that moisture enters the atmosphere and vigorous convection soon results in cumulonimbus and showers of rain or snow. Thus, you end up with a situation whereby an airmass that starts off at -10C and spends 24 hours over a sea-surface of +10C is likely to be about 2C in the lowest layers after 24 hours. This is why a frigid Siberian airmass, crossing Scandinavia at a temperature of -20C in the lowest layers in winter, is hardly ever likely to be below -5C at sea-level by the time it reaches the coast of eastern Scotland and North East England after its passage across the northern North Sea, the surface of which in January is typically about 8C. But this obvious warming is not the only way warm, ice-free high-latitude seas protect middle-latitude regions from the fiercest onslaughts of Arctic, Siberian or Antarcic cold: It is very clear that the vapour-pressure of warm sea-surfaces compared to a very cold airmass over-running it pumps large amounts of moisture into the air. A strong dry and icy wind at -10C over a sea-surface of 10C can extract over 1cm of water-equivalent, which is more than sufficient to result in deep cloud-cover in an airmass at (say) -5C in the lowest layers and -30C at 3,000 metres (the air would warm more near the surface though less quickly aloft and that would encourage strong convection currents in the still-frigid air). Much of the moisture would freeze out in cloudy convection resulting in sharp snow-showers, which are a feature of very cold airsreams over warm seas, but not all of it. With the moisture chiefly freezing out at higher levels the higher-level air (at 2 to 3 km) warms quite rapidly so after 24 hours this reduces the atmospheric temperature gradient to the extent that convection then proceeds at a lower rate. However, cloud-cover at 2-3 km would act as a stronger surface from which radiative heat-loss maintains the middle-level air at a low-enough temperature to ensure convection continues. Continued below

A wintry spell in late April - not exactly what one would expect! Many places in Western Europe have experienced wintry temperatures and even snowfall, for instance in the Netherlands people could greet a wintry landscape with snow actually settling! This caused quite some traffic jams and accidents on the road. Snowy landscape in the Netherlands on the 24th of April. Source: Weeronline. Snow did not only reach the Netherlands, but also in low-lying areas of Switzerland some snow fell, although only somewhat higher up the landscape got a wintry white colour. Slide of Arctic air The main culprit of the snow can be traced down to a slide of Arctic air which has come all the way down from areas near the North Pole. Fuelled by the relatively mild waters of the North Sea, this air becomes unstable and a lot of showers and small-scale low pressure areas spin up. Couple this with the unseasonably low temperatures and the snow is there! Slide of cold Arctic air envisaged by a satellite image as of 25-04 18Z. Source: Eumetrain. The image above nicely shows the cold air reaching all the way down into Western Europe. The red colours denote polar/arctic air which is flooding down over Europe from the north. Interestingly, though, this setup has been present for a number of days. This raises the question as to what is causing this persistent northerly airflow. A real traffic jam in the atmospere For the solution we look at a more global picture of the Northern Hemisphere in the midtroposphere. And that is not a usual one to say the least. Pressure at 500 hPa and anomalies (colours) as of Monday 18Z. Source: Tropicaltidbits. The most clear feature that shows up here is very strong ridging (high pressure) at 500 hPa which extends all the way towards Greenland. This is a nice example of a block which has been present for quite some time now. This feature is what is kind of causing the atmosphere to be 'locked'; pressure systems do not move at all or barely in a couple of weeks. Think of it as a traffic jam in the atmosphere. The counterpart of the high pressure area is located over Mid-Europe, which shows itself as a persistent trough even reaching Africa. This feature is partially responsible for the cold temperatures aloft and thereby a generation of a lot of showers. But this is not where the story ends. The same image, but now with airflow direction indicated in white. Source: Tropicaltidbits. The key for the cold air at low levels (and also partially at upper levels) is that between the high pressure area near Greenland, and low pressure over Scandinavia and Central-Europe, there is a deep northerly flow which starts off near Greenland and flows down all the way towards Tunesia. More cold to come? WIll the pressure situation remain locked, and will we continue to experience a brisk northerly airflow? The high pressure area near Greenland appears to be pulling away towards Russia, but this would still cause the cold to remain entrenched over Europe with also more unsettled weather to come. So one thing is for sure: the cold is going to stay around for some time still. Impressive for April to say the least! If we would only have had this in winter...