Cooler air is gradually being sucked in at the rear of a low-pressure area that will pass over the Benelux on Tuesday. It becomes a lot cooler both on the ground and higher up in the atmosphere. Due to the sharp contrast between the upper skies (-35°C at an altitude of 5.5 km) and the relatively warm sea water, winter showers easily occur. In addition to thunderstorms and grain hail, these can also have (wet) snow on board. During a fetch (street) of such winter showers, a temporary snow cover is therefore not excluded.
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Supply of polar air from the north
During the night from Tuesday to Wednesday and Wednesday, the shower carousel will start from the northwest. The upper air is becoming increasingly colder and fluctuates around -35 degrees at an altitude of 5.5 km. The sea water in the Channel and the North Sea is still relatively soft for the time of year with sea water temperatures between 12-14 degrees. This gives a difference of almost 50 degrees! This large difference causes upward movements, resulting in very heavy showers with thunder and/or grain hail at sea.
This great contrast is reflected in the formation of instability (CAPE). Above the sea, the instability is above 1000 J/kg, which are values that would not be out of place in the summer! The instability decreases above land, but there too there is still sufficient instability to maintain the showers (500 J/kg). The wind shear is also on the low side.
Not every region will receive the same amount of winter showers
In a northwesterly flow, the United Kingdom often lies in the way of Belgium, the so-called shadow effect of England. The showers from the north must therefore first pass over a large area of land (UK), causing the showers to lose strength and eventually disappear. The Channel is often too small to generate heavy showers, so the least precipitation is expected in the far west of Belgium and along the French border. For that region the current must come more from the north.
Most showers are therefore reserved for the Netherlands and the north/east of Belgium. Various weather models show a train of showers developing over Belgium and the south of the Netherlands (several showers over the same location) and in such cases surprises can arise. Isothermal energy can cause the mercury to drop to temperatures between 0 and 1 degree, allowing accumulation to take place temporarily. Of course, snow falls in the Ardennes. The chance of wet snow is also small along the coastal areas due to an onshore wind from the northwest that brings soft air.
- More information about isothermal: HERE
The timing of the rain showers is still highly uncertain. The Dutch weather model (HARMONIE) only arrives in the late morning with a train of showers in the south of the Netherlands/north of Belgium and then intensifies further in the afternoon. By the evening the showers would move away again towards the east.
The French weather model has a different timing. There, a first impulse of winter showers (hail, thunder, wet snow) will arrive in the morning of Wednesday. A temporarily drier period will follow later (especially in Belgium), but in the afternoon a new area will form with heavy showers that will gradually move towards the south/southeast. There is also a chance of thunderstorms, hail or wet snow.
During those showers, the mercury can drop quickly due to isothermal energy, as mentioned earlier. Both weather models drop the mercury in the shower areas towards 0-2 degrees. That is enough to generate wet snow, especially in the higher areas. Whether it actually gets that far depends of course on the degree of isothermal. A little more wind and that process happens less effectively, meaning that the precipitation retains a more liquid form. If the process is done optimally, a temporary (drap) cover is not excluded.
However, the German weather model is not very keen on showery streets and allows it to remain mainly dry in a large part of Belgium. The focus of the showers is further north, with winter showers especially in (the south of) the Netherlands.
Drier again on Thursday
The winter vicissitudes are short-lived. On Thursday it will temporarily calm down again with a decrease in showers in a large part of the Benelux. Only the north and the coastal areas remain sensitive to some winter showers (thunderstorms, hail, wet snow), although the chance of temporary accumulation is smaller.
We will of course monitor the situation closely in the coming hours and we will also update you via the comments at the bottom of this article!
