Meteorologist Luca Mathias from Meteolux provides insights into the intricacies of sleet formation, shedding light on why this phenomenon is relatively uncommon.
The key to sleet development lies in the presence of a warm front as the initial factor. However, before this warm front arrives, a crucial precondition is the proximity of cold air (ranging between -1 and -5 degrees) close to the ground, as was evident in Luxembourg on Wednesday morning.
Luca Mathias elaborates on the dynamics, emphasising that the cold air, being heavier and more resilient at ground level, creates a scenario where the warm air from the approaching front "glides" over it. This results in a situation where the warm air at altitudes between 700 and 1,500 metres moves faster than its ground-level counterpart. Essentially, there is a temperature disparity with higher altitudes experiencing warmer conditions than those at ground level.
When precipitation occurs, originating from higher altitudes in the form of snow, it traverses through the warmer layer, typically at positive temperatures ranging between 3 and 5 degrees. Consequently, the snow melts in this warmer layer and descends as rain.
Even if the rain subsequently returns to sub-zero temperatures, there is not enough time for it to refreeze. Instead, it falls in liquid form onto the cold ground, which remains very cold. The rain then rapidly freezes upon contact, resulting in the formation of black ice on surfaces, be it the ground or vehicles — a phenomenon known as "freezing rain."
These demanding requirements, cold frosts on the ground meeting a warm front, are the reason why this weather phenomenon is so rare. Mathias highlights the delicate balance required for sleet, often hanging on a difference of just 1-2 degrees of temperature within a small area of the warm front.
This precise condition contributes to the considerable uncertainty in Meteolux models and forecasts. The localisation of the phenomenon also poses a challenge. The recent zone that led to the formation of black ice lingered exceptionally long over Luxembourg, lasting around six hours, which is unusual.
As the warm air gradually retreats, the cold air takes its place, causing temperatures to drop back to sub-zero levels. If the ground temperatures remain negative, snowfall ensues. However, by Wednesday afternoon, temperatures had risen in many areas, leading to a return of rain.
What are the forecasts for Thursday?
Looking ahead to Thursday, Mathias predicts snowfall in the morning, lasting longer in the south due to the front's movement from northwest to southeast. The snowfall is expected to last until around 11am to 12pm. Meteolux anticipates up to 10 cm of snow in the north and 2-5 cm in the south, contingent on the front's intensity.
During the transition from rain to snow, there remains a potential for sleet in certain locations. Luca Mathias notes the possibility of brief instances of ice pellets followed by a shift to snow.
