Compared to the previous week, the weather was rather calm over the past days, but that is about to change.

Thunderstorms have been forecast and Meteolux initially issued a yellow alert, which has since been upgraded to an orange alert.

Meteolux mainly bases its forecasts on its weather models from France and Germany, which include forecasts for the Grand Duchy.

Prior to the floods last week, the German model forecasted more rain than the French one. In that case, Meteolux takes both models into account, but also conducts its own, separate analysis. While it is relatively easy to make long-term forecasts for heat or cold, this is much more difficult in the case of thunderstorms, according to Martina Reckwerth, the head of Meteolux.

This is mainly due to the fact that a thunderstorm is a "small-scale phenomenon, which occurs locally and is very dynamic". This means that while meteorologists are able to see that there is a potential for storms to occur locally, it is not always clear where they will occur, Reckwerth explains.

Due to Luxembourg's small size, a shift as small as a few metres to the east or west can have a significant effect and decide whether the country finds itself in the middle of a storm or escapes the weather front entirely.

Before an alert is issued, a detailed analysis is drawn up in order to back it up with as much data as possible. The closer a storm comes, the more precise the models become. If any doubts about the extent of a storm remain, Meteolux starts by issuing a yellow alert. But what do the different colours actually stand for?

Reckwerth explains that Meteolux uses a three-tier system:

Yellow Alert

This means that residents should be cautious and that there is the potential for a dangerous weather situation to occur. However, according to Reckwerth, when a yellow alert is issued, it is best to keep an eye on updated weather reports.

Orange Alert

An orange alert means that there is a dangerous situation at hand, which can result in material damage or even injuries.

Red Alert

This alert is only used for extreme weather situations which are "really dangerous".

Reckwerth is not in favour of every administration coming up with its own public alert system, instead preferring centralised communication. According to the Meteolux head, sending push notifications through cell broadcast to mobile phones would be a more efficient way to communicate alerts.

Meteolux uses radar and satellite images from the so-called "Nowcasting" to get a better idea of where thunderstorms occur. This information is then used to update the country's weather alerts.