RTL Fact Check: How reliable are weather forecasts? (2024)

Last week, meteorologists raised the alarm ahead of excessively hot conditions on Tuesday, whentemperatures reached around 39 degrees in Luxembourg.Six days earlier, on 13 July, the Ministry of the Environment, together with the Water Management Office, declared a phase of vigilance for drought, accompanied by a forecast of hot weather and little to no rain over the next fortnight.

RTL spoke to MeteoLux meteorologist Luca Mathias to answer a series of questions on weather, including: how far in advance can you actually make reliable weather forecasts?Can it be said in general terms that it will be a hot summer and relatively a wet winter?And is it easier to predict a heat wave, for example, than thunderstorms and floods?

How many days in advance can we reliably predict the weather?

According to Luca Mathias, it is not always possible to say that a forecast for the following day is likely to be 90% accurate, while a forecast two days in advance would be 75% accurate - this could not be proven on a global level. The accuracy of a forecast depends both on different weather parameters, such as temperature, precipitation or wind - as well as on the weather itself. "Sometimes you have stable major weather conditions with high pressure areas, which can allow you an extremely reliable forecast up to the 5th or even 7th day. In contrast to this, you also have to fight with weather conditions that make it difficult for you for the next two to three days to filter out a reliable forecast," says Mathias.

Overall, the meteorologist says forecasts in our region have a relatively acceptable amount of reliability for up to three days, while subsequent forecasts would become increasingly unreliable over time.

On RTL.lu's weather page,a detailed weather forecast for three days is on display, but only offers a more general view of the next five days, as the meteorologist Fabian Ruhnau of Kachelmannwetter explains.The data from the various weather stations operated by Kachelmannwetter in Luxembourg are also included in RTL's weather forecasts.

Are some weather phenomena easier to predict than others?

There is no concrete answer for this, says Mathias. For the Luxembourg's comparatively small surface area, it is generally easier to predict large-scale weather phenomena - for example a heat wave - than more local weather phenomena such as storms, wind or even hail.But even these predictions have their limits.

Ahead of last Tuesday's 39 degree-heat, the Ministry of the Environment issued a press release stating "The weather forecast predicts temperatures above 30°C for the next two weeks, as well as an absence of precipitation.". However, Mathias explains this statement had not been entirely correct, as forecasts showed significant uncertainties for the second week.

It is true that heatwaves can be predicted more reliably than other extreme weather conditions, while dry periods can be predicted some five to seven days ahead of time, but this depends heavily on the stability and complexity of the major weather conditions.

As for 2021's devastating floods which swept across Luxembourg, Germany, and Belgium, the rainy conditions could have been predicted some two to three days in advance. However, it was more difficult to predict the amount of rain that would fall, with fairly significant uncertainties until just before the rain began.
"In terms of continuous rain, which is often associated with a quasi-stationary front, small changes in the displacement of the front in relation to the forecast can have a large impact on the amount of rain for Luxembourg, because our country is so small," says Mathias.

Is it possible to predict a hot summer, or a wet winter?

These are not weather forecasts in the classic sense, but instead more like seasonal trends, says Mathias. In predicting whether a summer will be warm, for example, the forecast depends not on simulations, but on anomalies relating to a certain period of time - a month, or a season - examined in comparison to a climatological reference period. However, these models are not necessarily particularly reliable, even if they can give some indications."For the summer of last year the trends were largely wrong, but for this summer they have provided a good indicationso far", explains Mathias.In general, seasonal trends are more reliable in predicting conditions for winter than for summer, because seasonal weather models, for example, cannot take thunderstorms into account as well.

How has forecast reliability changed in recent years, and how are forecasts made?

Over the last 30 years, the weather models on which the forecasts are based have become gradually more precise and more efficient.A graphic from the European Centre for Medium-Range Weather Forecasts (ECMWF) clearly shows this evolution.For the Northern Hemisphere, a 5-day forecast is about as reliable today as a forecast over just three days in 1995. A forecast over seven days is now about as accurate as a forecast over five days was 20 years ago.

"In the last 30 years, enormous progress has been made, be it in terms of the computing capacity of the supercomputers, with the assimilation of weather observations around the globe or in general with the resolution and the physical parametrisations within the weather models", explains Mathias.

"The accuracy of the weather forecasts has become much better, even if you often hear otherwise,"adds Ruhnau.40 years ago, even forecasts for the next day were often uncertain, but today it is possibly to predict the temperature for the next day mostly to within 1 degree.

The national weather service MeteoLux does not calculate its own weather models, but uses those of neighbouring countries, referring to Germany and France and also the Netherlands.These have integrated Luxembourg's topography so well, that the country does not necessarily need its own high-resolution weather model.The models of the European Centre for medium-term weather forecasts are also considered in the Grand Duchy's forecasts.

Kachelmannwetter'sweather model divides the Grand Duchy into a high-resolution grid of 1x1 kilometre.The forecasts also include data from rain radars, satellite images and over 30 weather stations in Luxembourg.The topography of Luxembourg often ensures that there are different weather conditions in the south and in the north of the country, according to Fabian Ruhnau.For example, it often rains a little more in the north, because the clouds get stuck in the mountains of the Ardennes.

In conclusion

How reliable a weather forecast really is depends on a number of factors.A stable area of ​​high pressure is easier to calculate, for example, than storms, in which meteorologists often can't tell exactly when they will move along, or how strong they will become.Generally, weather forecasts covering the next three days are fairly reliable.

At the same time, weather forecasts cannot give an outlook on whether the summer will be particularly hot or the winter will be particularly cold.These are seasonal trends, which are often not particularly reliable and are subject to a number of variables.

In general, however, one can say that an incredible amount has been done in the field of weather forecasting in the last 30 years.Today, with modern supercomputers, calculations can be made that could only have been dreamed of in the 1990s.