• A publication, which has gone viral on social networks, announces temperatures ranging from 43 to 46 ° C across the whole of France on July 16.

  • This forecast, made on July 2, was captured from an English weather forecast site.

  • This is a measurement made by a meteorological center on an American model which produces four a day.

    Above all, it was made too long in advance to be reliable.

"France is going to burn this week..." This shocking sentence from Marc Hay, weather journalist at BFMTV, made an impression in June.

To the point that it has been taken up by many Internet users on social networks for two days.

The cause ?

A weather map of France covered in dark red, almost black, which would reveal a "terrifying" weather forecast for July 16.

A heat wave reaching 43 or even 46°C is expected to hit the entire country.

Enough to equal the national temperature record recorded in Vérargues in the Hérault on June 28, 2019. Should we prepare to suffer again?

We take stock.

FAKE OFF

The map presented in the publications comes from the site wxcharts.com.

This is an English site, weather data aggregator.

For many years, many national and continental meteorological monitoring centers have published their short, medium and longer term measurements.

This site brings them together, in order to offer the general public these results and the various forecasts.

The figures shown are therefore… real.

But that does not mean that France will know the "oven" announced by Internet users.

“This is just a 'run' of an American model called GFS, explains Frédéric Long, forecaster at Météo-France.

We cannot conclude that this is the temperature we will experience in a week.

»

51 hypotheses for a forecast

To understand the serenity of the scientist in the face of these figures, we must first understand how meteorological measurements work.

Millions of data are collected every day by Météo-France.

90% are provided by meteorological satellites, the remaining 10% by ground stations, radiosounding, sensors on board airliners and merchant ships or installed on moored and drifting buoys.

“We also exchange our data with other forecasting centers around the world,” says Frédéric Long.

The centers then use “models”, calculation methods, to establish forecasts.

Météo-France is based on the “European model ensemble forecast” from the European Center for Medium-Range Weather Forecasts (CEPMMT).

This means that Météo-France is not satisfied with the results of a model.

He “rotates” these models 51 times, resetting them each time, to consider different possibilities.

This is what forecasters call a “run”.

If many results converge, the forecast is reliable.

If they diverge, meteorologists will make less progress on a result (the famous “confidence indices” dear to Evelyne Dhéliat).

10 to 15°C less the next day

If the methods differ according to the forecast centers, Frédéric Long informs us that the Americans, with their GFS model, carry out four runs per day.

However, the result presented in the publication that has gone viral represents only one and only one run.

“Predictions can be very variable.

The day after this run, a new one, from the same model, gave 10 to 15°C less,” specifies Frédéric Long.

Above all, the forecaster insists on the very relative reliability of long-term measurements.

Here, the screenshot shows that the run was made on July 2, for a forecast on July 16, which is 14 days ahead.

“The most reliable forecasts are established only two to three days before.

Beyond that, it is much more uncertain.

»

It will be hot in the South

There is a lot of data available on the internet.

But as interesting as they are, they are not within everyone's reach.

“For the uninitiated, they are difficult to grasp.

This is why we must be wary of raw data published in this way.

»

But then, heat wave or no heat wave next week?

“We know that the week will be warmer than today.

But it's still too far to tell.

There is a possibility of a heat wave and very high temperatures in the South.

»

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