The quadricolor map of Météo-France is rarely a good omen. Since 2001, this is how the meteorological institute warns the population and public services of the risks associated with bad weather. The vigilance system considers four levels of risk: green (no particular vigilance), yellow (be attentive), orange (be very vigilant) and red (absolute vigilance).
Triggered on rare occasions, red vigilance concerned Tuesday, July 11, for a few hours, five departments of the East for a risk of “violent” or even “exceptional” thunderstorms. These caused material damage and deprived several thousand homes of electricity. Tuesday, July 18, seven departments were placed in heat wave orange vigilance. According to Météo-France, its vigilance system has a relevance rate of 87%.
How are these vigilance cards constructed? How do meteorologists decide the level of risk? With what tools and what success? Le Monde tried to answer these questions, with the help of Pierre Tabary, deputy director of the operations department for forecasting and head of the vigilance project.
The Météo-France vigilance system was created in 2001, two years after the violent storms that swept across France in 1999. If the meteorological service already had relevant information, it lacked, according to Mr. Tabary, “levers communication, especially to the general public”. The vigilance card, “as simple and understandable as possible”, transmitted simultaneously to the general public, the media and the authorities, then appeared to be a satisfactory answer.
When it was created, vigilance focused on five parameters: violent wind, heavy precipitation, thunderstorms, snow-ice and avalanches. In the aftermath of the 2003 heat wave – which caused the death of nearly 15,000 people –, in 2004, the heat wave was integrated into the vigilance system of Météo-France. Today, nine phenomena are covered: strong wind, rain-flood, floods, thunderstorms, snow-ice, avalanches, heat wave, extreme cold and waves-submersion.
To issue meteorological vigilance, Météo-France engineers rely on forecasts. Based on observations and the “initial state of the atmosphere” – centers of pressure, wind and precipitation conditions – models will predict future states of the atmosphere. Weather engineers have a number of observation tools at their disposal, including balloons carrying barometers, thermometers and other measuring instruments, numerous radars, and satellites providing essential data, geostationary satellites, which scan Europe and the world. Africa, transmitting news every five minutes.
We are talking about forecasts, in the plural: a hypothesis of weather situation is not enough, meteorologists “slightly vary the initial conditions and the parameterization of the model to have the range of possibilities in terms of scenarios and thus, to get an idea of the uncertainties “, explains the deputy director of forecasting. At Météo-France, we like to repeat that a forecast day is gained every ten years: “The three-day forecast has the same quality today as the two-day forecast ten years ago. »
However, forecasting storms remains a difficult exercise. “When you boil water in a pan, you know that there will be a first bubble that will appear, at some point, at the bottom of the pan, but you don’t know where exactly; thunderstorms are a bit the same, ”summarizes the chief forecaster. The conditions conducive to the outbreak of the storm – conditions of humidity, wind, instability, geographical relief – are known and identified, but it is still difficult to know where the storms will start with precision.
For example, on July 11, “we had all the ingredients in the Center-East and North-East for the development of very severe thunderstorms, exceptional in terms of gusts of wind, amount of precipitation, hail, activity electricity, but it was not possible to specify to the nearest minute, nor to the nearest ten kilometers, where the most severe storms would occur, “recounts Mr. Tabary. However, in view of these stormy conditions combined, red vigilance was decided for five departments in the second part of the afternoon.
The Météo-France vigilance map – since 2022, a double map covering the current day and the next day, until midnight – is entered by the forecasters of the institute. Twice a day, engineers based at headquarters in Toulouse and their colleagues in the regions exchange data and observations. It is during these conferences that they collectively make decisions on a level of vigilance for each department.
Decision-making responds to a “funnel effect”, according to Pierre Tabary. The meteorological characteristics of the phenomena are crossed with local sensitivities and vulnerabilities, or specific issues. Road traffic on a day of major vacation departures, strong winds blowing after heavy rainfall that has weakened the trees, or rains in already saturated areas are all factors taken into account when developing a vigilance , and the choice of color.
This color is determined according to thresholds, such as the amount of precipitation, the values of gusts of wind, the temperatures or the severity of the storms. These thresholds are objective, but they can “be subject to modulation depending on [some of these] aggravating factors, which are either known to forecasters or provided by exchanges with State services”. Vigilance thresholds are also established upstream in conjunction with Météo-France’s institutional partners, namely the General Directorate for Civil Security, the General Directorate for Risk Prevention and the General Directorate for Health.
They are discussed several times a year, also providing an opportunity to assess the performance of vigilance production. At each orange or red vigilance issued during the past three or four months, the relevance of the level of vigilance is checked. The maximum authorized rate of non-detection of situations deserving of orange or red vigilance is around 2%; that of false alarms, around 16%. With current tools, reducing the non-detection rate would be tantamount to agreeing to increase the false alarm rate, but “over-alerting is not desirable; this can have the effect of discrediting the system” and reducing the vigilance of the population.
If the occurrence of an episode that will have thwarted the forecasts is not enough for the adoption of an over-alarm policy, Météo-France draws many lessons from these shortcomings. In mid-August 2022, a powerful stormy episode over Corsica had killed five people, while the island only appeared in yellow vigilance. Orange vigilance was not triggered until 8:30 a.m., a quarter of an hour after the start of the phenomenon. The misfire was the subject of an internal investigation which identified a lack of sightings at sea. A buoy has since been deployed off Ajaccio.
Météo-France thus works closely with the State: upstream, for the implementation of forecast performance objectives and the setting of vigilance thresholds, then, at any time, as soon as an orange or red vigilance emerges . In Toulouse, the forecasters are in contact with the national level of the State services, the engineers in the regions with their counterparts in the civil security.
Telephone exchanges are constant and come to “complete” vigilance, explain the reasons for a change from one color to another or clarify the remaining uncertainties. Météo-France’s mission is then to provide “its best estimate of the possible weather risk” in order to warn populations and authorities, the latter then being able to take measures to evacuate, cancel events or close parks. .
Beyond 48 hours, forecasters also have tools to determine the level of risk for the next seven days. At such deadlines, the forecasts are certainly less precise but they allow “to wake up the security services”, explains Mr. Tabary. “A persistent signal is additional anticipation, also, an additional capacity for the State to put in place preventive measures or to preposition emergency means. »