The difficulty of getting the forecast right when storms and damage add up

Many residents of the capital were perplexed at the end of Sunday, since a flood of 120 liters had been predicted in Madrid and 36 had fallen in Retiro. What happened, has the prediction failed? The meteorological situation that occurred this weekend in Spain can be considered the hell of the men and women of the weather, as the two most difficult phenomena to predict come together, storms in the first place and a dana—an air pocket. in upper layers of the atmosphere—in second. In the case of storms, because they are very local, and in the case of storms, because they are isolated and erratic.

Eltiempo.es expert Roberto Granda explains that “the isolated depressions at high levels, as their name indicates, are separated from the general atmospheric circulation, from the jet stream, so they generate their own circulation of winds and, depending on Depending on how they are distributed, their movement and final position vary”, that is, they are phenomena with their particular internal dynamics.

“It is very difficult to predict what a dana is going to do because they have erratic behavior, unlike a storm, and the smaller the dana is, the more erratic,” he continues. In this case, it is not small, but medium in size, but enough to generate a lot of uncertainty in the prediction.

“As soon as the Dana moves 50 kilometers further east or west, the scenario changes completely, and we are also talking about the fact that in this case what it produces are storms,” indicates this expert. Another determining element is that the expected rains were “determined by a convergence of winds”, those generated by the dana at altitude and those from the eastern surface.

“This convergence was expected between the Toledo-Cuenca-Guadalajara-Madrid border and has occurred further west, which is why the rains have fallen there,” details Granda. “Local-scale and low-level factors have affected a lot,” adds the meteorologist.

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“In the probabilistic models there was dispersion, with very wide ranges of precipitation and, in the case of Madrid, although there was greater unanimity, the scenarios that had the least probability have been fulfilled,” he concludes. Overall, the prediction in the capital has not been accurate, because 120 liters were expected, but the quantities have been greater in the southwest of the region and in many other parts of Spain, where “it has rained a lot,” with accumulated between 100 and 200 liters in Pamplona, ​​Tarragona, Cádiz or Castellón.

For his part, the expert in supercomputing applied to meteorology Daniel Santos Muñoz indicates that “the damages are caused by an atmospheric situation at altitude, which varies greatly depending on the winds at altitude and these, at altitude, are very strong, therefore which moves very quickly and is very changeable.” For this reason, meteorological models are not “very accurate” when it comes to drawing its movement.

What the danas generate is “a reflection in the lower levels of the atmosphere, which in this case are storms,” continues Santos. But not all danas translate into storms. “This one is, because there is still a lot of heat stored. You have cold above and hot air below, so it rises very quickly and storms form,” he details.

Thus, what has happened is the concurrence of a double difficulty, in many cases insurmountable. “On the one hand, there is the location at altitude of the dana, which is quite complicated, it is very difficult to guess where it will be, and on the other, what it generates at this time of year and in these conditions are stormy phenomena. , intrinsically the most difficult to predict as they are very local.”

In fact, the great challenge for models today are storms, says Santos. “What the models and surveillance have detected is that there were favorable conditions for strong, recurrent and stationary storms to form in the Madrid area and surrounding areas, but since these storms have a very small size, it is very difficult to determine the locations. ”, continues this expert. “What the meteorologist sees is that there is a high probability in an area, in this case in the central area, but whether they end up falling in Madrid capital or in Alpedrete or in Toledo is impossible to determine,” he admits, to say that He himself was scared by what could happen in the capital.

“Luckily for the capital, the amount of precipitation has been less than expected and the location of the storms has not been close, they have fallen further south of the community,” which continued until midnight on Sunday under a red warning, the maximum. Santos adds a fact that explains why the maximum alert has been declared in Madrid capital: the red thresholds in the capital are lower than in the rest, because it rains little and because of the large number of population it concentrates. The damage from heavy rain can therefore be greater there.

From the Madrid delegation of the State Meteorological Agency (Aemet), its delegate, Miguel Ángel Pelacho, explains that “the precipitation has been more localized to the west of Madrid and in the mountains and not so much in the city due to the movement of the dana. “, while highlighting that the quantities dropped in the region are not at all negligible – at 10:00 p.m. on Sunday, 80.5 liters in Villanueva de la Cañada, 70 in Navalcarnero, 57.8 in Navacerrada, 52 in Colmenarejo, 49 in Móstoles, 45 in Pozuelo, 43 in Rascafría, 36 in the capital and in San Sebastián de los Reyes and 35.2 in Alcalá de Henares.

On days like today, caution is necessary and not putting lives at risk. But I also make a call for reflection: if a public body warns of “extreme danger” it must be very sure, because that has social and economic consequences.

All caution. Rigor, too.

— Juanma Moreno (@JuanMa_Moreno) September 3, 2023

In its special notice about the situation, Aemet already warned that it was necessary to take “into account the high uncertainty inherent in this type of situation to make an accurate prediction, since a slight variation in the position of the low can change the intensity.” and the geographical distribution of precipitation. With the danas, where it rains the most is not in the center of it, which on Sunday was in the Gulf of Cádiz, but in its front.

Despite all these problems in getting the predictions right, the day had not even ended when what began to rain was criticism of Aemet and, therefore, of the PSOE Government. The president of Andalusia, Juan Manuel Moreno (PP), reflected through social networks on the measures adopted in situations such as those that occurred this Sunday. “On days like today, caution is necessary and not putting lives at risk,” he wrote on his Twitter profile at 10:00 p.m. “If a public body warns of ‘extreme danger’ it must be very sure, because that has social and economic consequences. All caution. Rigor, too,” he added. Sources from the Ministry of the Interior consulted by EL PAÍS assure that “the alert was launched by 112 in Madrid.” According to these sources, Aemet issued a red notice, without qualifying, and the “extreme danger” was determined by the Community of Madrid, which is governed by the also popular Isabel Díaz Ayuso.

Pelacho defends the decision to decree the maximum warning: “When the (prediction) models give a situation between an orange and a red warning, as a precaution they tend to turn red, especially if it affects a large population.” “Stay at home,” which the mayor of Madrid, José Luis Martínez-Almeida, asked citizens to do, “is a decision external to Aemet. It is not our responsibility to decide that or suggest it,” he comments. And so it is. The agency issues weather warnings and the alerts and measures to be taken are decided by the regional Civil Protection.

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