The beginning of 2023 is marked for different temperature records, all associated with heat. February and March were The hottest “February” and “March” in Santiago and the central area since there are records, and the beginning of April was also governed by several records. The high temperatures lasted for many weeks, that the summer lasted for almost five months.
However, this weekend rain is expected in the Metropolitan Region and an important part of the central area, as has been indicated by the different weather models, which are updated daily and, on many occasions, offer different forecasts.
This is the case of the European model, whose acronym is ECMWF (European Center for Medium-Range Weather Forecasts), and of the US model. The latter, called Global Prediction System (GFS) and managed by NOAA, according to its latest report, signals a decrease in rain expectations for this weekend. “There is not much certainty about this rainfall. There are two main models: the European one and the US one. According to the European model, the rain would start on Friday night, at least in the Rancagua-Machalí area, until Sunday morning and accumulate about 60 millimeters,” says Raúl Valenzuela, an academic at O’Higgins University.
“While the US model indicates it about 30 millimeters would accumulate”, adds Valenzuela, which shows the difference in the projection of the storm that will affect the central area of Chile in the coming days.
Despite the above, the Ph.D. in Atmospheric Science explains it the rain will come hand in hand with a river of atmosphere, “a horizontal flow of water vapor concentration in long filaments, which controls precipitation in central Chile,” he says.
Valenzuela indicates that in 2019 a river atmospheric intensity scale was generated consisting of two dimensions: one is its duration and the other is the extent of the flow of water vapor it carries. “In these two dimensions, there is a combination. There are atmospheric rivers of short duration, but with a lot of flow, and can be equivalent – in terms of impact – to rivers that have less flow, but they last much longer,” he points out.
This scale ranges from category 1, which are atmospheric rivers of short duration or low flow, and category 5, which can be atmospheric rivers with the very intense flow in a short time or moderate flow over a long period, consider. “We expect this atmospheric river to reach the core on Friday and last for 48 hours. Given the magnitudes that the flow carries, it is considered to be between a category 3 and 4,” he adds.
“Category 3 atmospheric rivers have some advantages because they bring rainfall to areas with a Mediterranean climate, where it doesn’t rain permanently, which is beneficial rainfall. But in category 3, some adverse effects are already starting to be observed. Even if water is welcome, the intensity of the rain can lead to landslides, floods, or overflowing rivers”, emphasizes the academician.
The image shows the presence of precipitation associated with the humidity of the tropics, which would allow the arrival of a new atmospheric river in Chile.
“On this particular occasion, showers are expected, i.e. short periods with very heavy rain. Even so, much of the storm will be of more or less constant intensity,” Valenzuela states.
The latter indicates at the onset of the storm there will be relatively high temperatures and the isotherm, that imaginary line dividing the solid precipitations from the liquid ones (snow/water), would be relatively high, which implies that the basins, where all the water that falls in the upper sectors is drained, and the mountains will receive only water, for which rivers could increase their flow.
“At the beginning of the storm, when the atmospheric river touches the continent, at the height of Araucanía, the isotherm will be between 3,000 and 3,600 meters and as the isotherm advances, it will go down. In the O’Higgins region we expect snow on the mountain and water below 2,000 meters,” he indicates.
