Serious risk of flooding in southern Brazil due to extreme and frequent rain

MetSul Meteorologia reinforces its warns about a scenario of serious hydrological and meteorological risk in Rio Grande do Sul with excessive to extreme rain and a very high probability of river floods and floods in the state at the end of April and, especially, in the first week of May.

An exceptionally warm air mass over Brazil will intensify over the center of the country this week under an atmospheric blocking pattern. At the same time, colder air will be acting over Argentina for several days, generating a huge thermal contrast in the continent’s mid-latitudes.

Since Rio Grande do Sul will be in the transition zone between the two air masses, hot and cold, the Instability will be frequent with the expectation of a lot of rain. Sometimes colder air from the South will advance towards the state, sometimes warmer air from the North will enter, and this constant day-to-day variation will generate successive areas of instability with abundant precipitation.

O scenario is very similar to that of last year’s meteorological springduring the height of El Niño, when extreme rain events with floods hit Rio Grande do Sul in the months of September, October and November under atmospheric blockages associated with powerful heat waves in the Central-West and Southeast of Brazil.

The rain that will fall at the end of April and the beginning of May will add to what has already fallen this weekend and in recent days in Rio Grande do Sul, when it rained a lot and even excessively in some places. That is, they will be very high to extreme accumulated volumes that will add to already high volumes that occurred days before.

In the official network of the National Institute of Meteorology, the accumulated values ​​in the last 72 hours alone were 179 mm in Quaraí, 113 mm in Rio Grande, 110 mm in Caçapava do Sul, 87 mm in São Gabriel, 84 mm in Livramento, 80 mm in Capão do Leão and Vacaria, 78 mm in Campo Bom, 75 mm in Teutônia and 73 mm in Canguçu. Several other stations recorded more than 50 mm.

Several cities without official stations, but with monitoring by other bodies and private data, recorded accumulations close to and above 100 mm in the period. In some, rain amounts to 200 mm or more in just three days. In Greater Porto Alegre, only yesterday most cities in the region recorded rain between 50 mm and 80 mm in just six hours.

Rain projections from models

O Rain scenario indicated by numerical models for this week is extremely worrying with simulations indicating accumulated between 100% and 200% of the historical average rainfall for an entire month at this time of year in just seven days for a large number of locations in Rio Grande do Sul. The volumes indicated in the projections are excessively high to extreme.

The maps below show the projections of three global models that are used in Meteorology around the world and have a history of high accuracy rates. Although there are discrepancies in the distribution of volumes, everyone points to a scenario of excessive rain for a large part of Rio Grande do Sul and Santa Catarina. The maps show the rain projection for this week – just seven days – from the German Icon models, the European Meteorological Center (ECMWF) and the Canadian Meteorology (CMC).

As seen in the maps, the models indicate volumes above 100 mm in much of Rio Grande do Sul and part of Santa Catarina, but with accumulations exceeding 200 mm in many areas. The indicator is worrying at some volume points only this week in some areas 200mm to 300mmbut with even higher brands.

Rivers will overflow and there will be floods

As it has already rained a lot and the indications are that it will rain a lot more in the state, There will inevitably be river floods and floods. The tendency is that several basins (rivers) are expected to reach alert and flood quotas in Rio Grande do Sul and several regions this first week.

The map below shows the risk map from the Global Flood Awareness System (GloFAS), the hydrological model of the European Union’s Copernicus System that integrates rainfall forecast data from the European model with hydrological ones. The GloFAS projection points out that the vast majority of rivers in Rio Grande do Sul have a high (red) to extreme (purple) risk of flooding in the next ten days.

Residents of the valleys, especially Taquari and Caí, severely punished by last year’s floods, naturally ask whether there will be floods like the one in 2023. The risk of flooding in the Taquari and Caí Rivers is high, but in the short term (seven to ten days) the likelihood of a repeat of floods as severe as those in September or November is remote at low.

Extreme rain will bring floods and geological risks

Given the precipitation scenario presented, despite variations in volumes and distribution of rain in the numerical modeling package, it is certain that there will be impacts on the population and, in some municipalities, serious ones. The persistence of rain for several days, in addition to river flooding, will bring floods and flooding.

Rain with accumulations as extreme as expected also brings geological risks. It will be The risk of landslides in hillside areas is very high. There must still be barrier falls which can generate partial or total blockages of state and federal highways.

Some roads, particularly municipal and rural ones, must become impassable with likely sections and points covered by water. With the prospect of strong currents, Flooded stretches must be strictly avoided by drivers at risk of fatal accidents.

Another concern is that this episode of excessive rain occurs at a time when the summer harvest is still being harvested, compromising farmers’ activities and even causing losses. So many consecutive days of rain and such high volumes must cause serious problems for the countryside. Crops will even be underwater in several municipalities.

How to consult the maps

All rain maps in this bulletin can be consulted by our subscribers (subscribe here) in our maps section. The platform also offers maps of rain, frost, temperature, risk of hail, wind, humidity, atmospheric pressure, snow, soil moisture and risk of fire and lightning, among other variables, with updates two to four times a day, depending on with each simulation. In the maps section, you can also consult our very high resolution WRF model of MetSul.

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