**ALERT** Extreme rain and risk of flooding in southern Brazil

Episode of extreme rain in the last days of November and early December will bring a high risk of flooding in cities in the Northeast of Santa Catarina and East of Paraná in a situation of danger for the population | MAURICIO VIEIRA/SECOM/GOVERNMENT OF SANTA CATARINA/ARCHIVE

An episode of extreme rain will hit part of southern Brazil in the coming days and over the next week. It will be an event with several days of precipitation and some with very high accumulated rainfall in just 24 hours, which will contribute to extreme volumes in the sum of several very rainy days in a row.

The map shows the projection of rain for the next seven days from the Icon meteorological model, from the German Meteorological Service (Deutscher Wetterdienst). There is a tendency from significantly high accumulated to extremes in the eastern sectors of Santa Catarina and Paraná.

According to the projection of the Icon model, which is corroborated by other forecast models that are used by MetSul Meteorologia, the volumes in the Northeast of Santa Catarina, in the East of Paraná and in an area further south of the state of São Paulo, in the Ribeira Valley , they can reach marks as extreme as 200 mm to 300 mm with isolated higher accumulations.

The European meteorological model, for example, projects 280 mm in the next ten days for the city of Joinville. Yesterday, the same simulation, signaled around 300 mm. That is, despite small daily variations upwards or downwards in the indicator of volumes, the trend of extremely high accruals has been maintained.

The rain gains a lot of strength in these areas where the models project accumulated highs over the weekend and the instability persists at least until Wednesday or Thursday of next week. The rain would have the highest volumes in two moments. The first, this weekend, and the second around next Wednesday.

The scenario can be considered very worrying due to the huge agreement of the models around extremely high volumes of rain. Some simulations project even larger volumes than the Icon and European models for the Northeast of Santa Catarina, East of Paraná and the extreme south of São Paulo.

The most aggressive of the models in their rain projections for this episode is that of the Canadian Weather Service. This model indicates rain from months to days in these areas. As can be seen on the map, the Canadian model projects a cumulative 400 mm to 500 mm for the extreme Northeast of Santa Catarina and Paraná in the sum of ten days until December 3rd.

Rain in Santa Catarina

The extreme rain event in the next few days will not be widespread in Santa Catarina and should affect only part of Santa Catarina’s territory. The tendency is for the heaviest rain to be concentrated in the east of Santa Catarina, but, in particular, in an area further to the northeast of the state.

With that, the areas from Florianópolis to the North should concentrate the highest volumes of rain in the next seven to ten days. Accumulations can be especially extreme in the areas of Jaraguá do Sul, Joinville, Balneário Barra do Sul, São Francisco do Sul, Rio Negrinho, São Bento do Sul, Itapoá and Garuva.

In the Florianópolis area, the rain may be heavy in the sum of the next ten days, but without accumulations as extreme as forecast further north on the coast of Santa Catarina. There will be two moments of greater instability in the region of the capital of Santa Catarina. Initially, the rain should be stronger this weekend in Florianópolis. In the second moment, the rain can be very voluminous between Wednesday and Thursday of next week.

rain in Paraná

As in Santa Catarina, rainfall in Paraná can be excessive to extreme only in areas further east of the state. This includes municipalities such as Matinhos, Paraná and Guaratuba, Morretes, Guaratuba and Antonina. The rain should be extreme from the Serra do Mar region towards the coast.

The rain should intensify a lot in this area of ​​Paraná this weekend and will persist for most of next week with very high accumulated values ​​again around the middle of next week.

The city of Curitiba and its metropolitan region, due to its geographical position further east and next to the Serra do Mar, can also have very high volumes of rain and with a risk of disturbances. The highest accumulated starts to occur this weekend and at least until Thursday of next week the weather would continue to be rainy with many periods of moderate to heavy rain in the region of the capital of Paraná.

What aggravates the risk of extreme rain

Accumulations in several cities on the coast and in Serra do Mar will be expressive, totaling months in a few days and one of the reasons will be the orography of the region, that is, the relief and how it interacts with fluxes of humidity in the atmosphere coming from the quadrant East.

Moisture that comes from the ocean, brought by wind from the south to east quadrant, due to a cold air mass on the coast, when it encounters the barrier of the Serra’s relief, rises in the atmosphere and finds a lower temperature. This leads to condensation and relief-induced rainfall.

Episodes of orographic rain are of high risk because they tend to bring locally very high precipitation accumulations that often end up exceeding the projections of numerical models. The coasts of Santa Catarina, São Paulo and Rio de Janeiro are at greatest risk of extreme rainfall events of an orographic nature in Brazil, with a long history of disaster situations due to this type of precipitation.

Risk of disturbances and floods

It is inevitable, given the rain scenario projected by the numerical models, that there will be inconvenience and danger to the population. One of them includes flooding of rivers and, consequently, flooding. With accumulations of 300 mm to 500 mm in a few days, the risk is of major flooding in some areas.

The Northeast region of Santa Catarina, especially the Itajaí-Açu River Valley, is the one that will require more attention in terms of hydrological conditions. Cities like Rio do Sul and Blumenau will have to monitor the notices of the local Civil Defense very carefully.

The risk of overflowing streams and streams is also critical due to the high volumes added over several days and episodes of torrential rain, after all, at various times the rain must be intense with high accumulations of rain in a short period.

The succession of days of heavy rain and the extreme accumulations projected by the models will create the risk of critical landslides in the East and Northeast of Santa Catarina, as well as in the East of Paraná and the extreme South of São Paulo.

The Northeast of Santa Catarina, in particular, will have a greater risk of landslides on slopes due to the relief of the region. Falls of barriers are expected and there is a high probability that traffic will be affected on some roads with partial or total blockages, either by accumulated water or landslides.

How to consult the maps

All rain maps in this bulletin can be consulted by our subscriber (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, according to with each simulation. In the maps section, it is also possible to consult our very high resolution WRF model from MetSul.

The article is in Portuguese

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