In December heavy rain led to flooding in Paraguay and Uruguay.
By Nestor Santayana (INUMET) and Jose Prieto (EUMETSAT)
December was a very rainy month in both countries, with three heavy rain episodes.
The peak of the rain occurred between early 22nd and the afternoon of 23rd, affecting Paraguay and the north-west corner of Uruguay (mostly Artigas, Salto and Rivera).
Figure 1: Some images of the affected cities (Source: INUMET)
A warm front swept across Uruguay from north to south, setting the scene for convection. This was followed by a cold front crossing from south to north.
Download animation, Met-10 IR, 22 Dec 00:00 UTC–23 Dec 14:00 UTC
The fronts met very humid and unstable air (dew point around 20 °C, Lifted index -6 °C) and vertical ascent, plus divergence of the jet stream at high level.
At low level, north-westerly winds of 25–40 knots brought precipitable water from the Amazonia area into areas of Uruguay, Southern Brazil and Paraguay. The trade winds, which under normal circumstances prevent deep vertical development, were weakened at this phase of El Niño, so that generalised frontal precipitation affected those areas.
In 24 hours, 253 mm were measured in Artigas, with several instances above 100 mm in three hours in other towns, including Ciudad de Rivera.
The international bridge between Artigas and Brazil was closed after a sudden increase of 10 m in the river level in less than one day. Around 170,000 people had to be evacuated from their homes in Paraguay and 25,000 in Uruguay.
The Meteosat-10 infrared imagery (Figure 2 and animation), taken from channel 10.8µm with enhanced tops at brightness temperatures between -85 °C and -65 °C, shows the evolution of the systems over the areas affected by intense precipitation.
Figure 3 shows a comparison of the Isohyet map (top right) for the 22 December with the (main image) Meteosat infrared average image for the period of two weeks before 22nd December.
The actual precipitation in the worst period (22–23 December) roughly coincided with the areas north of Uruguay that had already received abundant rain, as reflected in the time-average image for the two weeks previous to 22nd December.
So, it was raining heavily on already saturated ground, leading to severe flooding in the region.
During the afternoon of 22 December north western areas of Uruguay suffered intensive convection, adding to the frontal precipitation.
View the KMZ file in Google Earth
In the animated gif (Figure 4) convective tops are presented without cirrus, using HRV imagery from Meteosat-10 (tinted with NIR channels).
The last frames show sun glint along the Parana river, in the southern Paraguay border.
Further analysis of the event can be found in this report (in Spanish) from the Instituto Uruguayo de Meteorología (PDF, 397 KB).
Previous case study
Heavy hail in northern Uruguay (21 Sept 2015)