Thunderstorms in Uruguay on 21 September brought around 80 mm of rain in two hours and large hailstones.
22 October 2020
21 September 2015
By Nestor Santayana (INUMET ) and Jose Prieto (EUMETSAT)
On 21 September 2015, a quasi-stationary front which was blocked over Brazil, reached northern Uruguay from the ocean. Short wave perturbations were triggered at mid level in the atmosphere on the divergence zone of the 250 hPa jet stream.
The Amazon Rainforest (Amazonia) was the main supplier of the humidity, with the east winds from the ocean at very low level also adding some more humidity. Strong instability and vertical shear completed the conditions for this severe precipitation episode.
Thunderstorms started at 09:00 UTC on 21 September in north west Uruguay. At around 11:00 UTC they reached their intensity peak with almost 80 mm recorded in two hours, accompanied by damaging hailstones of 1 to 5 cm diameter (credit: Lauri Becerra), and a lot of electrical storm (lightning) activity.
The GOES-13 animated gif (centred around 0.6 µ), from 11:00–14:45 UTC (Figure 1), provides the highest spatial detail with a resolution of 1 km at the sub-satellite point. There, the stretched shade of the storm anvil reached 50 km, indicating strong vertical convection.
On this full resolution image from 11:38 UTC at least six cells can be identified through their overshooting tops, above-anvil-level cirrus, flanking line for feeding from the west, and numerous gravity waves, all signs of weather severity and strong hail close to the ground.
After 13:00 UTC the systems dissolved, as can be seen on the animated gif.
Meteosat-10 Natural Colour RGB imagery based on solar imagery (Figure 2 and MP4), covering the same time period, fails to grab the horizontal details at this far position on its field of view, similar in resolution to the Baltic States in Europe.
However, it indicates the icy nature of the cloud tops, displaced by parallax and hardly showing any shades, since both Meteosat and the sun are located to the east of the scene.
The best horizontal resolution of Meteosat-10, the HRV and microphysics channels composite imagery (Figure 3 and MP4) retrieves part of the cloud top details offered by GOES-13.
Only images after 17:00 UTC are generated for this area, when the convection was already subsiding. The Meteosat HRV, coloured by other channels, continues where the GOES-13 visible loop stops.
The radar images from 06:20 and 08:40 local time (09:20 and 11:40 UTC) (Figure 4), indicate the location of the most intense precipitation for the times given.
The earlier image (right, top image) gave early warnings on the development of severe hail. The system in the centre of the radar images (Salto) developed further, as seen by the satellite images.
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