A shortwave trough will move through southern South America during the days Monday 18 and Tuesday 19. This system will interact with a very hot, moist and highly unstable air mass already in place and lead to the development of bad weather conditions over central, eastern and northern Argentina, as well as over Uruguay and extreme southern Brazil.

The Argentine provinces of Córdoba, La Pampa, Buenos Aires, Santa Fe and Ente Ríos will register very high temperatures and dew points during Monday, but the approaching of the disturbance from the West will trigger thunderstorms and showers there. Given the very high instability already dominating the area, with Lifted Indices of -6 to -8, an upper level jet blowing at 70/80 knots and the intensification of a low level jet (with maximum velocities of about 35 knots), some if these thunderstorms will likely develop into supercells, with the main threat becoming big hail (1 to 2 inches) and strong wind gusts. One or two isolated tornadoes can not be totally ruled out, especially over Córdoba and late in the afternoon, when the low level jet becomes more intense.

On Tuesday, all this activity will become organized into a mesoscale convective system over eastern and northeastern Argentina, increasing the potential for very heavy rains and storm totals locally surpassing 4 inches over Entre Ríos, Corrientes and northeastern Buenos Aires, as well as over western and northern Uruguay. The development of this system will be accompanied by the formation of a surface low pressure near Buenos Aires City... something very rare for this time of the year. This low will tend to move east and southeast during the day Tuesday. A cold front will then develop in association with this low pressure area and it will tend to affect northeastern Argentina and southern Brazil later on, bringing heavy rains and thunderstorms. A highly unstable environment along with 50 knots at low level will increase the chance for severe weather over that area, including the chance for some isolated tornadoes.