We haven't had any significant rain in Buenos Aires for nearly two months and the cause has been a blocking area of high pressure in the Atlantic to the east of the River Plate. As you know, a southern hemisphere high rotates air anticlockwise, so the air has been coming from the north. It's also true that highs tend to lurk for a long time, preventing cold fronts from bring cooler air and rain.
Here's a synoptic chart with the green arrow showing the high finally shifting NNE and the blue arrows indicating the direction of cold fronts from the SW.
There could be thunderstorms on Thursday night with more extensive rain and thunderstorms on Friday, beginning late morning.