I haven't been north of Piriapolis on that side in small boats... however I do know that intense lows can form offshore
just north of the Uruguay/Brazil border at very short notice. I know of two Argentino yachts lost
with all hands in that area in the last 5 years.
The River Plate can also serve up some very severe weather
... I remember it mainly for very bad lightning events
in the Delta
South of the Plate? I've been up twice, down twice.
First northbound was from Punta Arenas via the Narrows in May. ( DO NOT try and enter from the east and get up to Punta Arenas via the Narrows.. it will end in tears!)
A pleasant sail with westerlies until about 36 hours where we were hammered by a very short lived NW gale. The only time I have ever had heavy water
in the cockpit
...also lost solar panels
and pretty much everything else down the back... we were hove to at the time.
Mar del Plata... nice spot but a nasty entrance if there is any swell from N through to SE.
Daylight entry suggested.
Next voyage was from BA down to the Puerto Deseado area and back in April the following year. It was late in the season but we had been mucked around no end while trying to overhaul
in BA. A good run except for a major thunder and lightning
Blanca.... something must happen to the air as it comes over the Andes I reckon.
Ended up hove to offshore from Deseado for a few days with SW'lys and sleet.... forecast
offered more of the same but windier south of there for the next week so we turned tail and ran back to MdP.. the 14 day voyage to nowhere.
November that year sailed down from Uruguay to the Falklands, dream run in light westerlies apart from the last 12 hours which was windy... nothing more.
Following November .. Falklands via Le Maire to Pto Williams ... motored most of the way in a glassy calm, 12 hours sailing, 24 hours motor
Your mileage may vary.