At El Calafate the climate is dry, with only 300 mm. rainfall in a year. But inside the Park, which is located towards the west and where Perito Moreno Glacier is, the situation changes and rainfall easily gets to 1500mm./year.
As a matter of fact, El Calafate is situated at the equivalent latitude of the city of London, Europe.
Several particular conditions give it a much better microclimate than the one of the Patagonic plateau where it is settled: it is located on the coast of Lake Argentino, on a mountainside facing north and protected by a growing tree forest which the local inhabitants take care of.
It has a dry climate and the temperatures range from 20ºC (64.4F) in summer to -3ºC (28.4F) in winter. The length of the day varies a lot according to the time of the year: in summer you can enjoy about 17 hours of daylight and the shortest day in winter has nearly 8 hours of daylight.
In the glaciers area the weather becomes much more wet and the cold coming from the ices sets a difference to the weather of the city.
Keep this info in mind when packing.
Anyway, on the main street there are many shops where you can rent special winter clothing.