Patagonia weather is difficult to predict, and even any Internet forecast service can be erroneous. It's important to know, especially for this region, the further out a forecast predicts the more it plays with pure chance. These forecasts will vary almost always between sunny, partly cloudy, drizzle and rain. Wind alone rarely is a factor especially compared to southern Patagonia. See the When to go page.
Comparing the weather to other years. If you compare, for example, the month of January to its previous year, it could have completely different characteristics, other times similar. Some Januarys are completely dry, others very rainy and many somewhere between.
Getting the forecast once in the valley. If you're staying at the Camping La Junta or Refugio Cochamó, you can get updated forecasts there posted on their information board.
The local way. While in the valley locals follow one general rule that helps forecast weather. Clouds moving in a somewhat north-to-south direction means rain is coming. While south-to-north or directly west-to-east winds or cloud movement means stable weather will hold.