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(THE CONVERSATION) A few large weather systems make all the difference between a wet and a dry year in the Southwest. Coming during the winter and spring, they account for the bulk of the rain and snow the regions receives.

My colleagues and I were intrigued by the relationship between weather systems and rain and snowfall in the contiguous United States. We used historic weather data from the period 1979 to 2014 to find a representative set of weather types.

We found that during t...