This tells us the overall amount of precipitation will increase. But that has nothing to do with the relative proportion of rain vs. snow, which we already know about. The premise isn’t about amount, it’s about proportion of rain vs. snow.

This give us examples of what the author predicts in the conclusion. The fact (B) is about other mountainous regions doesn’t make it irrelevant. It’s still evidence that after a warmer winter, snowpacks will melt faster/earlier, leading to more flooding and less storable water.

This doesn’t help establish that mountain snowpacks will melt faster and earlier. It also doesn’t involve the same region after a warmer than usual winter. Contrast that with (B), which does involve a comparison within the same region after a warmer winter.

This concerns regions of the world with the “mildest” winters. But we’re interested in what happens to a region when it has a milder than normal winter. This isn’t necessarily the “mildest.” (D) also doesn’t help establish snowpacks will melt faster and earlier.

This creates a relationship between the size of a snowpack and the amount of flooding. But the premises don’t suggest the snowpack will be larger. The author predicts that the snowpack will melt faster and earlier because of the precipitation’s proportion of rain vs. snow.