As air rises, it's ability to hold water vapor reduces. So, moisture laden air at sea level that suddenly bumps into a mountain rises. As it rises, temperature falls and pressure falls causing the air to no longer carry the amount of moisture it did at lower elevations and releases it as clouds and rain. As a result, the windward (upwind) side of mountains tend to be greener than the leeward (downwind). A meteorologist can provide a more detailed explanation but that's the gist of it.
This isn't necessary what has happened to cause the northeastern side to be so much breener than the other sides but that would be my first guess.
