The geography of united states and north america is what gives us unique weather. The rocky mountains create a huge 'divide' in our geography and naturally ends up causing what is known as 'lee troughing' on the eastern side of rockies. Essentially the mountain range promotes low pressure development (storm development) on the eastern side. Which is why the other side of the mountain is so dry. Our side of the general storm system is wet, their side out west of rockies is dry.