Based on the map, which region of the united states has the driest climate? map showing the average annual precipitation through the united states from 1961 to 1990. states east of the mississippi river and in the midwest are primarily colored green and light blue; according to the legend, these regions receive between 30 to 70 inches of precipitation per year. regions in the rocky mountains and the southwest are primarily colored orange and red; according to the legent, these regions receive between 5 to 20 inches of precipitation per year. regions in the pacific northwest are primarily colored dark green and blue; according to the legend, these regions receive between 50 to 140 inches of precipitation per year. © 2010 noaa. gov northeast northwest southeast southwest