Excerpt from Spatial and Seasonal Precipitation Distribution in Southwest Idaho Located some 480 km from the Pacific Ocean, Idaho's major moisture source is maritime air from prevailing westerly winds. The westerly winds are able to carry more moisture into northern Idaho through the Columbia River Gorge than into southern Idaho, because of the mountain ranges to the west. During the summer, some moisture is brought in from the Gulf of Mexico at high levels, which produces thunderstorms, particularly in the eastern part of the State. Precipitation ranges widely in Idaho, with large areas of the northeastern valleys, Snake River plains, and southwestern valleys, such as lower Reynolds Creek, receiving less than 250 mm annually and some mountainous areas receiving over 1500 mm annually (1, 6.