Why do the mountainous northeastern areas of the Dominican Republic receive more rainfall than the southwestern valleys?

The mountainous northeastern areas of the Dominican Republic receive significantly more rainfall than the southwestern valleys due to the effects of orographic precipitation.

The northeast trade winds, which blow steadily from the Atlantic Ocean throughout the year, carry moisture across the country. As these winds encounter the island's mountain ranges, the air is forced upward. This rising air cools, causing moisture to condense and fall as heavy precipitation on the windward, northeastern slopes, where annual rainfall often exceeds 100 inches (2,540 mm).12

By the time these winds pass over the mountains and reach the far western and southwestern valleys, they have lost most of their moisture. Consequently, these regions lie in a rain shadow, resulting in an arid climate with less than 30 inches (760 mm) of annual precipitation.12