Why do Cfa (humid subtropical) climates along the east coast receive rain all year, but at the same latitude along the west coast, the Cs climates have dry summers?

The poleward shift of the STH in summer brings dry conditions.

Explanation:

Research has shown that there is a constant decline in average daily rainfall along the poleward latitude of the climate of the West Coast.

The constant lower amount of rainfall along the poleward latitude of the West Coast is attributed to the lower troposphere over the tropical Atlantic.

These changes in the amount of moisture in the region make the climates along the west coast Mediterranean, have dry summer.