The State Department policy, updated in an embassy memo last August, says men and women should dress conservatively. Women, it says, should wear long dresses and avoid trousers.
It adds, "Embassy women are not expected to wear abayas when out on official business. ... The guidance issued by the Saudi Embassy in Washington states that non-Muslim women are not required to wear an abaya but should dress conservatively" when in public, including loose-fitting dresses draped well below the knees with long sleeves and a high neckline.
The memo notes that some Western women, especially those living in the more conservative areas such as Riyadh, wear an abaya "in order to avoid harassment" by the mutawa. Even so, "harassment still occurs," the memo says.
Of course...bold added to highlight the appropriate section.