Flash flooding feared in already hurricane-ravaged Puerto Rico

— -- Hurricane-ravaged Puerto Rico is expecting heavy rain and flash flooding on Saturday as critical relief efforts continue on the island.

A tropical wave formed near the Lesser Antilles, and is expected to drift west over the next few days. Conditions are not favorable for development into a tropical storm, but locally heavy rainfall is expected over the Leeward Islands, including Dominica, and the U.S. Virgin Islands and Puerto Rico.

All of the islands were extremely hard hit by Hurricane Maria over a week ago, and all are struggling to recover from the blow.

The rain is typical in the tropics, but due to the hurricane, recovery efforts could be compromised.

Additional heavy rain will cause more stress on dams, more likelihood of river flooding, and more likelihood of street flooding. The Guajataca Dam in Puerto Rico continues to be under risk of failure, which could cause life-threatening flash flooding downstream on the Rio Guajataca.

A flash flood warning was in effect for Vieques, a separate island which is part of Puerto Rico, Friday night and watches were in effect for Puerto Rico and the U.S. Virgin Islands through Sunday.

Recovery efforts continued on Friday in Puerto Rico, which is still largely without power and in short supply of drinking water, medical supplies and food.

There are also 9,000 cargo containers with supplies at a port in San Juan but have yet to be distributed to those in need.

"The whole supply chain of distribution has been interrupted," Jose Ayala, vice president of Crowley Puerto Rico Services, told ABC News. "We need to restore the electricity power, we need to restore our roads, we need to restore communications, we need to start moving the diesel around the island."

The Federal Emergency Management Agency told ABC News none of their trailers were being held and they were distributing aid immediately.

ABC News' Morgan Winsor and Daniel Manzo contributed to this report.