Tropical Storm Dorian was swirling in the Caribbean Sea on Wednesday, cruising slowly north as it threatened to hit Puerto Rico within hours and possibly to strike Florida's Atlantic coast by Monday.

Dorian marks the second storm of the 2019 hurricane season to threaten mainland U.S. shores, after Hurricane Barry drenched New Orleans and the Gulf Coast on July 13, causing extensive flooding and road closures.

A blow to Puerto Rico would come as a particularly cruel twist, coming just two years after Hurricane Maria devastated the U.S. island territory in September 2017, causing thousands of deaths, running up tens of billions of dollars in damage, and hobbling logistics operations for months.

In preparation for the impact of Dorian, Port San Juan was closed on Wednesday by the U.S. Coast Guard for all freight and cruise ship operations. By setting "port condition zulu" for all maritime ports in the U.S. Virgin Islands and Puerto Rico, the Coast Guard has closed the region to all commercial inbound and outbound vessel traffic until the storm has cleared the area. Neither Florida's Port Miami nor the Port of Jacksonville had yet set similar restrictions at mid-day on Wednesday.

Humanitarian relief organization the American Logistics Aid Network (ALAN) was also laying the groundwork for a recovery effort on Wednesday, asking supply chain professionals to stand ready to donate transportation, space, services, and equipment. "The strength and path projections for Dorian are still very uncertain, but do suggest that it could be a destructive and dangerous event for Florida and potentially all of the Southeast and Gulf Coast," ALAN Executive Director Kathy Fulton said in a release.

"During our 14 years of operation, we've seen some potentially catastrophic hurricanes that have turned into relatively minor events while others have morphed into far more major events than originally anticipated," Fulton said. "Obviously, we hope Dorian will turn out to be the former. However, if it isn't, we want people to remember that we are here to assist--and that when it comes to these storms, there's no such thing as too ready."

The barrage of storms swamping U.S. ports in recent years has also gotten the attention of the American Association of Port Authorities (AAPA), which said today that its annual convention would include a new track titled "Coastal Resiliency." The session is intended to improve the ability of ports and coastal communities to prepare for, withstand, and bounce back from the impacts of severe weather events and sea-level rise, according to a preview of the group's 108th Annual Convention and Expo, scheduled to convene Oct. 13 in Norfolk, Va.

Editor's note: This story was revised on Aug. 28 to include more information from ALAN.

Islands that are in the path of #TropicalStormDorian, please heed this important safety message from FEMA: "Storm tracks can change quickly. Residents should monitor forecasts and listen to officials, be prepared to take shelter if ordered to do so." pic.twitter.com/Xpx6QKjOmi

— ALANAid (@ALANaid) August 28, 2019