Tropical Storm Melissa is barely moving through the Caribbean, and that’s exactly what makes it so dangerous. The longer it lingers, the more rain it dumps. Haiti, Jamaica and the Dominican Republic could face days of relentless downpours and mudslides.

Melissa is expected to significantly strengthen by late Friday and over the weekend, when the storm could become a hurricane as early as Saturday.

When Melissa departs the Caribbean later next week, it may contribute to stormy conditions along much of the United States East Coast with a new round of rough surf and beach erosion in some areas.

Tropical Storm Melissa is expected to meander the Caribbean at an agonizingly slow pace in the coming days, even as it’s forecast to strengthen into a hurricane, covering as little ground as 2 miles in an hour.