The storm has not become a hurricane, and none of the previous tropical storms this season have become hurricanes.
Gabrielle formed around August 25 off the African coast and has moved through the deep tropics since.
An Air Force reconnaissance plane reached it Wednesday afternoon and found a closed surface circulation. The storm was upgraded to Tropical Depression 7. By Wednesday evening, it was upgraded to tropical storm status and named Gabrielle.
Tropical storm warnings are in effect for all of Puerto Rico and the northeast coast of the Dominican Republic, as Gabrielle moves over the center of eastern Puerto Rico with estimated 40 m.p.h. winds.
"Gabrielle is a slow-moving tropical cyclone that is expected to bring strong winds and heavy rain to those in its path," the National Weather Service reported.
The National Hurricane Center predicts a gradual intensification over the next five days as it heads into the Atlantic, with a possible brush with Bermuda.