Two articles on the likely positive effects:

1. From Accuweather; August 29, and
2. From the Christian Science Monitor: Could tropical storm Isaac actually help break US drought?

Tropical storm Isaac is bearing down on the Gulf Coast, but once it gets inland, it is expected to bring much needed rain to drought-hit farmlands.

Although tropical storm Isaac is causing evacuations and is expected to lead to power outages when it comes ashore, there may be a silver lining for drought-pressed farmers farther inland.