Tropical Storm Isaac developed over the ocean, becoming the ninth named storm of the hurricane season. The storm formed rapidly from a tropical disturbance.

"A tropical disturbance is called a tropical depression whenever the winds are between 25 and 38 mph," says Annalisa Bracco, a professor in the School of Earth and Atmospheric Sciences. "When the wind speeds are between 39 and 73 mph it is classified as a tropical storm. Above 74 mph it is called a hurricane or typhoon." 

Hurricane Helene formed from an area of low pressure in the Caribbean and strengthened rapidly over a few days.

"Two main conditions behind intensification are warm surface waters (the cyclone can extract energy from it and the warmer the water, the more energy it can extract to grow), and low wind shear (high wind shear tends to disrupt vortices, both cyclones and anticyclones)," says Bracco.