If the conditions are favorable, a cluster of thunderstorms can develop a closed circulation.
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If the sustained winds are below 39mph, this is known as a Tropical Depression.
These systems typically form over warm (80-degree) waters of the tropical Atlantic Basin in areas where there are light winds aloft.
Strong winds in the mid and upper levels of the atmosphere tend to tear apart such circulations before they can really get going.
The Tropical Depression can continue to intensify as the circulation becomes better-organized.
If the sustained winds reach 39mph or higher, the depression will become a named Tropical Storm. If the winds reach 74mph or higher, the Tropical Storm becomes a Hurricane.
Tropical Storms and Hurricanes can, of course, cause significant damage, but even if the system just remains as a tropical depression, it can cause flooding rainfall along its path, especially if the system is slow-moving.
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