The first named tropical storm of the year was called Subtropical Storm Alberto. So why the Sub prefix?
The reason for the distinction has to do with how the system is formed. There is no difference in wind speeds or rainfall totals. Whether it's subtropical or not, it could still have winds up to 73 mph and result in flooding.
A subtropical system is assisted by upper level wind flow, but a tropical system is purely created from warm, ocean water. Upper level winds are flowing air out of the system, so lower level winds rise up to replace that exiting air. This means that warm, moist air from the ocean is being pulled into the center of the system. This helps strengthen the subtropical low pressure system.
In addition, winds in a subtropical low will be farther from the center than with a tropical low. A tropical low pressure system will have the strongest winds near the center.
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