NORFOLK, Va. — Tropical waves are sometimes called "invests" or "investigative areas." These are areas of unsettled weather that we want to monitor for the potential of tropical development in the short term, over the next two to seven days.
You'll see some other letters and numbers that are tacked onto the right side of an Invest.
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We start off hurricane season with "Invest-90L": the number cycles from 90 through 99 with each invest, before repeating. The letter "L" in this case is a basin indicator, signaling that this investigative area is located in the Atlantic basin. Other letters you may see in other basins include "E" for eastern Pacific, or "C" for central Pacific.
I know what you may be asking: "Why don't they use 'A' for a system in the Atlantic basin?" Well, that makes sense, however, the letter "A" is actually used to designate tropical systems in the Arabian Sea.
Tropical waves or "invests" are the first terms used to define tropical threats in all basins across the world. As a system begins to organize and strengthen other terms are used to define it. After a tropical wave or "invest," systems may strengthen enough to be called a tropical depression.
These are areas of low pressure with winds that are sustained below 39 miles per hour. Once winds cross that threshold it becomes a tropical storm, this is where a storm will be named. Further strengthening of winds to over 74 miles per hour gives us a hurricane.
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