Jacksonville, Fla. – The tropics have been busy, with the National Hurricane Center (NHC) designating storms with x’s on their Atlantic Tropical Cyclone Disturbance map. On Sept. 17, Tropical Storm Helene first appeared as a tropical disturbance and was marked Sept. 22 as “Investment 97.” Soon thereafter, it became a Tropical Depression and was a Tropical Storm as of Tuesday afternoon. According to the NHC, Tropical Hurricane Helene is forecasted to be a major hurricane.
Weather forecasting has changed throughout the decades.
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One of the changes involves the National Hurricane Center. It has recently introduced new terms for different storm categories, so let’s break down what each of the terms means.
What is an Invest?
An invest — short for “investigation” — is a naming convention used by the National Hurricane Center (NHC) to identify features they are monitoring for potential future development in a tropical depression or storm. However, if the system has been called an invest does not guarantee development into a tropical depression or storm, according to The Weather Channel.
Potential Tropical Cyclone (PTC)
Since 2017, NHC has had the option to issue advisories, watches, and warnings for disturbances that are not yet a tropical cyclone but which pose the threat of bringing tropical storm or hurricane conditions to land areas within 48 hours. Under previous longstanding NWS policy, it has not been permitted to issue a hurricane or tropical storm watch or warning until after a tropical cyclone had formed, according to the National Weather Service.
Tropical Disturbance
A tropical weather system with organized convection (generally 100-300 miles in diameter) originating in the tropics or subtropics, having a non-frontal migratory character and maintaining its identity for 24 hours or longer. It may or may not be associated with a detectable perturbation of the wind field.
Tropical Wave
An inverted trough (an elongated area of relatively low pressure) or cyclonic curvature maximum moving east to west across the tropics. These can lead to the formation of a tropical cyclone. Also known as an easterly wave.
Tropical Depression
A tropical cyclone that has maximum sustained surface winds (one-minute average) of 38 mph (33 knots) or less.
Tropical Storm
A tropical cyclone that has maximum sustained surface winds ranging from 39-73 mph (34 to 63 knots).
Hurricane
A hurricane is a tropical cyclone that has maximum sustained surface winds of 74 mph or greater (64 knots or greater).
Major Hurricane
A hurricane that is classified as Category 3 or higher.