Tropical Weather 101: What’s the difference between a “tropical disturbance”, potential tropical cyclone, tropical depression and a tropical storm?

Source: Chris Landsea, National Hurricane Center, NOAA

Source: Chris Landsea, National Hurricane Center, NOAA

Throughout the summer and into fall, you may hear meteorologists use phrases that sound like they mean the same thing to describe systems developing over the ocean. These phrases may include

  • Tropical Cyclone
  • Tropical Disturbance
  • Tropical Depression
  • Tropical Storm

While they all start with the word “tropical,” the phrases are distinctly different from each other.

Per the Atlantic Oceanographic and Meteorological Laboratory (AOML), a branch of NOAA within the Hurricane Research Division, the phrase “tropical cyclone” is a general term for a low pressure system that develops over the tropics and exhibits a definitely closed  center of circulation within the surface winds. The winds of the system swirl cyclonically, or in the counter-clockwise direction, which is why it’s called a tropical cyclone.

Keep in mind that the phrase “tropical cyclone” can describe tropical depressions, tropical storms, and even hurricanes. However, tropical disturbances are not  included because disturbances often lack a closed center of circulation.

What is a tropical disturbance?

According to the AOML, a tropical disturbance is defined as “a discrete tropical weather system of apparently organized convection – generally 200 to 600 km (100 to 300 nautical miles) in diameter – originating in the tropics or subtropics, having a non-frontal migratory character, and maintaining its identity for 24 hours or more. It may or may not be associated with a detectable perturbation of the wind field. Disturbances associated with perturbations in the wind field and progressing through the tropics from east to west are also known as easterly waves .”

Essentially, tropical disturbances can be described as a disorganized cluster of thunderstorms that form in the tropics. Often they do not cause much concern with the exception of localized heavy rain and flooding in the event that they move onshore. If a tropical disturbance is able to become more organized and show signs of low-level spin, then it may strengthen and develop into a tropical depression.

What is a potential tropical cyclone (PTC)?

All low pressure systems are cyclones (because the winds rotate around the low in the counter-clockwise direction; the term “cyclone” has been used to describe low pressure systems since the 1800s). In that sense, all of the following are “cyclones”: tropical depressions, tropical storms, hurricanes, typhoons, etc.

In the case of the tropics, the National Hurricane Center began issuing advisories in 2018 for tropical systems that have yet to truly form and meet the criteria to be classified as a depression/storm/hurricane, BUT have the potential to become one and produce impacts within 24-48 hours.

However, it has the potential to quickly strengthen and produce wind/flood/surge impacts, which is why the NHC has issued advisories for the Potential Tropical Cyclone.

What is a tropical depression?

A tropical depression is a tropical cyclone in which the maximum sustained wind speed measured is up to 39 mph.

Tropical depressions exhibit a closed center of circulation, but they are too unorganized to produce wind speeds stronger than 39 mph. If a tropical depression is able to become more organized and produce sustained wind speeds in excess of 39 mph, it would be upgraded to a tropical storm and it would then be named.

What is a tropical storm?

A tropical storm is a tropical cyclone in which the maximum sustained wind speed exceeds 39 mph but is less than 74 mph.

Stronger, more organized tropical storms can produce outer rain bands that can lash nearby islands and coast lines. Once they move onshore, tropical storms can produce very heavy rain and flash flooding, as well as gusty winds that can damage trees, powerlines, and some roofs and windows.

If a tropical storm were to strengthen to the point where it produced wind speeds of 74 mph or greater, it would be upgraded to a hurricane and maintain its name.

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