Hurricane Season Terms You Need To Know

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In the midst of hurricane season it's easy to be overwhelmed by the jargon meteorologists use.

We've put together this guide to explain the most common terms and also those used more rarely.

Types of Storms

Tropical cyclone - The catch-all name for any tropical depression, storm, hurricane, typhoon (what hurricanes are called in the Northwest Pacific basin) or cyclone (what hurricanes are called in the South Pacific and Indian Ocean basins). These systems with well-organized thunderstorms around a closed low-level center derive their energy from warm ocean water.

Tropical depression - This tropical cyclone has maximum sustained winds of 38 mph or less.

Tropical storm - When maximum sustained winds reach 39 mph, the system is called a tropical storm, and gets the next name in the official name list for that basin in that year. A tropical storm can have maximum sustained winds of up to 73 mph.

Hurricane - The term for tropical cyclones in either the Atlantic or Eastern Pacific basins which have maximum sustained winds of at least 74 mph.

Major hurricane - These are hurricanes with maximum sustained winds of at least 111 mph, at least Category 3 intensity on the Saffir-Simpson Hurricane Wind Scale. The term "major" does not imply hurricanes of a lower wind intensity, or even tropical storm or tropical depressions, can't have major impacts from storm surge, rainfall flooding, or winds.

Saffir-Simpson Hurricane Wind Scale - A method developed in the early 1970s to classify hurricanes based on their maximum sustained winds from Category 1 to 5. This scale does not take into consideration the hurricane's size - which can influence not only how large its wind field is, but its potential to produce storm surge flooding - nor its rainfall potential when striking land.

Potential tropical cyclone - This term, introduced by the National Hurricane Center in 2017, allows the NHC to issue tropical storm or hurricane watches or warnings for a system that has not yet developed, but is expected to impact land areas within 48 hours.

Subtropical depression or storm - This hybrid type of system has features of both tropical and non-tropical cyclones. Like tropical cyclones, they have a well-defined surface low with thunderstorms near the center. Unlike tropical cyclones, they get a significant fraction of their energy from contrasts between cold and warm air, and their strongest winds typically occur much farther from their center. The NHC issues forecasts for subtropical depressions and storms like their tropical counterparts. If sufficient thunderstorms build closer the center, these subtropical systems can warm enough to become tropical cyclones.

Post-tropical cyclone - Put simply, it's a low-pressure system that used to be a tropical cyclone. It can still contain strong winds and heavy rain, but it no longer has the characteristics of a tropical cyclone. This is a common occurrence in the North Atlantic Ocean east or southeast of the Canadian Maritimes after a former storm gets caught by the jet stream and loses its tropical nature.

Extratropical cyclone - This is a low-pressure system that is not a tropical cyclone, a type of storm system that is associated with warm and cold fronts, deriving its energy from

Watches and Warnings

Hurricane or tropical storm watch - This means hurricane (sustained winds of 74 mph or higher) or tropical storm (39 to 73 mph sustained winds) conditions are possible within the watch area, generally within 48 hours. Hurricane watches are typically issued by the NHC 48 hours in advance of the onset of tropical storm-force winds, which can make final preparations hazardous.

Hurricane or tropical storm warning - This means hurricane (sustained winds of 74 mph or higher) or tropical storm (39 to 73 mph sustained winds) conditions are expected within the warning area, generally within 36 hours. Hurricane warnings are generally issued by the NHC 36 hours in advance of the onset of tropical storm-force winds, which can make final preparations hazardous.

Storm surge watch - A life-threatening water rise from storm surge is possible in the area, generally within 48 hours. Areas in storm surge watches may be under at least voluntary evacuations by local emergency managers.

Storm surge warning - A life-threatening water rise from storm surge is expected in the area, generally within 36 hours. Areas in storm surge warnings may be under mandatory evacuation orders by local emergency managers.

Extreme wind warning - A rare warning issued by the National Weather Service to warn of the onset of at least Category 3 hurricane winds (at least 115 mph), typically in a hurricane's eyewall, capable of wind damage typical of stronger tornadoes.

Other Terms

Center - This is as it sounds, the center of the storm's circulation with the lowest pressure and lowest wind. In some weaker tropical storms or depressions, this can be challenging to locate in satellite imagery. In these cases, Hurricane Hunter reconnaissance aircraft can help locate the center, which they call a center fix or vortex fix. In stronger hurricanes such as 2018's Hurricane Michael shown above, the center is easily recognizable in satellite and radar imagery. The center is the position you will see in advisories from the NHC and other agencies both for the storm's current position and forecast track.

Eye - A circular area of lighter winds enveloping the center of intense tropical cyclones. The eye can be either cloud-free and very distinct or at least partially covered by high clouds. Those experiencing a hurricane on the ground sometimes report sunny skies within a hurricane's eye.

Eyewall - A ring of the most intense convection immediately surrounding the eye, in which the hurricane's strongest winds occur. At times, the eyewall may not be one complete ring of convection, but may have parts of it eroded by drier air. You can see the eyewall of Hurricane Michael from 2018 in the radar image shown earlier under the "extreme wind warning" section denoted by the orange and red ring surrounding the eye.

Eyewall replacement cycle - A process seen primarily in more intense tropical cyclones during which outer rainbands organize into a large ring of thunderstorms that contracts inward, choking off the inflow to the original, smaller eyewall, and eventually replacing it. The tropical cyclone tends to weaken a bit during the formation of the second outer eyewall, but then intensifies again as the new outer eyewall contracts inward. The net effect of this process is usually a larger wind field. Some intense hurricanes undergo multiple eyewall replacements.

Hurricane Hunters - These are reconnaissance missions requested by the NHC and flown by the U.S. Air Force Reserve and NOAA to gather data within an active tropical cyclone, investigate a disturbance to see if it is a tropical depression or storm, or gather data in the atmosphere surrounding a tropical cyclone to assist computer model forecasts of the storm.

Invest - Short for "investigation", this is a tropical system an agency such as the NHC is interested in for possible development. Once designated an "invest", special datasets and computer model guidance begin to be collected on that system. The naming convention for invests is a number from 90 to 99 followed by a letter, such as "L" for the Atlantic Basin or "E" for the Eastern Pacific Basin.

Landfall - The location where the center of a tropical cyclone first crosses a coastline. Not all tropical cyclones have to make landfall to impact the coast. Their strongest winds could scrape the coast while the center remains just offshore.

Rapid intensification - A period in which a tropical cyclone's maximum sustained winds increase by at least 35 mph in 24 hours or less.

Remnant - This can refer to either a post-tropical cyclone with winds less than 39 mph without enough convection to be deemed a tropical cyclone, or is sometimes used with the upper-level spin or moisture left over from a previous tropical cyclone.

Tropical wave - Also known as African easterly waves, these are areas of low pressure, without a closed circulation, that are generated over Africa and migrate westward across the Atlantic Basin. According to the National Hurricane Center, 60 tropical waves track across the Atlantic Ocean each year. Roughly one in five of these tropical waves becomes an Atlantic Basin tropical cyclone, and a few of these waves become tropical storms or hurricanes in the eastern Pacific.

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