There’s a lot of different terms we’ll be using in this year’s discussion regarding the upcoming hurricane season, so we would like to clarify a few of them. Some of these definitions below come right from the National Hurricane Center’s glossary.
The National Hurricane Center generally issues an advisory on tropical cyclones every six hours, or every three hours if there are any watches are warnings in effect for the region. According to the National Hurricane Center, advisories contain “official information issued by tropical cyclone warning centers describing all tropical cyclone watches and warnings in effect along with details concerning tropical cyclone locations, intensity and movement, and precautions that should be taken. Advisories are also issued to describe: (a) tropical cyclones prior to issuance of watches and warnings and (b) subtropical cyclones.”
The roughly circular area of comparatively light winds that encompasses the center of a severe tropical cyclone. The eye is either completely or partially surrounded by the eyewall cloud.
An organized band or ring of cumulonimbus clouds that surround the eye of the storm. Eyewall and wall cloud are used synonymously.
A tropical cyclone in which the maximum sustained surface wind (using the U.S. 1-minute average) is 74 mph or greater. The term hurricane is used for Northern Hemisphere tropical cyclones east of the International Dateline to the Greenwich Meridian. The term typhoon is used for Pacific tropical cyclones north of the Equator west of the International Dateline.
An invest is an area of disturbed weather that the National Hurricane Center is closely tracking. It is not a tropical cyclone, but an area of thunderstorms that have a chance of organizing into a tropical cyclone. Once an invest is designated, this area of thunderstorms gets its own satellite sector and ‘spaghetti’ models.
The intersection of the surface center of a tropical cyclone with a coastline. Because the strongest winds in a tropical cyclone are not located precisely at the center, it is possible for a cyclone’s strongest winds to be experienced over land even if landfall does not occur.
An increase in the maximum sustained winds of a tropical cyclone of at least 35 mph in a 24-hour period.
An abnormal rise in sea level accompanying a hurricane or other intense storm, and whose height is the difference between the observed level of the sea surface and the level that would have occurred in the absence of the cyclone. Storm surge is usually estimated by subtracting the normal or astronomic high tide from the observed storm tide.
A non-frontal low-pressure system that has characteristics of both tropical and extratropical cyclones. Like tropical cyclones, they are non-frontal, synoptic-scale cyclones that originate over tropical or subtropical waters, and have a closed surface wind circulation around a well-defined center. In addition, they have well-organized moderate to deep convection, but lack a central dense overcast. Unlike tropical cyclones, subtropical cyclones derive a significant proportion of their energy from baroclinic sources. They are generally cold-core in the upper troposphere, often being associated with an upper-level low or trough. In comparison to tropical cyclones, these systems usually have a radius of maximum winds occurring relatively far from the center (usually greater than 60 nautical miles), and have a less symmetric wind field and distribution of convection.
The term “cyclone” can sometimes be confusing. It is used whenever we discuss a tropical cyclone, but it is also a more broad term that can cover tropical depressions, storms, or hurricanes. A tropical cyclone is a warm-core, non-frontal, synoptic-scale (large) cyclone, originating over tropical or subtropical waters, with organized deep convection and a closed surface wind circulation around a well-defined center. Once formed, a tropical cyclone is maintained by the extraction of heat energy from the ocean at high temperature and heat export at the low temperatures of the upper troposphere. In this they differ from extratropical cyclones, which derive their energy from horizontal temperature contrasts in the atmosphere (baroclinic effects).
A trough or cyclonic curvature in the trade-wind easterlies. The wave may reach maximum amplitude in the lower middle troposphere.