Tropical Cyclones [Index]

Review of Global Tropical Cyclone Terminology by Gary Padgett

  Revised : August 2009
 
     The following is a synopsis of the operational terminology used by
  the various TCWCs to describe the different stages of tropical cyclone
  development and intensification.   This gives the formal terminology
  used to refer to a given cyclone in warnings, public advices, and
  discussion bulletins.  The adjective "severe" is used in several regions
  with quite different meanings, and these are defined below if the term
  is formally applied to designate an intensity range.  In the U. S., the
  term may be frequently used to describe the character of a hurricane,
  but is not formally used as the descriptor for a given cyclone intensity
  range.

     In the Atlantic and Northeast Pacific basins, the Saffir/Simpson
  Hurricane Wind Scale is utilized to further classify tropical cyclones
  with winds exceeding hurricane intensity.  This is the same as the older
  Saffir/Simpson Scale but with the central pressure and storm surge
  descriptors removed.  Information on the scale may be found at the
  following URL:

  http://www.nhc.noaa.gov/aboutsshs.shtml

     The TCWCs in Australia and Fiji also employ a five-category scale to
  further classify all tropical cyclones above gale force.  The Australian
  Cyclone Severity Scale is based upon the peak gusts expected to occur
  within a cyclone.  Information on this scale may be found at:

  http://www.bom.gov.au/weather/cyclone/about/about-tropical-cyclones.shtml#severity


  1.  ATLANTIC and NORTHEAST PACIFIC Basins - TPC/NHC (also CPHC)
      MSW Averaging Period: 1 minute

      Tropical Cyclone - generic term for systems of all intensities
      Tropical Disturbance - distinct area of disturbed weather but
         usually with no well-defined low-level circulation apparent/
         regular advisories not issued/ usually corresponds to Dvorak
         ratings less than T2.0
      Tropical Depression - fairly well-defined low-level circulation/
         MSW less than 34 kts/ Dvorak rating usually T2.0
      Tropical Storm - MSW in range of 34-63 kts/ Dvorak rating T2.5,
         T3.0, or T3.5
      Hurricane - MSW exceeding 63 kts/ Dvorak rating T4.0 or higher

     The point at which regular advisories are initiated is a little
  subjective.  A system with a Dvorak rating of T1.5 might be upgraded
  to a tropical depression if it were in a position to affect a populated
  area and/or if it seemed to be rapidly intensifying and its development
  potential was considered excellent.

    
  2.  JTWC for NORTHWEST PACIFIC Basin Only
      MSW Averaging Period: 1 minute

      Tropical Cyclone - generic term for systems of all intensities
      Tropical Disturbance - distinct area of disturbed weather but
         usually with no well-defined low-level circulation apparent/
         regular advisories not issued/ usually corresponds to Dvorak
         ratings of T1.0 or less/ MSW generally less than 25 kts
      Tropical Depression - fairly well-defined low-level circulation/
         MSW 25-34 kts/ Dvorak rating usually T1.5 - T2.0
      Tropical Storm - MSW in range of 34-63 kts/ Dvorak rating T2.5,
         T3.0, or T3.5
      Typhoon - MSW exceeding 63 kts/ Dvorak rating T4.0 or higher
      Super typhoon - MSW reaching or exceeding 130 kts


  3.  JTWC for NORTH INDIAN OCEAN and SOUTHERN HEMISPHERE
      MSW Averaging Period: 1 minute

     For the North Indian Ocean and all Southern Hemisphere regions, JTWC
  uses only the generic term "Tropical Cyclone" to refer to systems of all
  intensities in warning status.   Warnings are usually initiated when the
  system is forecast to produce gale/tropical storm force winds within
  48 hours.  In many cases winds are already approaching this threshold
  when the first warning is issued and frequently the initial MSW is set
  at 35 kts.


  4.  NORTHWEST PACIFIC Basin - JMA (Japan) 
      MSW Averaging Period: 10 minutes

      Tropical Cyclone - generic term for systems of all intensities
      Low-pressure Area - distinct area of disturbed weather but
         usually with no well-defined low-level circulation apparent/
         regular warnings not issued
      Tropical Depression - fairly well-defined low-level circulation/
         MSW less than 34 kts
      Tropical Storm - MSW in range of 34-47 kts/ Dvorak rating T2.5
         or T3.0
      Severe Tropical Storm - MSW in range of 48-63 kts/ Dvorak rating
         T3.5 or T4.0
      Typhoon - MSW exceeding 63 kts/ Dvorak rating T4.5 or higher

     The Japanese Meteorological Agency (JMA) is the World Meteorological
  Organization's (WMO) official Regional Specialised Meteorological
  Centre (RSMC) for the Northwest Pacific Basin.   While adhering to
  a 10-min averaging period for MSW, JMA normally equates 34 kts to a
  Dvorak rating of T2.5; thus, JMA and JTWC agree in principle on the
  threshold of tropical storm intensity.   However, for very intense
  typhoons, JMA's MSW estimates are usually far below those assigned
  by JTWC due to a much lower 1-min to 10-min conversion factor for
  extreme intensities.  

     The meteorological services of other Asian nations issue tropical 
  cyclone warnings for portions of the Northwest Pacific region,
  including the Philippines, Hong Kong, China, Vietnam, Taiwan, and
  Korea.   Warnings from these weather services are issued independently
  of JMA but utilize the same terminology and are usually reasonably
  close to JMA's positions and intensity estimates.


  5.  NORTH INDIAN OCEAN Basin - IMD (Indian Meteorological Department)
      MSW Averaging Period: 3 minutes

      Zone of disturbed weather - a zone in which the pressure is low
         relative to the surrounding region and there are convective
         cloud masses which are not organized
      Low-pressure Area - an area enclosed by a closed isobar with mean
         surface winds less than 17 kts
      Depression - well-defined low-level circulation but with MSW
         generally less than 28 kts/ Dvorak rating of T1.5
      Deep Depression - depression with MSW in range of 28-33 kts/ 
         Dvorak rating of T2.0
      Cyclonic Storm - tropical cyclone with MSW in range of 34-47 kts/
         Dvorak rating T2.5 - T3.0
      Severe Cyclonic Storm - tropical cyclone with MSW in range of
         48 - 63 kts/ Dvorak rating of T3.5
      Very Severe Cyclonic Storm - tropical cyclone with MSW in the range
         of 63 kts - 119 kts/ Dvorak rating T4.0 or higher
      Super Cyclonic Storm - tropical cyclone with MSW 120 kts or higher
      
     The Indian Meteorological Department (IMD) is the WMO's RSMC for
  the North Indian Ocean (Bay of Bengal and Arabian Sea), although the
  meteorological services of other nations may also issue warnings for
  portions of the basin.   The MSW values reported in warnings are
  considered to be over a 3-min averaging period; however, IMD does not
  modify the Dvorak scale.


  6.  SOUTHWEST INDIAN OCEAN Basin (West of 90E)
      MSW Averaging Period: 10 minutes

      Zone of Disturbed Weather - term used to describe weak, ill-defined
         systems with winds generally less than 25 kts and corresponding
         to Dvorak ratings of less than T2.0
      Tropical Disturbance - MSW usually 25 kts near center/ Dvorak
         rating usually T2.0 (Beaufort Force 6) / such systems usually
         classified as tropical depressions by most TCWCs
      Tropical Depression - MSW in range of 28-33 kts (Beaufort Force 7 -
         Dvorak T2.5)
      Moderate Tropical Storm - MSW in range of 34-47 kts/ Dvorak rating
         of T3.0 - weak T3.5
      Severe Tropical Storm - MSW in range of 48-63 kts/ Dvorak rating
         of strong T3.5 - T4.0
      Tropical Cyclone - MSW in range of 64-89 kts/ Dvorak rating of
         T4.5 - T5.5
      Intense Tropical Cyclone - MSW in range of 90-115 kts/ Dvorak
         rating T6.0 - T6.5
      Very Intense Tropical Cyclone - MSW exceeding 115 kts/ Dvorak
         rating T7.0 - T8.0

     The WMO's RSMC for the South Indian region is Meteo France on the 
  French island of La Reunion; however, names are actually assigned by the
  Sub-regional Tropical Cyclone Advisory Centres on Mauritius (east of 55E)
  and Madagascar (west of 55E).   The La Reunion TCWC employs a conversion
  factor of 0.88 to convert the 1-minute MSW Dvorak scale to an equivalent
  10-minute average scale.  (Prior to the 1999-2000 season, the conversion
  factor used was 0.80.)


  7.  AUSTRALIAN REGION (longitude 90E eastward to longitude 160E)
      MSW Averaging Period: 10 minutes

      Tropical LOW - term is used to describe disturbances ranging from
         diffuse, ill-defined low-pressure areas all the way to well-
         organized tropical depressions with MSW up to 33 kts
      Tropical Cyclone - MSW in range of 34-63 kts/ Dvorak rating ranging
         from a strong T2.5/weak T3.0 to T4.0
      Severe Tropical Cyclone - MSW exceeding 63 kts/ Dvorak T4.5 or
         higher

     Warnings in the Australian Region are issued by three separate TCWCs
  at Brisbane (Queensland), Perth (Western Australia), and Darwin
  (Northern Territory) with Darwin being the RSMC for the region.  In
  addition to these, a TCWC at Port Moresby, Papua New Guinea (formerly
  an Australian territory) issues warnings for a small portion of the
  region near and east of the island of New Guinea, and a TCWC at Jakarta,
  Indonesia, issues warnings for the region west of 125E and north of
  10S.   The Papua New Guinea and Indonesian regions have an extremely low
  incidence of tropical cyclone occurrences.   

     The Australian centres avoid use of the term "tropical depression" in
  public advices primarily to reduce possible confusion with the use of 
  the term "depression" in association with extratropical systems; and 
  also possibly because until recently (early 1990's), in the Southwest 
  Indian Ocean Basin, a "tropical depression" meant any system with winds 
  up to 63 kts (hurricane force).     The Australian TCWCs utilize a 
  conversion factor of 0.88 or 0.90 to modify the 1-minute Dvorak scale to
  an equivalent 10-minute average scale.

     In the Australian Region to qualify as a tropical cyclone a tropical
  LOW must be accompanied by gales surrounding more than 50% of the center
  of circulation for a period of at least six hours.   In order to insure
  that adequate warnings are provided, it is not at all unusual for a
  system to be named as a tropical cyclone, but later reduced to tropical
  LOW status after a careful post-storm analysis reveals that this spatial
  distribution of gales criterion was not met, e.g., Isobel and Odette,
  2007, and Gabrielle, 2009.


  8.  SOUTH PACIFIC Basin (east of longitude 160E)
      MSW Averaging Period: 10 minutes

      Tropical Disturbance - distinct area of disturbed weather but
         usually with no well-defined low-level circulation apparent/
         regular advisories not issued/ usually corresponds to Dvorak
         ratings less than T2.0
      Tropical Depression - fairly well-defined low-level circulation/
         MSW less than 34 kts/ Dvorak rating usually T2.0 or weak T2.5
      Tropical Cyclone - MSW exceeding 33 kts/ Dvorak rating of
         strong T2.5/T3.0 to T4.0
      Severe Tropical Cyclone - MSW exceeding 63 kts/ Dvorak T4.5 or
         higher

     The TCWC at Nadi, Fiji, is the RSMC for the South Pacific basin
  but the meteorological service of New Zealand (Wellington) issues
  warnings for systems that move (or very rarely form) south of latitude
  25S.    The Fiji TCWC employs a 1-minute to 10-minute MSW conversion
  factor of 0.90 when adjusting the Dvorak scale.

     A tropical system can have associated gales and still be classified
  as a tropical depression if the gales are well-removed from the center.
  In such cases the gales are usually found in only one or two quadrants. 
  Prior to 2000, the WMO Region V definition of a tropical cyclone required
  that gales surround the center, but the definition was changed to allow 
  classification as a tropical cyclone if gales were present near the 
  center and likely to persist, even if in only one quadrant.