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Joint Typhoon Warning Center (JTWC)

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Q: What is the difference between my local time and the time on JTWC's products?

A: The time referenced in all JTWC products is called Coordinated Universal Time (UTC). The U.S. military refers to UTC time as "Zulu" (Z) time. The use of UTC time is standard practice for most weather organizations as recommended by the World Meteorological Organization (WMO), an agency of the United Nations. To convert from Z to your local time, go to
this site .

(Note: clicking on this link will redirect your browser to a website unaffiliated with JTWC).

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Q: What do "Low", "Medium", and "High" mean on the ABIO and ABPW messages?

A: "Low" formation potential describes a tropical disturbance that is being monitored for development, but is unlikely to develop into a significant tropical cyclone within the next 24 hours.

"Medium" formation potential describes a tropical disturbance that is being monitored for development and has an elevated potential to develop, but development of a significant tropical cyclone will likely occur beyond 24 hours.

"High" formation potential describes an area that is being monitored for development and is either expected to develop within 24 hours or development has already started, but warning criteria have not yet been met. All areas designated as "High" will be accompanied by a Tropical Cyclone Formation Alert.

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Q: What is a "Tropical Cyclone Formation Alert (TCFA)"?

A: A TCFA designates a disturbance that is likely to become the subject of a JTWC tropical cyclone warning within the following 24 hour period. This information is provided to the DoD and other U.S. government agencies for operational planning.

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Q: Why are some TCFA areas shaped like a circle and others like a rectangle?

A: Rectangular TCFA areas are issued when a developing cyclone's speed and direction of motion can be predicted with relatively high confidence. Circular TCFA areas are issued when a developing cyclone's speed and direction of motion is relatively difficult to predict or if the developing cyclone is nearly stationary.

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Q: When does JTWC initiate warnings on tropical disturbances?

A: JTWC initiates tropical cyclone warnings when one or more of the following four criteria are met:

  • Estimated maximum sustained wind speeds within a closed tropical circulation meet or exceed a designated threshold of 25 knots in the North Pacific Ocean or 35 knots in the South Pacific and Indian Oceans.
  • Maximum sustained wind speeds within a closed tropical circulation are expected to increase to 35 knots or greater within 48 hours.
  • A tropical cyclone may endanger life and/or property within 72 hours.
  • USPACOM directs JTWC to begin tropical cyclone warnings.

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Q: How often are warnings updated by JTWC?

A: North Pacific and North Indian Ocean tropical cyclone warnings are routinely updated every six hours. South Indian and South Pacific Ocean tropical cyclone warnings are routinely updated every twelve hours, although the frequency of tropical cyclone warning issuance in these areas can be increased to six-hourly when deemed necessary by JTWC. The REMARKS section of each warning bulletin lists the times when the next warnings are scheduled. When JTWC issues the final warning for a tropical cyclone, no additional warnings are issued unless the cyclone regenerates.

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Q: When are warnings available?

A: For tropical cyclones occurring in the western Pacific and Indian Oceans, JTWC products are transmitted no later than 3 hours past the synoptic hour. Because the synoptic hours are 00Z, 06Z, 12Z, and 18Z, warnings will be available by 03Z, 09Z, 15Z, or 21Z. For the eastern North Pacific Ocean, JTWC products are transmitted no later than 4 hours after the synoptic hour (04Z, 10Z, 16Z, or 22Z).

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Q: What do wind radii represent? In which instances are they included or not included in JTWC forecast products?

A: Wind radii represent the maximum distance in nautical miles, measured from the center of a tropical cyclone, to which the annotated threshold surface wind speed values (34, 50, and 64 knots) associated with the tropical cyclone extend, by quadrant. Note that wind speed values that equal or exceed the indicated threshold values will be observed at some, but not all, locations within each radius because the radii extend to encompass threshold wind speed values even if they are observed within a limited geographic area. Due to the complex effects of land interaction on surface winds, JTWC forecast wind radii are valid only over water. Wind radii accompany the current analysis and all forecast positions through the entire forecast period (maximum 120 hours), including subtropical/extratropical positions and positions over land whenever any portion of the forecast wind radii extends over water. Wind radii are NOT included with current analysis or forecast positions if no portion of those wind radii extends over water.

Click here for an overview of changes that accompanied implementation of current wind radii forecasting procedures in November 2016.

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Q: Why does JTWC use 34, 50, and 64-knot thresholds for wind radii criteria? Why are these thresholds different from other agencies? 

A: U.S. tropical cyclone directives require tropical cyclone wind radii to be reported at those thresholds.

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Q: What is the shaded area around the forecast track on your warning graphic?

A: The shaded area around the forecast track is called the "area of potential gale force winds." This area does not denote forecast track uncertainty. It is calculated by adding the JTWC 5-year running mean forecast track error to the forecast 34-knot wind radii at each forecast time. Thus, the shading highlights the area that may be affected by wind speeds exceeding 34 knots for a given JTWC forecast, based on historical track forecast errors.

Note: Other U.S. and international forecast agencies apply various methods to derive the danger area "cones" associated with their official tropical cyclone forecasts.  Because these methods do not follow the process described above, danger area "cones" depicted on other agencies' tropical cyclone forecast graphics will typically not match the size and shape of the areas of potential gale force winds displayed on JTWC forecast graphics prepared for the same tropical cyclone at the same forecast time.  Consult each agency's data and/or information outlets for details about its processes.

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Q: How are the geographical references determined in the warning text remarks?

A: Various locations are predetermined by U.S. Government agencies, which are the official customers of JTWC products.

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Q: What does "DTG" mean in the Closest Point of Approach (CPA) box?

A: DTG stands for Date Time Group. It indicates the date and time the tropical cyclone will be closest to the specified location.

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Q: How are CPA locations and Bearing/Distance locations determined for the warning graphic?

A: CPA locations are determined by U.S. Government agencies, who are the official customers of JTWC products. Bearing and distance data are routinely computed for these locations.

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Q: Why are prognostic reasoning messages only available for the western North Pacific?

A: Per JTWC directive, prognostic reasoning messages are only required for the western North Pacific, where there are increased U.S. Government asset concerns. Detailed tropical cyclone analysis and forecast discussions are included in the remarks section of warning text messages for tropical cyclones that occur in the Indian Ocean and South Pacific Ocean.

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Q: What is an "Amended" tropical cyclone warning?

A: JTWC issues an amended warning whenever a significant change is made to the forecast track, intensity, and/or tropical cyclone best track position before the next regular warning is issued. The reason for the amendment is stated in the REMARKS section of the amended warning.

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Q: What is a "Corrected" tropical cyclone warning?

A: JTWC issues a corrected warning when administrative or typographical errors are noted in the current warning.

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Q: What is a "Relocated" tropical cyclone warning?

A: JTWC issues a relocated warning to indicate a significant re-assessment of the tropical cyclone's location and movement. The term "relocated" is used when a vector drawn from the previous warning position to the current warning position does not reasonably represent the actual cyclone's movement.

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Q: What is the average size of a tropical cyclone?

A: There is no "average" tropical cyclone. Tropical cyclones are measured radially from the center to the outermost closed isobar. This distance, in degrees latitude, determines the system's relative size (see table below):

Radius    Type    Example
< 2 °    Very Small/Midget    TC Tracy (1994)
2 – 3 °    Small  
3 – 6 °    Medium  
6 – 8 °    Large  
> 8 °    Very Large    STY Tip (1979)

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Q: Can JTWC provide me historical data on tropical cyclones?

A: JTWC provides two areas of information for historical purposes. Historical tropical cyclone tracks are available from the
Tropical Cyclone Best Track page. Tropical cyclone narratives are available from the Annual Tropical Cyclone Report (ATCR).

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Q: Can I view JTWC forecast performance (statistics)?

A: Forecast performance is calculated following a thorough post-storm review of all available data. Statistical information is available in the ATCR.

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Q: What is meant by a "climatological" rate of intensification or weakening?

A: A climatological rate is defined as one Dvorak T-number per 24 hour period. See the following question and answer for more information on the Dvorak scale and its relationship to intensity.

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Q: How is tropical cyclone intensity determined?

A: JTWC uses several tools and techniques to estimate tropical cyclone intensity, including subjective Dvorak estimates, objective fix data, and observations. Over most of the JTWC AOR, the Dvorak technique is the primary means to estimate tropical cyclone intensity. The Dvorak technique is based on the analysis of cloud patterns in visible and infrared imagery from geostationary and polar-orbiting satellites. The Dvorak technique results in a decimal number, called a T-number, which in turn corresponds to an intensity estimate.

T-Number   Estimated Intensity (kt)
1.0    25
1.5    25
2.0    30
2.5    35
3.0    45
3.5    55
4.0    65
4.5    77
5.0    90
5.5    102
6.0    115
6.5    127
7.0    140
7.5    155
8.0    170

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Q: What are the description labels used with tropical cyclones by JTWC?

A: In the North Pacific Ocean, a tropical cyclone with an estimated intensity of less than 34 knots is designated a "Tropical Depression" and a tropical cyclone with an estimated intensity between 34 and 63 knots is designated a "Tropical Storm." Within the eastern and central North Pacific, a tropical cyclone with an estimate intensity of 64 knots or greater is called a "Hurricane." Within the western North Pacific, however, a tropical cyclone with an estimated intensity between 64 and 129 knots is called a "Typhoon," while a tropical cyclone of 130 knots or greater is designated a "Super Typhoon." In the Indian Ocean and South Pacific, JTWC labels ALL tropical cyclones as "Tropical Cyclone," regardless of estimated intensity.

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Q: How are tropical cyclones named?

A: JTWC does not name tropical cyclones. JTWC uses the names determined by the World Meteorological Organization (WMO) Tropical Cyclone Program. JTWC will add the tropical cyclone name in parentheses after the JTWC-designated tropical cyclone number only after the WMO-designated Regional Specialized Meteorological Center (RSMC) or Tropical Cyclone Warning Center (TCWC) names a cyclone. If the RSMC/TCWC has not yet named a cyclone, JTWC uses its TC number, spelled out, as a placeholder, i.e. "TS 16P (SIXTEEN)."

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Q: How are JTWC forecasts different than forecasts issued by tropical cyclone warning centers (TCWCs) of other countries?

A: JTWC and RSMC/TCWC tropical cyclone warnings may differ for several reasons. One difference is the measurement of maximum sustained surface winds. JTWC reports the maximum sustained surface winds in tropical disturbances and cyclones in terms of 1-minute mean wind speed, as required by the U.S. National Hurricane Operations Plan. Other nations, however, report maximum sustained surface winds averaged over a different time interval, which in many cases is 10-minutes. The difference generally means that JTWC will report higher maximum sustained surface wind speeds than non-U.S tropical cyclone forecasting centers for the same cyclone. Another difference is that JTWC will issue forecasts out to 120 hours as required by U.S. DoD. Several TCWCs will transmit forecasts that extend to a period less than 120 hours. Finally, JTWC does not apply the same tropical cyclone numbering scheme used by the regional centers. Hence, the cyclone number assigned by JTWC may not match the numerical designation assigned to the same cyclone by the responsible RSMC/TCWC.

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Q: Does JTWC have a subscription service to automatically send me warnings?

A: No, JTWC does not offer a subscription-based service.

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Q: What numerical models does JTWC use for track forecasting?

A:  The current (April 2017) JTWC consensus forecast track, CONW, is computed by averaging five global dynamic model, three mesoscale dynamic model, and three ensemble mean TC track forecasts. The CONW member tracks are interpolated from the original model TC track forecasts, which are available only after the model runs have completed (several hours after the corresponding model start times). The CONW is computed from the following interpolated model trackers: NVGI, AVNI, JGSI, EGRI, ECMI, COTI, GFNI, HWFI, AEMI, EEMI, and JENI, which are derived from the NAVGEM, GFS, JGSM, UKMET, ECMWF, COAMPS-TC, GFDN, HWRF, GEFS, EC-EPS, and JMA GEPS models, respectively. In order to be considered for the consensus, a model must be consistently available, demonstrate sufficient forecast skill, and add value to the consensus. Models are normally evaluated for one to two years before being included in the consensus. Models that fail to meet these criteria are either not accepted into, or are removed from, the consensus.

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Q: What are three-hourly position update graphics?

A: When a tropical cyclone closes to within 180 nautical miles of six designated installations in the western North Pacific, when one of these installations sets TCCOR 2 or higher, or when directed by USPACOM, JTWC is required to produce three-hourly updated position graphics on the "off-hour" tropical cyclone fix cycles (03Z, 09Z, 15Z, 21Z). The six designated installations are:

  • Anderson AFB, Guam
  • Kadena AB, Japan (Okinawa)
  • Cheju Do, Republic of Korea
  • Sasebo NS, Japan
  • MCAS Iwakuni, Japan
  • Yokosuka NB, Japan

The updated position is derived in a similar manner to the standard best track position. All available satellite and radar imagery and fixes and synoptic data are used to determine the most accurate position of the tropical cyclone. A green triangle labeled "JTUP," which is placed on the graphic alongside the current forecast track, represents the "off-hour" estimated position. It is important to note that tropical cyclones move erratically/wobble in the short term, so there may be some deviation between the estimated position and the forecast track.

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Commonly Used Acronyms:

AOR - Area of Responsibility
C - Celsius
CAA - Cold Air Advection
CDO - Central Dense Overcast
CCC - Convective Cold Cover (i.e., arrested development)
CI - Current Intensity (Dvorak)
CONV - Convergence
CPA - Closest Point of Approach
DAA - Dry Air Advection
DIV - Divergence
DT - Data T-number (Dvorak)
E - East
EI - Explosive Intensification (50 knots / 24 hours)
EIR - Enhanced Infrared satellite imagery
ENE - East-northeast
ENSO - El Niño Southern Oscillation
ERC - Eyewall Replacement Cycle
ESE - East-southeast
ET - Extratropical
ETT - Extratropical Transition
FT - Final T-Number (Dvorak)
IR - Infrared satellite imagery
KT - knots
LLCC - Low Level Circulation Center
MB - millibars
MI - Microwave Imagery
MJO - Madden-Julian Oscillation
MSI - Multispectral Satellite Imagery
NER - Near Equatorial Ridge
N - North
NE -Northeast
NM - Nautical Miles
NNE - North-northeast
NNW - North-northwest
NVA - Negative Vorticity Advection
NW - Northwest
OHC - Ocean Heat Content
PFJ - Polar Front Jet
PVA - Positive Vorticity Advection
QS - Quasi-stationary
RI - Rapid Intensification (30 knots / 24 hours)
S - South
SBC - Spiral Banding Curvature
SE - Southeast
SFC - Surface
SLP - Sea Level Pressure
SSE - South-southeast
SST - Sea Surface Temperature
SSW - South-southwest
STJ - Subtropical Jet
STR - Subtropical Ridge
STY - Super Typhoon
SW - Southwest
TAU - A time variable, used to indicate amount of HOURS beyond a certain time (ie TAU 0 is equal to valid time, TAU 24 is 1 day beyond valid time).
TC - Tropical Cyclone
TCB - Tightly Curved Banding
TD - Tropical Depression
TEJ - Tropical Easterly Jet
TS - Tropical Storm
TUTT - Tropical Upper Tropospheric Trough
TY- Typhoon
IR - Infrared Satellite Imagery
ULCC - Upper Level Circulation Center
ULL - Upper-Level Low
VIS - Visible satellite imagery
VWS - Vertical Wind Shear
W - West
WAA - Warm Air Advection
WNW - West-Northwest
WSW - West -Southwest
XT - Extratropical
XTT - Extratropical Transition

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