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Frequently Asked Questions

Last updated 14 August 2015


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

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 commercial 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 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 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. Since JTWC does not forecast wind radii for the 96- and 120-hour forecast times, the area of gale force winds is calculated by adding the 72-hour 34 knot radii to the forecast track error at those times. 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.

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

A: Various locations are per-determined by U.S. Government agencies, who 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):




< 2 °

Very Small/Midget

TC Tracy (1994)

2 - 3 °



3 - 6 °



6 - 8 °



> 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.


Estimated Intensity (kt)































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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 (November 2014) JTWC consensus forecast track, CONW, is computed by averaging eight dynamic model and two ensemble mean TC track forecasts. The CONW member tracks are interpolated from the model TC track forecasts as the original (vice interpolated) track forecasts are always produced at or after the forecast time (unavailable to the JTWC Typhoon Duty Officer at forecast production time); thus, an earlier numerical track forecast must be interpolated, then used for CONW computations. The CONW is computed from (listed are the interpolated track forecasts): NVGI, GFNI, AVNI, JGSI, EGRI, ECMI (or, more likely, another available version of the ECMWF forecast such as EMXI or ECM2), COTI, HWFI, and the two interpolated ensemble mean members JENI and AEMI.  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 or removed from 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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