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Frequently Asked Questions

Last updated 2 August 2009

 

 



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 Universal Time Coordinate (UTC), previously known as Greenwich Mean Time (GMT). The U.S. military uses UTC but calls it 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

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Q: What do "Poor", "Fair", and "Good" mean on the ABIO and ABPW messages?

A:
"Poor" describes an area of convection or low level inflow where conditions are not favorable for a tropical cyclone to develop within 24 hours. "Fair" describes a tropical disturbance for which conditions have improved but development has not yet commenced. "Good" describes a tropical disturbance where conditions are favorable for development and development has begun. "Good" descriptions are applied to tropical disturbances covered by a TCFA.

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

A: A TCFA designates an area where JTWC is most likely to begin issuing tropical cyclone warnings within the next 24 hours. This information is provided to the DoD and other U.S. government agencies for operations planning.

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

A: Rectangular TCFAs are used when the developing cyclone's location and movement can be reasonably predicted. Circular TCFAs are used if the movement cannot be reasonably predicted or if the developing cyclone is nearly stationary.

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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. 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: JTWC products are transmitted not later than 3 hours past the synoptic hour from which a warning has been created. As the synoptic hours are 00Z, 06Z, 12Z, and 18Z, warnings will be available by 03Z, 09Z, 15Z, or 21Z.

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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 uncertainty." The area of uncertainty is calculated by adding the JTWC 5-year running mean forecast track error to the forecast 35 knot wind radii at each forecast time. Since JTWC does not forecast wind radii at the 96- and 120-hours, the area of uncertainty is calculated by adding the 72-hour 35 knot radii to the forecast track error at each time.

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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 pre-determined by the DoD.

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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 were determined by the DoD. Bearing and Distance data are routinely computed for these locations.

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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 and/or intensity 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/Average  
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 in 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 on 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 answer and diagram below for intensification/weakening trends for a normal-, less-than-, and greater-than-climatological rate.

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Q: How is tropical cyclone intensity determined?

A: JTWC primarily uses the Dvorak technique 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: Anywhere in the Northern Pacific Ocean, a tropical cyclone of 33kt intensity or less is designated a "Tropical Depression." Throughout that same area, a tropical cyclone between 34 and 63kt intensity is designated a "Tropical Storm." Within the Northeastern and North-central Pacific, a tropical cyclone of 64kt intensity or greater is called a "Hurricane." Within the Northwestern Pacific, however, a tropical cyclone between 64 and 129 kt intensity is called a "Typhoon." A tropical cyclone of 130kt or greater in the Northwestern Pacific is designated a "Super Typhoon." Elsewhere (Indian Ocean and the South Pacific), JTWC describes all tropical cyclones as "Tropical Cyclone," regardless of 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. Only after the WMO-designated Regional Specialized Meteorological Center (RSMC) names a cyclone will JTWC products include a name. If the RSMC 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: One difference is the measurement of maximum sustained surface winds. JTWC uses the 1-minute mean wind speed to determine maximum sustained surface winds, as required by the U.S. National Hurricane Operations Plan. Other countries, however, use the 10-minute mean wind speed to determine maximum sustained surface wind speeds. The difference generally means that JTWC will report higher maximum sustained surface wind speeds than non-U.S. TCWCs for the same cyclone. Another difference is that JTWC will issue forecasts out to 120 hours in the North Pacific Ocean as required by U.S. DoD.

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Q: Why isn't minimum central pressure included in the best track data?

A: Because central pressure is rarely measured within tropical cyclones in JTWC's area of responsibility (AOR), central pressure is not included in JTWC best track data or warnings.

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Q: My Internet Explorer window is resizing JTWC graphics, which makes the text illegible. How do I correct this?

A: This problem occurs when using Microsoft Explorer version 6.0+. To prevent Explorer from automatically resizing the graphics, select the 'Tools' menu, 'Internet Options'. From there, select the 'Advanced' tab and scroll down to 'Multimedia'. Find the box next to 'Enable Automatic Image Resizing' and uncheck the box.

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

A: JTWC does not offer a subscription-based service.

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Q: What is the purpose of the Wind Probability products?

A: Wind probability products are intended to convey the likelihood of significant winds occurring at specific locations. The products are currently under development and are only being created for the Northwestern Pacific basin. JTWC welcomes comments from its customers concerning the format and utility of the wind probability products.

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Q: What is depicted on the Wind Probability Graphics?

A: Wind probability graphics show the cumulative probability (from Tau 00 through the forecast time) that max sustained surface wind speeds will equal or exceed the given threshold (34-, 50-, or 64-kt) at the locations on the map.

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Q: How are the Wind Probabilities generated?

A: Centered on the latest JTWC forecast, 1000 plausible alternate forecasts are created, each with a different evolution of track, intensity, and size. The track and intensity variations are determined by random sampling of historical track and intensity errors over the past 5 years. Size variations are determined by a climatology and persistence (CLIPR) model and its error components. For each alternate forecast, swaths of particular wind speeds (34-, 50-, 64KT) are calculated and saved. Finally, probabilities are calculated on a ½ degree latitude/longitude grid by counting the fraction of alternate forecasts in which each grid point falls within a swath of each wind speed.

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Q: What numerical models does JTWC use?

A: The current consensus forecast track used at JTWC, called CONW, is composed of seven baroclinic dynamical models and one barotropic model. The dynamical model tracks included (labeled with their first interpolation) are: NGPI, GFNI, AVNI, TCLI, JGSI, EGRI, and ECMI. The sole barotropic track model is WBAI. In order to be considered for the consensus, a model must be consistently available, show 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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