5-88 Examples:
23010KT Wind from 230 degrees “true” (southwest) at 10 knots.
28020G35KT Wind from 280 degrees “true” (west) at 20 knots gusting to 35 knots.
VRB05KT Wind variable at 5 knots.
This e xample depicts a forecast for light winds that are expected to variable in direction.
VRB15G30KT Wind variable at 15 knots gusting to 30 knots. This example depicts winds that are forecast to be variable with Convective activity.
00000KT Wind calm 090105KT W ind from 90 degrees at 105 knots
5.13.2.6 Visibility Group (VVVV).
The initial time period and any subsequent FM groups include a visibility forecast (VVVV) in statute miles appended by the contraction sm.
When the prevailing visibility is forecast to be less than or equal to 6 sm, one or more significant weather groups are included in the TAF. However, drifting dust (DRDU), drifting sand (DRSA), drifting snow (DRSN), shallow fog (MIFG), partial fog (PRFG), and patchy fog (BCFG) may be forecast with prevailing visibility greater than or equal to 7 sm.
When a whole number and a fraction are used to forecast visibility, a space is included between them (e.g., 1 1/2SM). Visibility greater than 6 sm is encoded as P6SM.
If the visibility is not expected to be the same in different directions, prevailing visibility is used.
When volcanic ash (VA) is forecast in the significant weather group, visibility is included in the forecast, even if it is unrestricted (P6SM). For example, an expected reduction of visibility to 10 sm by volcanic ash is encoded in the forecast as P6SM VA.
Although not used by the NWS in U.S. domestic TAFs, the contraction “CAVOK” (Ceiling and Visibility OK) may replace the visibility, weather, and sky condition groups if all of the following conditions are forecast:
Visibility of 10 kilometers (km) (6 sm) or more, no clouds below
5-89 1500 meters (m) (5,000 ft) or below the highest minimum sector altitude (whichever is greater), no cumulonimbus, and no significant weather phenomena.
Examples:
P6SM Visibility unrestricted 1 1/2SM Visibility 1 and ½ sm 4SM Visibility 4 sm
5.13.2.7 Significant Weather Group (w’w’ or NSW).
The significant weather group (w’w’ or NSW) consists of the appropriate qualifier(s) and weather phenomenon contraction(s) or NSW (no significant weather).
If the initial forecast period and subsequent FM groups are not forecast to have explicit significant weather, the significant weather group is omitted.
NSW is not used in the initial forecast time period or FM groups.
One or more significant weather group(s) is (are) required when the visibility is forecast to be 6 sm or less. The exceptions are: volcanic ash (VA), low drifting dust (DRDU), low drifting sand (DRSA), low drifting snow (DRSN), shallow fog (MIFG), partial fog (PRFG), and patchy fog (BCFG).
Obstructions to vision are only forecast when the prevailing visibility is less than 7 sm or, in the opinion of the forecaster, is considered operationally significant.
Volcanic ash (VA) is always forecast when expected. When VA is included in the significant weather group, visibility is included in the forecast as well, even if the visibility is unrestricted (P6SM).
NSW is used in place of significant weather only in a TEMPO group to indicate when significant weather (including in the vicinity (VC)) included in a previous subdivided group is expected to end.
Multiple precipitation elements are encoded in a single group
(e.g., -TSRASN). If more than one type of precipitation is forecast, up to three appropriate precipitation contractions can be combined in a single group (with no spaces) with the predominant type of precipitation being first. In this single group, the intensity refers to the total precipitation and can be used with either one or no intensity qualifier, as appropriate. In TAFs, the intensity qualifiers (light, moderate, and heavy) refer to the intensity of the precipitation and not to the intensity of any thunderstorms associated with the precipitation.
5-90 Intensity is coded with precipitation types, except ice crystals and hail, including those associated with thunderstorms and those of a showery nature (SH). No intensity is ascribed to blowing dust (BLDU), blowing sand (BLSA), or blowing snow (BLSN). Only moderate or heavy intensity is ascribed to sandstorm (SS) and duststorm (DS).
5.13.2.7.1 Exception for Encoding Multiple Precipitation Types.
When more than one type of precipitation is forecast in a time period, any precipitation type associated with a descriptor (e.g., FZRA) is encoded first in the precipitation group, regardless of the predominance or intensity of the other precipitation types. Descriptors are not encoded with the second or third precipitation type in the group. The intensity is associated with the first precipitation type of a multiple precipitation type group. For example, a forecast of moderate snow and light freezing rain is coded as -FZRASN, although the i ntensity of the snow is greater than the freezing rain.
Examples:
Combinations of one prec ipitation and one non-precipitation weather phenomena:
-DZ FG Light drizzle and fog (obstruction that reduces visibility to less than 5/8 sm) RA BR Moderate rain and mis t -SHRA FG L ight rain showers and fog +SN FG Heavy snow a nd fo g Combinations of more than one type of precipitation:
-RASN FG HZ Light rain and snow (light rain predominant), fog and haze TSSNRA Thunderstorm with moderate snow and rain (moderate snow predominant)
FZRASNPL Moderate freezing rain, snow and ice pellets (freezing rain mentioned first due to the descriptor, followed by other precipitation types in order of predominance) SHSNPL Moder ate snow showers and ice pellets
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5.13.2.7.2 Thunderstorm Descriptor.
The TS descriptor is treated differently than other descriptors in the following cases:
• When non-precipitating thunderstorms are forecast, TS may be encoded as the sole significant weather phenomenon; and
• When forecasting thunderstorms with freezing precipitation (FZRA or FZDZ), the TS descriptor is included first, followed by the intensity and weather phenomena.
Example:
TS -FZRA When a thunderstorm is included in the significant weather group (even using vicinity—VCTS), the cloud group (NsNsNshshshs) includes a forecast cloud type of CB. See the following example for encoding VCTS.
Example:
-FZRA VCTS BKN010CB
5.13.2.7.3 Fog Forecast.
A visibility threshold must be met before a forecast for fog (FG) is included in the TAF. When forecasting a fog-restricted visibility from 5/8 sm to 6 sm, the phenomena is coded as BR (mist). When a fog-restricte d visibility is forecast to result in a visibility of less than 5/8 sm, the code FG is used. The forecaster never encodes weather obstruction as mist (BR) when the forecast visibility is greater than 6 sm (P6SM).
The follow ing fog-related terms are used as described below:
5-92 Table 5-16. TAF Fog Terms
TERM DESCRIPTION Freezing Fog (FZFG) Any fog (visibility less than 5/8 SM) consisting predominantly of water droplets at temperatures less than or equal to 32 ˚F / 0 ˚C, whether or not rime ice is expected to be deposited. FZBR is not a valid significant weather combination and will not be used in TAFs.
Shallow Fog (MIFG) The visibility at 6 feet above ground level is greater than or equal to 5/8 sm and the apparent visibility in the fog layer is less than 5/8 sm.
Patchy Fog |(BCFG) Fog patches covering part of the airport. The apparent visibility in the fog patch or bank is less than 5/8 sm, with the foggy patches extending to at least 6 feet above ground level.
Partial Fog (PRFG) A substantial part of the airport is expected to be covered by fog while the remainder is expected to be clear of fog (e.g., a fog bank). Note: MIFG, PRFG and BCFG may be forecast with prevailing visibility of P6SM.
Examples:
1/2SM FG Fog is reducing visibilities to less than 5/8 sm, therefore FG is used to encode the fog.
3SM BR Fog is reducing visibilities to between 5/8 and 6 sm, therefore BR is used to encode the fog.
5.13.2.8 Vicinity (VC).
In the United States, vicinity (VC) is defined as a donut-shaped area between 5 and 10 sm from the center of the airport’s runway complex. The FAA requires TAFs to include certain meteorological phenomena, which may directly affect flight operations to and from the airport. Therefore, NWS TAFs may include a prevailing condition forecast of fog, showers, and thunderstorms in the airport’s vicinity. A prevailing condition is defined as a greater than or equal to 50 percent probability of occurrence for more than half of the subdivided forecast time period. VC is not included in TEMPO or PROB groups.
The significant weather phenomena in Table 5-17, TAF Use of Vicinity (VC), are valid for use in prevailing portions of NWS TAFs in combination with VC:
5-93 Table 5-17. TAF Use of Vicinity (VC)
PHENOMENON CODED Fog* VCFG Shower(s)** VCSH Thunderstorm VCTS *Always coded as VCFG regardless of visibility in the obstruction, and without qualification as to intensity or type (frozen or liquid) **The VC group, if used, should be the last entry in any significant weather group (w’w’)
5.13.2.9 Cloud and Vertical Obscuration Groups (NsNsNshshshs or VVhshshs or SKC).
The initial time period and any subsequent FM groups include a cloud or obscuration group (NsNsNshshshs or VVhshshs or SKC), used as appropriate to indicate the cumulative amount (NsNsNs) of all cloud layers in ascending order and height (hshshs), to indicate vertical visibility (VVhshshs) into a surface-based obstructing medium, or to indicate a clear sky (SKC). All cloud layers and obscurations are considered opaque.
5.13.2.9.1 Cloud Group (NsNsNshshshs).
The cloud group (NsNsNshshshs) is used to forecast cloud amount in Table 5-18, TAF Sky Cover.
Table 5-18. TAF Sky Cover
SKY COVER CONTRACTION SKY COVERAGE SKC 0 oktas FEW 0 to 2 oktas SCT 3 to 4 oktas BKN 5 to 7 oktas OVC 8 oktas
5-94 When 0 oktas of sky coverage is forecast, the cloud group is replaced by SKC. The contraction CLR, which is used in the METAR code, is not used in TAFs. TAFs for sites with ASOS/AWOS contain the cloud amount and/or obscurations, which the forecaster expects, not what is expected to be reported by an ASOS/AWOS.
Heights of clouds (hshshs) are forecast in hundreds of feet AGL.
The lowest level at which the cumulative cloud cover equals 5/8 or more of the celestial dome is understood to be the forecast ceiling. For example, VV008, BKN008, or OVC008 all indicate an 800 ft ceiling.
5.13.2.9.2 Vertical Obscuration Group (VVhshshs).
The vertical obscuration group (VVhshshs) is used to forecast, in hundreds of feet AGL, the vertical visibility (VV) into a surface-based total obscuration.
VVhshshs is this ceiling at the height indicated in the forecast. TAFs do not include forecasts of partial obscurations (i.e., FEW000, SCT000, or BKN000).
Example:
1SM BR VV008 Ceiling is 800 ft due to vert ical visibility into fog
5.13.2.9.3 Cloud Type (CB).
The only cloud type included in the TAF is CB. CB follows cloud or obscuration height (hshshs) without a space whenever thunderstorms are included in significant weather group (w’w’), even if thunderstorms are only forecast in the vicinity (VCTS). CB can be included in the cloud group (NsNsNshshshs) or the vertical obscuration group (VVhshshs) without mentioning thunderstorm in the significant weather group (w’w’). Therefore, situations may occur where nearly identical NsNsNshshshs or VVhshshs appear in consecutive time periods, with the only change being the addition or elimination of CB in the forecast cloud type.
Examples :
1/2SM TSRA OVC010CB Thunderstorms are forecast at the airport
5.13.2.10 Low-Level Wind Shear Alert System (LLWAS).
The LLWAS system was originally developed by the FAA in the 1970s to detect large-scale wind shifts (sea breeze fronts, gust fronts, and cold and warm fronts). It was developed by the FAA in response to an accident at JFK Airport in New York. The aircraft (Eastern 66) landed during a wind shift caused by interacting sea breeze and thunderstorm outflows.
5-95 Figure 5-50. Microburst Effect on Landing Aircraft
Artist rendition of a microburst and its effect on a landing aircraft (Image Credit:
National Center for Atmospheric Research (NCAR)).
This Phase-1 LLWAS was very simple. It compared a center field wind to five other sensors around the airport. When there was a 15-knot vector difference, it would flash the wind data to the air traffic controller and the controller would read the raw winds (e.g., 120/35 (120 degrees at 35 kts), 110/20, 350/15, etc.) from each sensor to the pilot landing or about to take off and the pilot had to do the vector addition in his head to determine the headwind/tailwind components.
This simple system worked for large-scale weather features, but it also had a serious false alarm problem and the sensors were too far apart to capture small, but intense windshear events important to aircraft. Also, the center field wind could be variable and this would trigger windshear alarms at all the outer sensors since all the other sensor winds were compared to center field.
Research conducted at the NCAR in the 1980s indicated that microburst windshear was very dangerous to aircraft below 1,000 ft. Several major accidents during the 1980s also implicated windshear as a factor.
5.13.2.10.1 LLWAS History.
In 1983, the FAA asked NCAR to develop a version of LLWAS that could detect microbursts. Between 1983 and 1988, NCAR developed and tested a new LLWAS system, called enhanced LLWAS, or LLWAS–Network Expansion, that detected microbursts, determined the strength in terms of headwind/tailwind gains or losses (in knots), and located the event (on the runway, at 1, 2, or 3 NM on departure or arrival). The system was designed to provide alerts specific to each runway operation. It was designed to have a probability of detection of 90 percent or greater and a false alarm rate of 10 percent or less.
5-96 This system was later improved and is now called the Phase-3 LLWAS. A typical Phase-3 LLWAS will have enough sensors to be spaced 2 km apart (∼1 NM apart) and cover out to 2 NM from the end of each major runway.
The largest LLWAS is at Denver International Airport, which has 32 wind sensors. Most Phase-3 systems have between 12 and 16 wind sensors. A siting evaluation is done for each airport to determine the network geometry since it depends on terrain, number of runways, obstructions, etc.
The Phase-3 LLWAS alert information is described below.
If a pilot is landing on runway 08, and there is a microburst on his path, the controller would have a display that reads: 08A MBA 30K–3MF 350/25.
This is read to a pilot arriving on runway 08 (08A) by a final controller as “microburst alert (MBA), expect a thirty knot loss (30K–) at three miles final (3MF), threshold wind three–five–zero at 25 (knots).”
If there was a windshear with a wind speed gain at 1 mile departure (headwind gain), for a pilot departing runway 25 left, the final controller’s LLWAS display would show: 25LD WSA 15k+ 1MD. This would be read as “windshear alert, expect a fifteen knot gain at one mile departure.” There are Phase-3 LLWAS systems at 9 U.S. airports and Phase-2 LLWAS at more than 100 airports. Taiwan, Korea, Singapore, Saudi Arabia, and Kuwait are now implementing Phase-3 LLWAS systems. Note, the FAA also has Terminal Doppler Weather Radars (TDWR) for windshear detection at 45 airports and has Airport Surveillance Radar-9 (ASR-9)-based windshear detection systems at another 37 airports. The FAA originally had 110 Phase-1 LLWAS systems, which were upgraded to Phase-2 systems.
A Phase-2 LLWAS has the same number of sensors (5 to 6) as a Phase-1 system (described above), but the windshear algorithm was upgraded to significantly decrease the number of false alarms. As mentioned above, a Phase-1 or Phase-2 LLWAS was not designed to detect microbursts per se, but if the flow is large and strong, it may send an alert.
When NCAR developed the Phase-3 LLWAS, it gave the specifications to the FAA. The University Corporation for Atmospheric Research (UCAR) Foundation owned the intellectual property for the windshear algorithm during the lifetime of the patent. A license agreement was required for companies to implement LLWAS technology until early 2013 when the patent exclusion expired. A license from UCAR Foundation is no longer required to utilize the LLWAS algorithm. The UCAR Foundation does, however, provide technical materials such as test datasets, test airport configuration files, test alert outputs, etc. to aid companies in the implementation and testing of the Phase-3 LLWAS algorithm.
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5.13.2.10.2 Frequently Asked Questions (FAQ).
1. Q. What is LLWAS?
A. A typical Phase-3 LLWAS system includes a network of anemometers (wind sensors) atop tall poles located around the airport (i.e., remote stations) out to no more than 3 NM from the end of the runways, a master station that processes system data and communicates with the remote stations, an archiving system, operator console, alphanumeric alarm displays, and in some instances, graphical displays.
2. Q. How does it work?
A. The wind data at each remote station is processed every 10 seconds to determine if there is divergence or convergence within the network, or station-to-station wind differences between stations aligned with the runways. The divergence/convergence information is processed and if the intensity of the event is large enough, the system will calculate the strength of the along-runway wind losses or gains and generate windshear or microburst alerts (depending on strength), and identify the location of the event.
3. Q. What is a microburst?
A. A microburst is an intense windshear. By definition:
Microburst n: A small, very intense downdraft that descends to the ground resulting in a strong wind divergence. The size of the event is typically less than 4 km across. Microbursts are capable of producing winds of more than 100 mph, causing significant damage. The life span of a microburst is around 5 to 15 minutes.
4. Q. What causes microbursts?
A. Microbursts are strong windshears (greater than 30-knot wind speed losses over distances of 1 to 4 km) that are primarily generated by evaporative cooling and rain loading. When precipitation (rain or snow) descends below the cloud base or is mixed with dry air, it begins to evaporate and this evaporation process cools the air. The cool air descends and accelerates as it approaches the ground. When the cool air approaches the ground, it spreads out in all directions and this divergence of the wind is the signature of the microburst. In humid climates, microbursts can also generate from heavy precipitation. The weight and drag associated with the falling precipitation can result in a downdraft that will descend to the ground and spread out.
5. Q. Why is it a problem for airplanes?
A. Windshear is a rapid change of wind speed or direction over a short distance. In general, windshear becomes a hazard for aircraft if the wind changes more than 20 kts over a distance of 1 to 4 km
(0.5 to 2.5 NM). On either takeoff or landing, aircraft are near stall speeds. When going through a windshear, the headwind decreases
5-98 resulting in a loss of lift. If the aircraft is near stall, then a little loss of lift can make all the difference to whether the aircraft can continue the flight.
6. Q. How and what kind of decisions do airports or airlines make using LLWAS?
A. LLWAS provides information on windshear type, location, and intensity. Windshear alerts are issued via radio to arriving and departing aircraft by final air traffic controllers. In the United States, most airlines require that pilots not continue their arrival or departure if there is a microburst alert valid for their operation.
Although most aviation authorities do not close the runways when microbursts are occurring, the air traffic controllers will work with the pilots to reroute aircraft away from the event to a runway that is not impacted by the windshear.
7. Q. How and what kind of decisions do airports make using LLWAS?
A. LLWAS provides information on windshear type, location, and intensity. The pilots get the alert from the controller and the pilot is supposed to determine if they feel comfortable continuing the operation. In the United States, airlines require that pilots do not continue if there is a microburst alert.
8. Q. Where have LLWAS systems been implemented?
A. Phase-3 LLWAS systems have been implemented throughout the United States and at numerous international airports including, but not limited to, Taiwan, Korea, Saudi Arabia, Hong Kong, Kuwait, Italy, Singapore, and Spain. Additional LLWAS procurements are in progress.
9. Q. What are some of the windshear problems associated with airports?
A. Any airport that has thunderstorms will be exposed to Convective windshear. Any airport near mountains will experience terrain-induced windshear from time to time. Any airport near a coast will experience windshear due to sea breezes. In the drier climates, even a light shower (or virga) can produce severe windshears, so in some places, the convection does not have to be as strong as a thunderstorm (e.g., Denver, Phoenix, Reno, Albuquerque, etc.).
Discussion Credit: NCAR.
For m ore information readers can go to:
https://www.ral.ucar.edu/projects/low-level-wind-shear-alert-system-llwas.
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5.13.2.11 Non-Convective Low-Level Wind Shear (LLWS) Group (WShwshwshws/dddffKT).
Windshear (WS) is defined as a rapid change in horizontal wind speed and/or direction, with distance and/or a change in vertical wind speed and/or direction with height. A sufficient difference in wind speed, wind direction, or both can severely impact airplanes, especially within 2,000 ft AGL because of limited vertical airspace for recovery.
Forecasts of LLWS in the TAF refer only to Non-Convective LLWS from the surface up to and including 2,000 ft AGL. LLWS is always assumed to be present in Convective activity. LLWS is included in TAFs on an “as-needed” basis to focus the aircrew’s attention on LLWS problems that currently exist or are expected. Non-Convective LLWS may be associated with the following: frontal passage, inversion, low-level jet, lee side mountain effect, sea breeze front, Santa Ana winds, etc.
When LLWS conditions are expected, the Non-Convective LLWS code WS is included in the TAF as the last group (after cloud forecast). Once in the TAF, the WS group remains the prevailing condition until the next FM change group or the end of the TAF valid period if there are no subsequent FM groups. Forecasts of Non-Convective LLWS are not included in TEMPO or PROB groups.
The format of the Non-Convective LLWS group is:
WShwshwshws/dddffKT WS Indicator for Non-Convective LLWS Hwshwshws Height of the top of the WS layer in hundreds of feet AGL Ddd True direction in 10-degree increments at the indicated height -- VRB is not used for direction in the Non-Convective LLWS forecast Ff Speed in knots of the forecast wind at the indicated height KT Unit indicator for wind Example:
TAF…13012KT…WS020/27055KT Windshear from the surface to 2,000 ft. Surface winds from 130 (southeast) at 12 kts changes to 270 (west) at 55 knots at 2,000 ft.
In this example, the indicator WS is followed by a three-digit number that is the top of the windshear layer. LLWS is forecast to be present from the surface to this level. After the solidus (/), the five-digit wind group is the wind direction and speed at the top of the windshear layer. It is not a value for the amount of shear.