11/14/16 Aviation Weather Services AC 00-45H Figure 5-42. Alaskan Graphical Area Forecast (FA)-Convective Outlook VALID 30 Jul 2013 01 PM to 31 Jul 2013 01 AM AKDT LEGEND TCU 060 / FL280 060 / FL ISOL TS SCT TS
WDSPRD TS (BASE / TOP in 100s of Feet (040 / FL1
VALID CONVECTIVE OUTLOOK ISSUED 30 Jul 2013 2100 to 30 Jul 2013 31 Jul 2013 0900 UTC NOAA - NWS ALASKA AVIATION WEATHER UNIT 04 AM AKDT
5.10.3 Use.
The Alaska Graphical FAs are a series of forecasts intended to complement, not replace, the text-based FA for Alaska.
5.11 Terminal Aerodrome Forecast (TAF).
A TAF is a concise statement of the expected meteorological conditions significant to aviation for a specified time period within 5 sm of the center of the airport's runway complex (terminal). The TAFs use the same weather codes found in METAR weather reports (see paragraph 3.1) and can be viewed on http://www.aviationweather.gov.
5.11.1 Responsibility.
TAFs are issued by NWS WFOs.
5-75
11/14/16 Aviation Weather Services AC 00-45H 5-76
5.11.2 Generic Format of the Forecast Text of an NWS-Prepared TAF.
Table 5-13. Generic Format of NWS TAFs TAF or
TAF AMD or
TAF COR Type of report
CCCC YYGGggZ Y1Y1G1G1/Y2Y2G2G2 dddffGfmfmKT Location identifier Date/time of forecast origin group Valid period Wind group
VVVV w'w' or NSW NsNsNshshshs or VVhshshs or SKC WShwshwshws/dddftKT Visibility group Significant weather group Cloud and vertical obscuration groups Non-Convective low-level wind shear (LLWS) group
TTGGgg Forecast change indicator groups
FMY1Y1GGgg TEMPO Y1Y1GG/YeYeGeGe PROB30 Y1Y1GG/YeYeGeGe From group Temporary group Probability group
11/14/16 Aviation Weather Services AC 00-45H 5-77
5.11.2.1 Type of Report (TAF or TAF AMD or TAF COR).
The report-type header always appears as the first element in the TAF and is produced in three forms: a routine forecast, TAF, an amended forecast, TAF AMD, or a corrected forecast, TAF COR.
TAFs are amended whenever they become, in the forecaster’s judgment, unrepresentative of existing or expected conditions, particularly regarding those elements and events significant to aircraft and airports. An amended forecast is identified by TAF AMD (in place of TAF) on the first line of the forecast text.
5.11.2.2 Location Identifier (CCCC).
After the line containing either TAF, TAF AMD, or TAF COR, each TAF begins with its four-letter ICAO location identifier.
Examples:
KDFW Dallas-Fort Worth PANC Anchorage, Alaska PHNL Honolulu, Hawaii
5.11.2.3 Date/Time of Forecast Origin Group (YYGGggZ).
The date/time of forecast origin group (YYGGggZ) follows the terminal’s location identifier. It contains the day of the month in two 2 digits (YY) and the time in four digits (GGgg in hours and minutes) in which the forecast is completed and ready for transmission, with a Z appended to denote UTC. This time is entered by the forecaster. A routine forecast, TAF, is issued 20 to 40 minutes before the beginning of its valid period.
Examples:
061737Z The TAF was issued on the 6th day of the month at 1737 UTC.
121123Z The TAF was issued on the 12th day of the month at 1123 UTC.
5.11.2.4 Valid Period (Y1Y1G1G1/Y2Y2G2G2).
The TAF valid period (Y1Y1G1G1/Y2Y2G2G2) follows the date/time of forecast origin group. Scheduled 24- and 30-hour TAFs are issued four times per day, at 0000, 0600, 1200, and 1800Z. The first two digits (Y1Y1) are the day of the month for the start of the TAF. The next two digits (G1G1) are the starting hour (UTC). Y2Y2 is the day of the month for the end of the TAF, and the last two digits (G2G2) are the ending hour (UTC) of the valid period.
A forecast period that begins at midnight UTC is annotated as 00. If the end time of a valid period is at midnight UTC, it is annotated as 24. For example,
11/14/16 Aviation Weather Services AC 00-45H 5-78 a 00Z TAF issued on the 9th of the month and valid for 24 hours would have a valid period of 0900/0924.
Whenever an amended TAF (TAF AMD) is issued, it supersedes and cancels the previous TAF. That is, users should not wait until the start of the valid period indicated within the TAF AMD to begin using it.
Examples:
1512/1612 The TAF is valid from the 15th day of the month at 1200 UTC until the 16th day of the month at 1200 UTC.
2306/2412 This is a 30-hour TAF valid from the 23rd day of the month at 0600 UTC until the 24th day of the month at 1200 UTC.
0121/0218 This is an amended TAF valid from the 1st day of the month at 2100 UTC until the 2nd day of the month at 1800 UTC.
0600/0624 This TAF is valid from the 6th day of the month at 0000 UTC until the 6th day of the month at 2400 UTC (or 7th day of the month at 0000 UTC).
5.11.2.5 Wind Group (dddffGfmfmKT).
The initial time period and any subsequent FM groups begin with a mean surface wind forecast (dddffGfmfmKT) for that period. Wind forecasts are expressed as the mean three-digit direction (ddd—relative to true north) from which the wind is blowing, rounded to the nearest 10 degrees, and the mean wind speed in knots (ff) for the time period. If wind gusts are forecast (gusts are defined as rapid fluctuations in wind speeds with a variation of 10 kts or more between peaks and lulls), they are indicated immediately after the mean wind speed by the letter G, followed by the peak gust speed expected. KT is appended to the end of the wind forecast group. Any wind speed of 100 kts or more will be encoded in three digits. Calm winds are encoded as 00000KT.
The prevailing wind direction is forecast for any speed greater than or equal to 7 kts. When the prevailing surface wind direction is variable (variations in wind direction of 30 degrees or more), the forecast wind direction is encoded as VRBffKT. Two conditions where this can occur are very light winds and Convective activity. Variable wind direction for very light winds must have a wind speed of 1–6 kts inclusive. For Convective activity, the wind group may be encoded as VRBffGfmfmKT, where Gfmfm is the maximum expected wind gusts. VRB is not used in the Non-Convective LLWS group.
Squalls are forecast in the wind group as gusts (G) but must be identified in the significant weather group with the code SQ.
11/14/16 Aviation Weather Services AC 00-45H 5-79 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 example 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 Wind from 90 degrees at 105 knots
5.11.2.6 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
11/14/16 Aviation Weather Services AC 00-45H 5-80 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
5.11.2.7 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.
11/14/16 Aviation Weather Services AC 00-45H 5-81 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).
5.11.2.7.1 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 intensity of the snow is greater than the freezing rain.
Examples:
Combinations of one precipitation 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 mist -SHRA FG Light rain showers and fog +SN FG Heavy snow and fog 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 Moderate snow showers and ice pellets
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5.11.2.7.2 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
5.11.2.7.3 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-restricted 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 following fog-related terms are used as described below:
11/14/16 Aviation Weather Services AC 00-45H 5-83 Table 5-14. 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.
5.11.2.8 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 5-15, TAF Use of Vicinity (VC), are valid for use in prevailing portions of NWS TAFs in combination with VC:
11/14/16 Aviation Weather Services AC 00-45H 5-84 Table 5-15. 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’)
5.11.2.9 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.
5.11.2.9.1 Cloud Group (NsNsNshshshs).
The cloud group (NsNsNshshshs) is used to forecast cloud amount in Table 5-16, TAF Sky Cover.
Table 5-16. 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
11/14/16 Aviation Weather Services AC 00-45H 5-85 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.
5.11.2.9.2 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 vertical visibility into fog
5.11.2.9.3 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
5.11.2.10 Non-Convective Low-Level Wind Shear (LLWS) Group (WShwshwshws/dddffKT).
Wind Shear (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.
11/14/16 Aviation Weather Services AC 00-45H 5-86 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 Wind shear 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 wind shear 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 wind shear layer. It is not a value for the amount of shear.
A Non-Convective LLWS forecast is included in the initial time period or a FM group in a TAF whenever:
• One or more Pilot Weather Reports (PIREP) are received of Non-Convective LLWS within 2,000 ft of the surface, at or in the vicinity of the TAF airport, causing an indicated air speed loss or gain of 20 kts or