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
11/14/16 Aviation Weather Services AC 00-45H 5-82
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
11/14/16 Aviation Weather Services AC 00-45H 5-87 more, and the forecaster determines the report(s) reflect a valid Non-Convective LLWS event rather than mechanical turbulence; or
• When Non-Convective vertical WS of 10 kts or more per 100 ft in a layer more than 200 ft thick are expected or reliably reported within 2,000 ft of the surface at, or in the vicinity of, the airport.
5.11.2.11 Forecast Change Indicator Groups.
Forecast change indicator groups are contractions that are used to subdivide the forecast period (24 hours for scheduled TAFs; less for amended or delayed forecasts) according to significant changes in the weather.
The forecast change indicators FM, TEMPO, and PROB are used when a change in any or all of the elements forecast is expected.
5.11.2.11.1 From (FM) Group (FMYYGGgg).
The change group FMYYGGgg (voiced as “from”) is used to indicate when prevailing conditions are expected to change significantly over a period of less than 1 hour. In these instances, the forecast is subdivided into time periods using the contraction FM, followed, without a space, by six digits, the first two of which indicate the day of the month and the final four, which indicate the time (in hours and minutes Z) the change is expected to occur.
While the use of a four-digit time in whole hours (e.g. 2100Z) is acceptable, if a forecaster can predict changes and/or events with higher resolution, then more precise timing of the change to the minute will be indicated. All forecast elements following FMYYGGgg relate to the period of time from the indicated date and time (YYGGgg) to the end of the valid period of the terminal forecast, or to the next FM if the terminal forecast valid period is divided into additional periods.
The FM group will be followed by a complete description of the weather
(i.e., self-contained), and all forecast conditions given before the FM group are superseded by those following the group. All elements of the TAF (surface wind, visibility, significant weather, clouds, obscurations, and when expected, Non-Convective LLWS) will be included in each FM group, regardless if they are forecast to change or not. For example, if forecast cloud and visibility changes warrant a new FM group but the wind does not, the new FM group will include a wind forecast, even if it is the same as the most recently forecast wind.
The only exception to this involves the significant weather group. If no significant weather is expected in the FM time period group, then significant weather group is omitted. A TAF may include one or more FM groups, depending on the prevailing weather conditions expected. In the interest of clarity, each FM group starts on a new line of forecast text, indented five spaces.
11/14/16 Aviation Weather Services AC 00-45H 5-88 Examples:
TAF KDSM 022336Z 0300/0324 20015KT P6SM BKN015 FM030230 29020G35KT 1SM +SHRA OVC005 TEMPO 0303/0304 30030G45KT 3/4SM -SHSN FM030500 31010G20KT P6SM SCT025...
A change in the prevailing weather is expected on the 3rd day of the month at 0230 UTC and the 3rd day of the month at 0500 UTC.
TAF KAPN 312330Z 0100/0124 13008KT P6SM SCT030 FM010320 31010KT 3SM -SHSN BKN015 FM010500 31010KT 1/4SM +SHSN VV007...
Note the wind in the FM010500 group is the same as the previous FM group, but is repeated since all elements are required to be included in a FM group.
5.11.2.11.2 TEMPO (YYGG/YeYeGeGe) Group.
The change-indicator group TEMPO YYGG/YeYeGeGe is used to indicate temporary fluctuations to forecast meteorological conditions that are expected to:
• Have a high percentage (greater than 50 percent) probability of occurrence;
• Last for one hour or less in each instance; and
• In the aggregate, cover less than half of the period YYGG to YeYeGeGe.
The first two digits (YY) are the day of the month for the start of the TEMPO. The next two digits (GG) are the starting hour (UTC). After the solidus (/), the next two digits (YeYe) are the ending day of the month, while the last two digits (GeGe) are the ending hour (UTC) of the TEMPO period.
Each TEMPO group is placed on a new line in the TAF. The TEMPO identifier is followed by a description of all the elements in which a temporary change is forecast. A previously forecast element that has not changed during the TEMPO period is understood to remain the same and will not be included in the TEMPO group. Only those weather elements forecast to temporarily change are required to be included in the TEMPO group.
TEMPO groups will not include forecasts of either significant weather in the vicinity (VC) or Non-Convective LLWS.
11/14/16 Aviation Weather Services AC 00-45H 5-89 Examples:
TAF KDDC 221130Z 2212/2312 29010G25KT P6SM SCT025 TEMPO 2215/2217 30025G35KT 1 1/2SM SHRA BKN010...
In the example, all forecast elements in the TEMPO group are expected to be different than the prevailing conditions. The TEMPO group is valid on the 17 from 1500 UTC to 1700 UTC.
TAF KSEA 091125Z 0912/1012 19008KT P6SM SCT010 BKN020 OVC090 TEMPO 0912/0915 -RA SCT010 BKN015 OVC040...
In this example the visibility is not forecast in the TEMPO group. Therefore, the visibility is expected to remain the same (P6SM) as forecast in the prevailing conditions group. Also, note that in the TEMPO 0912/0915 group, all three cloud layers are included, although the lowest layer is not forecast to change from the initial time period.
5.11.2.11.3 PROB30 (YYGG/YeYeGeGe) Group.
The probability group, PROB30 YYGG/YeYeGeGe, is only used by NWS forecasters to forecast a low probability occurrence (30 percent chance) of a thunderstorm or precipitation event and its associated weather and obscuration elements (wind, visibility, and/or sky condition) at an airport.
The PROB30 group is the forecaster’s assessment of probability of occurrence of the weather event that follows it. The first two digits (YY) are the day of the month for the start of the PROB30. The next two digits (GG) are the starting hour (UTC). After the solidus (/), the next two digits (YeYe) are the ending day of the month, while the last two digits (GeGe) are the ending hour (UTC) of the PROB30 period. PROB30 is the only PROB group used in NWS TAFs. Note that U.S. military and international TAFs may use the PROB40 (40 percent chance) group as well.
The PROB30 group is located within the same line of the prevailing condition group, continuing on the line below if necessary.
NWS TAFs do not use the PROB30 group in the first 9 hours of the TAF’s valid period, including amendments. Also, only one PROB30 group may be used in the initial forecast period and in any subsequent FM groups. Note that U.S. military and international TAFs do not have these restrictions.
PROB30 groups do not include forecasts of significant weather in the vicinity (VC) or Non-Convective LLWS.
11/14/16 Aviation Weather Services AC 00-45H 5-90 Example:
FM012100 18015KT P6SM SCT050 PROB30 0123/0201 2SM TSRA OVC020CB In this example, the PROB30 group is valid on the 1st day of the month at 2300 UTC to the 2nd day of the month at 0100 UTC.
5.11.2.11.4 TAF Examples.
TAF KPIR 111140Z 1112/1212 13012KT P6SM BKN100 WS020/35035KT TEMPO 1112/1114 5SM BR FM111500 16015G25KT P6SM SCT040 BKN250 FM120000 14012KT P6SM BKN080 OVC150 PROB30 1200/1204 3SM TSRA BKN030CB FM120400 14008KT P6SM SCT040 OVC080 TEMPO 1204/1208 3SM TSRA OVC030CB TAF Terminal Aerodrome Forecast KPIR Pierre, South Dakota 111140 Prepared on the 11th at 1140 UTC 1112/1212 Valid from the 11th at 1200 UTC until the 12th at 1200 UTC 13012KT Wind 130 at 12 kts P6SM Visibility greater than 6 sm BKN100 Ceiling 10,000 broken WS020/35035KT Wind shear at 2,000 ft, wind from 350 at 35 kts TEMPO 1112/1114 Temporary conditions between the 11th day of the month at 1200 UTC and the 11th day of the month at 1400 UTC 5SM Visibility 5 statute miles BR Mist FM111500 From the 11th day of the month at 1500 UTC 16015G25KT Wind 160 at 15 kts gusting to 25 kts P6SM Visibility greater than 6 sm SCT040 BKN250 4,000 scattered, ceiling 25,000 broken FM120000 From the 12th day of the month at 0000Z 14012KT Wind 140 at 12 kts P6SM Visibility greater than 6 sm BKN080 OVC150 Ceiling 8,000 broken, 15,000 overcast PROB30 1200/1204 30 percent probability between the 12th day of the month at 0000 UTC and the 12th day of the month at 0400 UTC 3SM Visibility 3 sm TSRA Thunderstorm with moderate rain showers BKN030CB Ceiling 3,000 broken with cumulonimbus FM120400 From the 12th day of the month at 0400 UTC
11/14/16 Aviation Weather Services AC 00-45H 5-91 14008KT Wind 140 at 8 kts P6SM Visibility greater than 6 sm SCT040 OVC080 4,000 scattered, ceiling 8,000 overcast TEMPO 1204/1208 Temporary conditions between the 12th day of the month at 0400 UTC and the 12th day of the month at 0800 UTC 3SM Visibility 3 sm TSRA Thunderstorms with moderate rain showers OVC030CB Ceiling 3,000 overcast with cumulonimbus
TAF AMD KEYW 131555Z 1316/1412 VRB03KT P6SM VCTS SCT025CB BKN250 TEMPO 1316/1318 2SM TSRA BKN020CB FM131800 VRB03KT P6SM SCT025 BKN250 TEMPO 1320/1324 1SM TSRA OVC010CB FM140000 VRB03KT P6SM VCTS SCT020CB BKN120 TEMPO 1408/1412 BKN020CB TAF AMD Amended Terminal Aerodrome Forecast KEYW Key West, Florida 131555Z Prepared on the 13th at 1555 UTC 1316/1412 Valid from the 13th at 1600 UTC until the 14th at 1200 UTC VRB03KT Wind variable at 3 kts P6SM Visibility greater than 6 sm VCTS Thunderstorms in the vicinity SCT025CB BKN250 2,500 scattered with cumulonimbus, ceiling 25,000 broken TEMPO 1316/1318 Temporary conditions between the 13th day of the month at 1600 UTC and the 13th day of the month at 1800 UTC 2SM Visibility 2 sm TSRA Thunderstorms with moderate rain showers BKN020CB Ceiling 2,000 broken with cumulonimbus FM131800 From the 13th day of the month at 1800 UTC VRB03KT Wind variable at 3 knot s P6SM Visibility greater than 6 sm SCT025 BKN250 2,500 scattered, ceiling 25,000 broken TEMPO 1320/1324 Temporary conditions between the 13th day of the month at 2000 UTC and the 14th day of the month at 0000 UTC 1SM Visibility 1 sm TSRA Thunderst orms with moderate rain showers OVC010CB Ceiling 1,000 overcast with cumulonimbus FM140000 From the 14th day of the month at 0000 UTC VRB03KT Variable wind at 3 knots P6SM Visibility greater than 6 sm VCTS T hunderstorms in the vicinity SCT020CB BKN120 2,000 scattered with cumulonimbus, ceiling 12,000 broken