Chapter 27, Forecasts 27-10
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 km (6 SM) or more ; no clouds below 1,500 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.
27.4.2.7 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) included 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 temporary (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.
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 a
sandstorm (SS) and dust storm (DS).
Chapter 27, Forecasts 27-11
27.4.2.7.1 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 phenomenon:
• -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.
27.4.2.7.2 Thunderstorm Descriptor
The TS descriptor is treated differently than other descriptors in the following cases:
• When nonprecipitating 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.
Chapter 27, Forecasts 27-12
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
27.4.2.7.3 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 to 6 SM, the phenomenon 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).
Fog-related terms are used as described in Table 27-4.
Table 27-4. 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 ft AGL 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 ft AGL.
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.
27.4.2.8 Vicinity (VC)
In the United States, vicinity ( VC) is defined as a donut -shaped area between 5 SM and 10 SM from the
center of the airport ’s runway complex. 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
Chapter 27, Forecasts 27-13
equal to 50 percent probability of occurrence for more than half of the subdivided forecast time period. VC
is not included in temporary (TEMPO) or probability (PROB) groups.
The significant weather phenomena in Table 27-5 are valid for use in prevailing portions of NWS ’ TAFs
in combination with VC.
Table 27-5. 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.
27.4.2.9 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.
27.4.2.9.1 Cloud Group (NsNsNshshshs)
The cloud group (NsNsNshshshs) is used to forecast cloud amount as indicated in Table 27-6.
Table 27-6. 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
When 0 oktas of sky coverage are forecasted, 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 an ASOS or AWOS contain
the cloud amount and/or obscurations, which the forecaster expects, not what is expected to be reported by
an ASOS/AWOS.
Chapter 27, Forecasts 27-14
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.
27.4.2.9.2 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.
27.4.2.9.3 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 the 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 a 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.
27.4.2.9.4 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, espec ially 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.
Chapter 27, Forecasts 27-15
The format of the non-convective LLWS group is:
WShwshwshws/dddffKT
WS Indicator for non-convective LLWS.
Hwshwshws Height of the top of the wind shear 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 kt changes to 270° (west)
at 55 kt 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 an FM group in a TAF whenever:
• One or more PIREPs 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 kt or more, and the
forecaster determines the report(s) reflect a valid non-convective LLWS event rather than
mechanical turbulence; or
• Non-convective vertical wind shear of 10 kt or more per 100 ft in a layer more than 200 ft thick is
expected or reliably reported within 2,000 ft of the surface at, or in the vicinity of, the airport.
27.4.2.10 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
forecast elements is expected.
27.4.2.10.1 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 peri od of less than one 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 indicate the time (in hours and minutes Z) the
change is expected to occur. While the us e 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 prec ise 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 ( e.g., surface wind, visibility, significant weather, clouds, obscurations, and when expected,
Chapter 27, Forecasts 27-16
non-convective LLWS) will be included in each FM group, regardless of 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.
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 third day of the month at 0230 UTC and the third 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 that the wind in the FM010500 group is the same as the previous FM group but is repeated since all
elements are to be included in a FM group.
27.4.2.10.2 Temporary (TEMPO) Group (TEMPO YYGG/YeYeGeGe)
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 included in the
TEMPO group.
TEMPO groups will not include forecasts of either significant weather in the vicinity ( VC) or
non-convective LLWS.
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 22nd day of the month from 1500 UTC to 1700 UTC.
Chapter 27, Forecasts 27-17
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.
27.4.2.10.3 Probability (PROB) Group (PROB30 YYGG/YeYeGeGe)
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 (e.g., 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: The 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.
Only one PROB30 group may be used in the initial forecast period and in any subsequent FM groups. Note
that the 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.
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 second day of
the month at 0100 UTC.
27.4.2.10.4 TAFs for Joint-Use (Joint Civilian/Military) Airports
The TAF format at some joint-use airports is different from the NWS TAF format as follows:
• Visibility is in meters instead of statute miles.
o Example: BECMG 0504/0505 21006KT 9000 BR SKC QNH3005INS.
• Includes a forecast of the lowest barometric altimeter setting (QNH) during the forecast period in
inches of mercury.
o Example: BECMG 0504/0505 21006KT 9000 BR SKC QNH3005INS.
• Includes a forecast of the maximum temperature (in whole degrees C elsius) and expected time of
occurrence.
o Example: BECMG 0511/0512 16005KT 9999 NSW SKC QNH3006INS TX28/0420Z
TN22/0410Z.
• Includes a forecast of the minimum temperature (in whole degrees C elsius) and expected time of
occurrence.
Chapter 27, Forecasts 27-18
o Example: TN22/0410Z BECMG 0511/0512 16005KT 9999 NSW SKC QNH3006INS
TX28/0420Z TN22/0410Z.
27.4.3 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 day of the month at 1140 UTC.
1112/1212 Valid from the 11 th day of the month at 1200 UTC until the
12th day of the month at 1200 UTC.
13012KT Wind 130° true at 12 kt.
P6SM Visibility greater than 6 SM.
BKN100 Ceiling 10,000 ft broken.
WS020/35035KT Wind shear at 2,000 ft, wind from 350° true at 35 kt.
TEMPO 1112/1114 Temporary conditions between the 11 th day of the month at
1200 UTC and the 11th day of the month at 1400 UTC.
5SM Visibility 5 SM.
BR Mist.
FM111500 From the 11th day of the month at 1500 UTC.
16015G25KT Wind 160° true at 15 kt gusting to 25 kt.
P6SM Visibility greater than 6 SM.
SCT040 BKN250 4,000 ft scattered, ceiling 25,000 ft broken.
FM120000 From the 12th day of the month at 0000Z.
14012KT Wind 140° true at 12 kt.
P6SM Visibility greater than 6 SM.
BKN080 OVC150 Ceiling 8,000 ft broken, 15,000 ft overcast.
PROB30 1200/1204 30 percent probability between the 12 th 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 ft broken with cumulonimbus.
FM120400 From the 12th day of the month at 0400 UTC.