MANAIR Manual of Standards and Procedures for Aviation Forecasts: chapter 2


2.1 Definition

The aerodrome forecast (TAF) is the forecaster’s best judgment of the most probable weather conditions expected to occur at an aerodrome together with their most probable time of occurrence. It is designed to meet the preflight and in-flight requirements of flight operations. Aerodrome forecasts are intended to relate to weather conditions for flight operations within five nautical miles of the centre of the runway complex depending on local terrain.

2.2 Aeronautical Meteorological Forecaster (AMF) responsibility

An AMF responsible for the production of TAFs for Canada shall produce those forecasts using the standards and recommended practices concerning weather forecasts and observations identified in the Canadian Aviation Regulations (CAR).

2.3 Observational requirements

Aerodrome forecasts are issued for aerodromes for which regular hourly and special weather reports meeting standards for observations, as stated in the Manual of Surface Weather Observations (MANOBS), are available. Aerodrome forecasts based on automated observations shall be issued only for acceptable Automated Weather Observation System (AWOS) sites. They are recognizable by the label “AUTO” in the corresponding METAR or SPECI report.

2.3.1 Parameters required to establish a TAF service

The meteorological parameters required to establish a TAF service are the following:

  • sky condition
  • visibility
  • present weather and obstructions to vision
  • air temperature
  • dew point temperature
  • wind speed, direction and character
  • mean sea level (MSL) pressure

Regular hourly and special weather observations are only one of the data sources available to forecasters. Therefore, when it comes to maintaining an already established aerodrome forecast, no single element is necessarily critical.

After analyzing available data from other sources (e.g. satellite imagery, radar pictures, profiler data), if, in the forecaster’s judgment, one missing observation or a missing element will have no impact on the quality of the aerodrome forecast, the forecast can be maintained.

2.4 Time references

All time references shall be stated in Coordinated Universal Time (UTC).

2.4.1 Issue time

The issue time of an aerodrome forecast is the time that it is released to the telecommunication system. To facilitate flight planning, aerodrome forecasts shall be transmitted on telecommunication system as close as possible to, but not later than 20 minutes prior to the beginning of their period of validity.

For those aerodromes which do not have a 24-hour observing program, two consecutive hourly observations immediately prior to the issue time of the forecast are required before issuing a forecast. Depending on forecast validity, the above situation can be accomplished in one of two ways:

  1. After receiving the second hourly observation, the TAF is issued as soon as possible, for example, 15 minutes after the second observation is received. The period of validity of such a TAF is backdated to begin on the whole hour prior to the issue time of the forecast. The forecast, however, is valid from the time that it is received. For example, after receiving the 13Z and 14Z observations for CZFA, the TAF is issued as: TAF  CZFA 201415Z 2014/2101 …
  2. After receiving the second hourly observation, the TAF is issued approximately 40 minutes after the hour of the observation. In this case, the period of validity of such a TAF is postdated to begin on the whole hour following the issue time of the forecast. For example, after receiving the 10Z and 11Z observation for Muskoka, a TAF is issued as: TAF  CYQA  201140Z  2012/2100

If two consecutive hourly observations are not available immediately prior to the issue time of a TAF, a nil TAF shall be issued, e.g. TAF  CYTS  051635Z 0517/0605 FCST NOT AVBL DUE NO OBS. Alternatively, if only one observation is available, the nil TAF would read TAF CYTS 051635Z 0517/0605 FCST NOT AVBL DUE INSUFFICIENT OBS.

In exceptional circumstances (e.g. where there is a nearby weather station providing routine hourly weather reports) forecasts may be issued after receipt of the first hourly report.

TAFs are normally scheduled to be updated six hours after the previous issue time. Under no circumstance shall a TAF be scheduled more than seven hours after the previous issue time.

Once a TAF for an aerodrome has been cancelled due to unavailable or insufficient observations, the TAF for that aerodrome shall not be reissued until a minimum of one hourly observation (in VFR situations) is received or a maximum of two consecutive hourly observations (in below VFR situations) representative of the aerodrome are received. The two observations must be consecutive and must be not more than one hour apart (e.g. two regular hourly observations). In both cases, the forecast shall be issued within 20 minutes after the appropriate observation is received.

2.4.2 Time of validity

An aerodrome forecast shall be valid from the moment it is issued (e.g. a TAF with an indicated period of validity from 11Z to 23Z issued at 1040Z is considered to be valid from 1040Z) until it is amended, or until the next scheduled TAF for the same aerodrome is issued, or until the valid period is ended with no new TAFs issued. This avoids amending the old TAF 10 to 15 minutes before the valid time of the new one.

Note: When a new TAF is issued, it automatically cancels the previous one.

2.5 Format of the Canadian Aerodrome Forecast (TAF)

The information included in a Canadian aerodrome forecast (TAF) is presented in a fixed order as described in 2.5.1 and 2.5.2. For a description of the WMO international TAF code, refer to the Manual on Codes, Volume I.1 - Part A - Alphanumeric Codes (WMO-No. 306).

2.5.1 Telecommunication header

TAF bulletins begin with a WMO abbreviated telecommunication header (as required by the Manual on Global Telecommunication System [WMO-No. 386]). The normal source input header for a TAF bulletin is of the following form “FTCNii  CWAO  ddhh00  (BBB)” where:

  • ii is the bulletin number;
  • CWAO refers to the Canadian Meteorological Centre/Network Operations (CMC/NETOPS) as the centre compiling the bulletins;
  • ddhh00 is the date and time of the forecast, in whole hours UTC, that precedes the time of entry to the collection circuit, as required by the Global Telecommunication System (GTS) of the WMO (e.g. FTCN32 CWAO 101800); and
  • BBB is an indicator used for amendments, corrections, or delayed bulletins and will be added automatically by the national bulletin preparation software to all TAFs scheduled for transmission to the GTS. The inclusion of this term is required by international protocol, as outlined in the Manual on the Global Telecommunication System, Volume I, Attachment II-12 - Instructions for the Use of the Indicator BBB (WMO-No. 386), BBBwill take the form AA(x) for amendments, CC(x) for corrections, and RR(x) for delayed bulletins. The letter (x)will take the value of “A” for the first amendment, correction or delayed bulletin, the value “B” for the second and so on (e.g. FTCN32 CWAO  201800  AAA,  FTCN33  CWAO  182100  CCA, etc.).

Note: The time that appears in the telecommunication header shall always be in whole hours Universal Coordinated Time (UTC) preceding the transmission time, even for amended, corrected or delayed bulletins. This rule is outlined in the Manual on the Global Telecommunication System, Volume I - Global Aspects, Part II, p. 2.3.2.2 (WMO-No. 386).

2.5.2 Symbolic form

The symbolic form of a Canadian aerodrome forecast in TAF code is:

TAF  bbb CCCC  YYGGggZ  Y1Y1G1G1/Y2Y2G2G2 dddffGfmfmKT
WShxhxhx/ddffKT  VVVV {w’w’
or NSW}  {NSNSNShShShSor VVhShShS or SKC }
PROBC2C2 YbYbGbGb/YeYeGeGe
TEMPO  YbYbGbGb/YeYeGeGe
{BECMG  YbYbGbGb/YeYeGeGe}
or {FMYYGGgg}
RMK

Note (1): Groups included in braces { } and separated by the word “or” represent mutually exclusive options.

Note (2): Each individual time period beginning with FMYYGGggshall start a new line and indented four spaces, while continuation lines shall be indented five spaces.

Note (3): The length of a line shall not exceed 69 characters. A period is not required at the end of every TAF. An equal “=” sign, indicates the end of the message to comply with the GTS rules.

2.5.2.1 Partial program format

For an aerodrome operating a partial observing program (e.g. no nighttime observations), an appropriate remark from the list below shall be included in the last regular issue of the TAF as follows:

  • NO FCST COVERAGE d1d1h1h1m1m1-d2d2h2h2m2m2Z(e.g. NO FCST COVERAGE 130500-131000Z)
  • NXT FCST WILL BE ISSUED AT ddhhmmZ (e.g. NXT FCST WILL BE ISSUED AT 181000Z)
  • NO FCST ISSUED UFN

Where:

  • d1d1h1h1m1m1is the time (UTC, date, hour and minutes) the observing program is no longer available;
  • d2d2h2h2m2m2 is the time (UTC, date, hour and minutes) at which the TAF will be available;
  • dd is the date in which the next TAF will be issued; and
  • hhmm is the time (UTC, hour and minutes) at which the next TAF will be issued.

Note: The preceding list is not meant to be exhaustive; forecasters may utilize other expressions which properly describe the situation. If, for unexpected reasons, the observing program should end earlier or start later than expected, an amended TAF shall be issued as described in 2.10.1.

2.5.3 Definition of symbols

Only a broad overview of the description of the terms is provided in this section. More details regarding the preparation and the coding of aerodrome forecast are provided in 2.6.

Table 1: Definition of symbols
Symbol Description
TAF Bulletin identifier (refer to 2.6.1).
bbb Amendment or correction indicator (refer to 2.6.2).
CCCC International aerodrome identifier (refer to 2.6.3).
YYGGggZ Date/time (UTC) of origin of the TAF (refer to 2.6.4).
Y1Y1G1G1/Y2Y2G2G2 Date and period of validity of the forecast (refer to 2.6.5).
dddffGfmfmKT Surface wind speed and direction with gusts (refer to 2.6.6)
WShXhXhX/dddffKT Low level wind shear group (refer to 2.6.7)
VVVV Horizontal prevailing visibility at the surface (refer to 2.6.8)
w’w’ Significant forecast weather (refer to 2.6.9)
NSW Abbreviation for no significant weather (refer to 2.6.10)
NSNSNShShShS Cloud group (refer to 2.6.11.1&2)
VVhShShS Vertical visibility (refer to 2.6.11.3)
SKC Abbreviation for sky clear (refer to 2.6.11.4)
PROBC2C2 YbYbGbGb/YeYeGeGe Probability group (refer to 2.6.12)
TEMPO  YbYbGbGb/YeYeGeGe Temporary change group indicator (refer to 2.6.13)
FMYYGGgg Rapid and permanent change group indicator (refer to 2.6.14.1)
BECMG  YbYbGbGb/YeYeGeGe Gradual but permanent change group indicator (refer to 2.6.14.2)
RMK Remarks (refer to 2.6.15)

2.6 TAF Forecast preparation and coding

The description of forecast conditions shallalways contain, as a minimum, information about the following elements:

  • wind
  • visibility
  • weather
  • cloud

2.6.1 Aerodrome forecast message identifier (TAF)

The code name “TAF” (Aerodrome Forecast) shall be included at the beginning of each TAF (e.g. TAF  CYUL  191635Z  1917/2017 …).

2.6.2 Amendment/Correction indicator (bbb)

The indicator “bbb” takes the form of “AMD” and is used for both amendments and corrections and is placed after the term TAF followed by one space (e.g. TAF  AMD  CYUL…).

2.6.3 Location identifier group (CCCC)

The group “CCCC” is the international four-letter aerodrome identifier (for a full list of international identifiers, see ICAO Document 7910: ICAO Location Indicators (DOCS 7910). For Canadian aerodromes the first letter of this group is always a “C” (e.g. TAF  CYUL …).

2.6.4 Date/Time group (YYGGggZ)

The day/time group “YYGGggZ” is the date and time of origin of the forecast (e.g. TAF  CYUL  191635Z …). This term is considered optional by WMO; however, Canada and the USA have agreed to include it. This group will be inserted automatically by the national bulletin preparation software after the forecaster sends the forecast.

2.6.5 Date and period of validity group (Y1Y1G1G1/Y2Y2G2G2)

The group “Y1Y1G1G1/Y2Y2G2G2” represents the period of validity of the forecast, where:

  • Y1Y1G1G1is the date and hour (UTC) of the beginning of the period of validity; and
  • Y2Y2G2G2is the date and hour (UTC) of the end of the period of validity (e.g. TAF CYUL 191640Z 1917/2017).

2.6.6 Wind group (dddffGfmfmKT)

The mean direction and speed of the forecast wind shall be indicated by “dddff” where ddd is the wind direction given to the nearest 10 degrees true, and ffis the wind speed given in knots. Except as stated below, the group dddff is immediately followed, without a space, by the letter code indicator KT to indicate the wind speed unit (knots).

When it is forecast that the maximum wind speed (gust, symbol G) will exceed the mean speed by 10 KT or more, the maximum wind speed shall be indicated by adding Gfmfm between dddff and KT. There is no space either before or after Gfmfm (e.g. 32020G30KT).

The code “dddshall normally be encoded as variable (VRB) only when the mean wind speed is three knots or less (e.g. VRB03KT). A variable wind at higher speeds may also be indicated during strong convective activity (e.g. when a thunderstorm passes over an aerodrome).

A north wind of 20 KT shall be indicated as 36020KT, while a calm wind shall be indicated as 00000KT.

For wind speeds (mean and/or maximum gust) of 100 KT or greater, the exact value of the speed shall be given using three digits instead of two digits (e.g. 320105G130KT).

2.6.7 Low level wind shear group (WShXhXhX/dddffKT)

While the main effect of turbulence is related to erratic changes in altitude and/or attitude of the aircraft, the main effect of wind shear is the rapid gain or, more critical, loss of air speed. Therefore, for forecasting purposes, any cases of non-convective low level wind shear within 1,500 feet AGL will be labeled as WS. This group is not part of the international code but it will be used in North American TAFs (this difference has been filed with the ICAO by Canada).

The low level wind shear group shall be included in a TAF whenever the forecaster has strong evidence to expect significant, non-convective wind shear which could adversely affect aircraft operation within 1,500 feet AGL.

To a large extent, wind shear is an element which, for the time being, cannot be satisfactorily observed from the ground. As a result, aircraft observations and radiosonde reports represent the only available evidence. However, the following guidelines could be used at the forecaster’s discretion to establish whether significant non-convective wind shear hazardous to aircraft exists:

  1. Vector magnitude exceeding 25 KT within 500 feet AGL
  2. Vector magnitude exceeding 40 KT within 1,000 feet AGL
  3. Vector magnitude exceeding 50 KT within 1,500 feet AGL
  4. A pilot report of loss or gain of indicated air speed of 20 KT or more within 1,500 feet AGL

The low level wind shear group, when included, shall be encoded as WShXhXhX/dddffKTwhere:

  • WS means low level wind shear;
  • hXhXhXrepresents the height (above ground) of the layer in which the low level wind shear is expected (e.g. 015 means 1500 feet above ground);
  • ddd is the wind direction at level hXhXhX (e.g. 240 means 240 degrees); and
  • ffKT is the wind speed, in knots (KT), at level hXhXhX (e.g. 45KT means 45 KT).

Note: When the wind speeds at hXhXhX is expected to be 100 KT or higher, a three-digit figure shall be used (e.g. 120KT). Wind gusts are not allowed in this group.

2.6.8 Visibility group (VVVV)

The horizontal prevailing visibility group “VVVVshall always be indicated in statute miles and fractions up to three miles, then in whole miles up to six miles (e.g. 21/2SM SHSN, meaning visibility two and a half statute miles in light snow shower). The letters SM (statute miles) shall be added without a space to each forecast visibility to identify the unit.

Visibility values allowed are the following:

  1. zero to 3/4SM (in increments of one eighth [1/8] statute mile)
  2. 3/4SM to 21/2SM (in increments of one quarter [1/4] statute mile)
  3. 3SM to 6SM (in increments of one [1] statute mile)
  4. P6SM (for visibility greater than six [6] statute miles)

When whole miles and fractions of a mile are used in combination, no space shall be placed between the whole mile and the fraction (e.g. 11/2SM; 21/2SM).

2.6.9 Significant weather group (w’w’)

The significant weather group “w’w’ ” is comprised of weather phenomena which may contain one or more forms of precipitation, obscuration or other (refer to Table 2, columns 3 to 5). Weather phenomena are preceded by one or more qualifiers, one of which describes the intensity or the proximity to the station of the phenomena. The other qualifier is a descriptor and describes the quality of the weather phenomena (refer to Table 2, columns 1 to 2).

Table 2: Significant present and forecast weather

Qualifier

Intensity or proximity

Column 1

Qualifier

Descriptor

Column 2

Weather
phenomena

Precipitation

Column 3

Weather
phenomena

Obscuration

Column 4

Weather
phenomena

Other

Column 5

- Light

Moderate
(no qualifier)

+ Heavy

VC In the Vicinity

MI Shallow

BC Patches

PR Partial (covering part of the aerodrome)

DR Low Drifting

BL Blowing

SH Shower(s)

TS Thunderstorm

FZ Freezing (supercooled)

DZ Drizzle

RA Rain

SN Snow

SG Snow Grains

IC Ice Crystals (Vis ≤ 6 SM)

PL Ice Pellets

GR Hail

GS Small hail and/or snow pellets

BR Mist (Vis ≥ 5/8 SM)

FG Fog (Vis < 5/8 SM)

FU Smoke
(Vis ≤ 6 SM)

DU Dust (Vis ≤ 6 SM)

SA Sand (Vis ≤ 6 SM)

HZ Haze (Vis ≤ 6 SM)

VA Volcanic Ash (with any visibility)

PO Dust/Sand Whirls (dust devils)

SQ Squalls

+FC Tornado or water-spout

FC Funnel Cloud(s)

SS Sandstorm (Vis ½ SM and 3/8 SM)

+SS Sandstorm
(Vis ¼ SM or less)

DS Duststorm (Vis ½ SM and 3/8 SM)

+DS Duststorm (Vis ¼ SM or less)

Note: Precipitation intensity refers to all forms combined.

Note: The forecast w’w’ group(s) shall be indicated by considering columns 1 to 5 in the table above in sequence, that is, intensity followed by weather phenomena (e.g. +SHRA for heavy shower of rain).

A w’w’ group shall be indicated by selecting appropriate items from column 1 to 5 of Table 2 (in sequence) which describe the forecast weather. The w’w’group shall be ordered as follows:

  1. first, if appropriate, the qualifier for intensity or proximity; followed without a space by, if appropriate,
  2. the abbreviation for the descriptor; followed without a space by
  3. the abbreviation for the forecast weather phenomenon or combination thereof.

The w’w’ group shall be omitted if no significant weather is expected. The w’w’ group shall also be omitted after a change group of the form TEMPO/BECMG YbYbGbGb/YeYeGeGewhen it is not expected to differ from the preceding value. For example …3SM BR SKC BECMG 0815/0816 …BKN020 …This implies that the visibility/weather group 3SM BR remains unchanged after the BECMG.

When two or three weather elements are indicated, the combined weather group will be considered as one entity. When one element is expected to change, the entire group shall be repeated. For example: …4SM BR … TEMPO …4SM -RA BR. The BR is repeated because another element, namely -RA, is introduced. In addition, the visibility is also repeated because is considered to be an integral part of the weather group.

In case of a “significant change” in visibility, the entire weather group shall also be repeated. A significant change (visibility, weather, cloud amounts, etc.) is a change for which, under normal circumstances, an amendment would be required.

In this example, ...3SM -RA BR TEMPO 2108/2112 1SM -RA BR... -RA and BR are indicated before and after the change group TEMPO, because a significant change in visibility is forecast.

Similarly, in the case of ...5SM -RA BR TEMPO 2108/2112 4SM -SN BR..., the BR is repeated because a significant change in precipitation is forecast.

2.6.9.1 Qualifiers and descriptors
Note: When associated with the proximity ( VC ) indicator, the intensity of precipitation shall not be specified.
Note : When SN and BLSN are expected together, they shall be indicated in separate w’w’ groups (e.g. 5SM -SN BLSN ).
  1. The “intensity qualifier” (Table 2 column 1) refers to the total precipitation of the weather group and is coded as plus “+” or minus “-”, meaning heavy or light as described in MANOBS. When the intensity of the phenomena is expected to be moderate, no indicator sign shall be indicated in the group.
  2. In “precipitation types where the intensity is not applicable such as ice crystals (IC), no intensity shall be included in the TAF. The intensity qualifier shallbe indicated only with the following:
    • precipitation
    • blowing dust/sand/snow (BLDU/BLSA/BLSN), only when the intensity is moderate or heavy
    • dust storm or sand storm (DS/SS)
  3. The “proximity (VC) qualifier (Table 2, column 1) is used only when the following phenomena are expected to occur within 5 to 10 nautical miles from the centre of the runway complex:
    • dust storm (DS)
    • sand storm (SS)
    • fog (FG)
    • funnel cloud(s) (FC)
    • tornadoes or waterspouts (+FC)
    • thunderstorm (TS)
    • shower (SH)
    • dust/sand whirls (PO)
    • blowing dust/sand/snow (BLDU/BLSA/BLSN)
  4. “Descriptors” (Table 2, column 2): Not more than one descriptor shall be included in any w’w’ group (e.g. -FZDZ). When more than one group is indicated, only one descriptor per group may be used (e.g. -FZDZ, SHSNRA, +BLSN, etc.).
    • The descriptors shallow (MI), patches (BC) and partial (PR) shall be used only to forecast fog (FG). For example:
      • the term MIFG is used when the visibility within the fog layer between the ground and two metres above the ground level is expected to be less than 5/8 SM while the visibility above that layer is greater than 5/8 SM;
      • the term BCFG is used to forecast fog patches covering only part of the aerodrome and the visibility in the fog patch is expected to be less than 5/8 SM with the fog layer extending at least to two meters above ground level; and
      • the term PRFGshall be used to forecast fog covering a substantial part of the aerodrome while the remainder of the aerodrome is expected to be free of fog PRFG (fog bank).
    • The descriptors low drifting (DR) shall be used only in combination with dust (DU), sand (SA) or snow (SN) when these are raised by the wind to less than two meters above the ground.
    • The descriptor BL (blowing) shall be used only in combination with DU, SA or SN to forecast dust, sand or snow raised by the wind to a height of two meters or more above the ground.
    • Shower (SH) shall be used only in combination with one or more of the precipitation types, rain (RA), snow (SN), ice pellets (PL), snow pellets (GS), and/or hail (GR).
    • Thunderstorm (TS) must either be stated alone or in combination with one or more of the precipitation types RA, SN, PL, GS, and GR.
    • Super-cooled, freezing (FZ) is used in combination with the weather types FG (e.g. FZFG when the visibility is expected to be less than 5/8 of a mile and the temperature is expected to be below freezing, except when the fog is composed entirely of ice crystals), drizzle (DZ), and RA (e.g. -FZRA).

Note: For a more detailed description of the above terms refer to the Manual of Surface Weather Observations (MANOBS).

2.6.9.2 Weather phenomena (Table 2, columns 3 to 5)
2.6.9.2.1 Precipitation

Types of “Precipitation” (Table 2, column 3) which are allowed in the TAF are the following:

  • drizzle (DZ)
  • rain (RA)
  • snow (SN)
  • snow grains (SG)
  • ice crystals or diamond dust (IC) when the visibility is expected to be 6 SM or less
  • ice pellets (PL)
  • hail (GR)
  • snow pellets (GS)

When more than one type of precipitation (Table 2, column 3) is forecast, the appropriate abbreviations shall be combined in a single group, according to regulations stated in this section, with the most significant type of precipitation being stated first. Not all precipitation types can be combined. Only RA, SN, SG, PL, GR, and GS(e.g. SGPL, SNRA, PLSN, etc.) can be combined, while DZand IC cannot (e.g. RADZ, SNIC, etc. are not allowed). In such a single group, the intensity shall refer to the total precipitation (-SHSNRA) where SN is the most significant type of precipitation.

When more than one significant weather phenomenon other than a precipitation combination mentioned above is forecast, separate w’w’ groups, but not more than three, shall be included in the forecast in accordance with Table 2 (e.g. -FZDZ -SGSN BR, etc.).

2.6.9.2.2 Obscuration

Obscuring phenomena (Table 2, column 4) are forecast whenever the prevailing visibility is expected to be six statute miles or less. The only exception is volcanic ash (VA) which, when expected, shall always be indicated regardless of visibility.

2.6.9.2.3 Other

Other weather phenomena (Table 2, column 5) are classified as:

  • dust/sand whirls (PO)
  • squalls (SQ)
  • funnel clouds(s) (FC)
  • tornadoes or waterspouts (+FC)
  • sand storm (SS)
  • dust storm (DS)

2.6.10 Alternative term NSW

The abbreviation “NSW” (no significant weather) is an alternate term for w’w’. It is used to replace the w’w’ group after a change group of the form TEMPO/BECMGYbYbGbGb/YeYeGeGeto indicate the end of significant weather phenomena indicated in the w’w’ group prior to the change. The term NSWshall not be used in the initial forecast period.

2.6.11 Cloud and obscuration groups (NSNSNShShShSand VVhShShS)

The cloud or obscuration group “NSNSNShShShS” or “VVhShShSshall be used in the initial time period and in any subsequent FM group(s) to indicate cumulative cloud amounts NSNSNS and the height above ground hShShS of the base of cloud layers in units of 100 feet. When the sky is totally obscured, the cloud group is replaced with the vertical visibility group VVhShShS. In the absence of clouds, the term SKC (sky clear) shall be used.

2.6.11.1 Cloud amount (NSNSNS)

The cloud amount “NSNSNSshall be given as SKC, FEW, SCT, BKN, and OVC as indicated in Table 3.

The code “NSNSNS”represents the total (cumulative) amount of cloud that the forecaster expects to occur at the level up to and including hShShS.

The group “NSNSNShShShS” is limited to a maximum of three significant layers of cloud. The only exception is for cumulonimbus clouds (CB), which shall always be indicated when expected.

In case of a significant change in a cloud layer, the entire cloud group, including those cloud layers not expected to change, shall be repeated. A significant change in the clouds is intended to mean a change for which an amendment would be required.

Table 3: Sky cover terms
Abbreviation Cloud amount in oktas
SKC No Cloud
FEW Trace to 2 Inclusive
SCT 3 to 4 Inclusive
BKN 5 to 7 Inclusive
OVC 8

Towering cumulus (TCU) and altocumulus castellanus (ACC) shall not be forecast. Only CBs shall be forecast. When expected, a CB cloud shall be indicated by adding the letter abbreviations “CB” to the cloud group without a space (e.g. ...BKN020CB...).

2.6.11.2 Cloud height (hShShS)

The code “hShShS” represents the height of the base of the cloud layers, or the vertical visibility in a surface based layer in hundreds of feet above the ground. Cloud height (hShShS) shall be forecast according to the following intervals:

  1. 100-foot increments from the surface to 1,500 feet
  2. 500-foot increments from 1,500 feet to 3,000 feet
  3. 1,000-foot increments above 3,000 feet
2.6.11.3 Vertical visibility group (VVhShShS)

When the sky is expected to be obscured, the group “VVhshshsshall be used in lieu of NSNSNShShShS. In this case, VV is the indicator for vertical visibility and hshshs is the value of the vertical visibility in units of 100 feet, e.g. VV005, meaning vertical visibility is five hundred feet or obscured ceiling at five hundred feet above ground level (AGL).

2.6.11.4 Alternative term SKC

The term sky clear “SKC" shall be used in Canada to forecast the absence of cloud or obscuration at the beginning of any self-contained part period. The reason for this practice is to avoid giving the impression that the group was inadvertently left out. In addition the term SKCshall be used to replace the cloud or vertical visibility group after a change of the form TEMPO/BECMG YbYbGbGb/YeYeGeGeto indicate that the cloud or the obscuration is no longer expected.

2.6.12 Probability group (PROBC2C2YbYbGbGb/YeYeGeGe)

In order to indicate the probability of occurrence of an alternative value(s) of a forecast condition, the group PROBC2C2YbYbGbGb/YeYeGeGewill be stated immediately before the alternative value(s).

The code “C2C2” represents the numerical probability, in percent, of the alternative value of the weather condition. There is no space between PROB and C2C2. Only the values 30 and 40 shall be allowed to indicate the probability 30% and 40% respectively.

A probability of less than 30% of actual values deviating from those forecasts is not considered to justify the use of the group PROBC2C2. When the probability of an alternative value is 50% or more, this shall be indicated by the use of BECMG, TEMPO, or FM as appropriate.

The probability group shall be used to forecast weather phenomena that may adversely affect aircraft operations. These include the following:

  • thunderstorm
  • freezing precipitation, ice pellets, and snow grains
  • low level wind shear (below 1500 feet AGL)
  • ceiling and visibility values important to aircraft operations (e.g. thresholds such as alternate and lowest approach limits)

Only one PROBC2C2 group is allowed per self-contained part period. The probability group PROBC2C2shall not be used as a direct modifier of BECMG or TEMPO YbYbGbGb/YeYeGeGe(e.g. …PROB40 TEMPO 0915/0918… or …PROB30 BECMG 0915/0916…is not allowed).

The combination FM (condition A) PROB (condition B) BECMG (condition C) is allowed, provided that the time period of the PROB group ends on or before the time the BECMG group begins, e.g. (condition A) PROB40 0918/0922(condition B) BECMG 0922/0923(condition C). The combination FM(condition A) PROB (condition B) TEMPO (condition C) is not allowed.

The time period of the PROBgroup cannot cross two self-contained part periods, i.e., the combination … FM271900(condition A) PROB 2719/2723(condition B) FM272200Z(condition C)… is not allowed.

An exception to this rule, however, is allowed when hours and fraction of one-hour are used in the FM group. For example, the combination ….(condition A) PROB 2718/2720(condition B) FM271930 (condition C) … is allowed to indicate that the fluctuation in the weather condition stated in the PROBgroup will last until the beginning of the next self-contained part period, in this case, 271930Z.

Similarly, the combination … FM271930 (condition A) PROB 2719/2723 (condition B) … is allowed to indicate that the fluctuation in the weather condition stated in the PROB group is expected to start at the beginning of the self-contained part period, in this case, 271930Z.

Note: The reason for the above exceptions is that in a PROB group only whole hours are allowed to identify the period during which the possibility of some weather event may occur. In both the above cases, the overlap shall be less than one hour. This practice will give forecasters greater flexibility in the use of PROB in conjunction with an FM group not starting on a whole hour.

In any of the above and following examples, the ending of a group is to be understood as one minute prior to the time stated. For example, (condition A) PROB30 2718/2721 (condition B) BECMG 2723/2800 (condition C) is to be understood as a 30% probability of condition B existing from 271800Z to 272059Z  and a permanent change from condition A to condition C between 272300Z and 272359Z.

2.6.13 Transitory change groups TEMPO YbYbGbGb/YeYeGeGe

The transitory change group “TEMPO YbYbGbGb/YeYeGeGeshall be used when a temporary fluctuation in some or all of the elements forecast is expected to occur during the period YbYbGbGbto YeYeGeGe. This group shall be used only when the modified forecast condition is expected, in each instance, to last less than one hour and, if expected to recur, will not in the aggregate cover more than half of the forecast period during which the modified condition is expected to occur, i.e. the period indicated by YbYbGbGb/YeYeGeGe. When the modified forecast condition is expected to last more than one hour, a new change group of the form FM or BECMGshall be used. This transitory group shall be followed by a description of only those elements for which a change is forecast to occur. In other words, when an element is not indicated after TEMPO YbYbGbGb/YeYeGeGe, it shall be considered to be the same as it was prior to YbYbGbGb.

Example: ...FM271100 VRB03KT 3SM -RA BR OVC020 TEMPO 2712/2715 1SM -RA BR FM271500

In the above example, the cloud group OVC020 is not repeated after TEMPO because it is forecast to remain unchanged. On the other hand, the weather group -RA BR is repeated after TEMPO because a significant change in the visibility is forecast.

The time period “YbYbGbGb/YeYeGeGeshall always be stated following TEMPO, even in those cases where it spans the same time period as the self-contained part period
(e.g. …FM271500 (condition A) TEMPO 2715/2720 (condition B) FM272000 …)

The time period of the TEMPOgroup cannot cross two self-contained part periods, e.g., the combination … FM271900(condition A) TEMPO 2721/2801(condition B) FM272300 (condition C)… is not allowed.

An exception to this rule, however, is allowed when hours and fraction of one-hour are used in the FM group. For example, the combination … (condition A) TEMPO 2718/2720 (condition B) FM271930 (condition C) … is allowed to indicate that the fluctuation in the weather condition stated in the TEMPO group will last until the beginning of the next self-contained part period, in this case, 271930Z.

Similarly, the combination … FM271930 (condition A) TEMPO 2719/2723 (condition B) … is allowed to indicate that the fluctuation in the weather condition stated in the TEMPO group is expected to start at the beginning of the self-contained part period, in this case, 271930Z.

Note: The reason for the above exceptions is that in a TEMPO group only whole hours are allowed to identify the period during which some weather fluctuations are expected. In both the above cases, the overlap shall be less than one hour. This practice will give forecasters greater flexibility in the use of TEMPO in conjunction with a FM group not starting on a whole hour.

The combination FM (condition A) TEMPO (condition B) PROB (condition C) is allowed, provided that the time period specified in the PROB group is the same as or a subset of the time period specified in the TEMPO group. For example: FM (condition A) TEMPO 2719/2723 (condition B) PROB30 2720/2723 (condition C)…, or the PROB group begins at the same time or after the TEMPO group ends. For example: FM (condition A) TEMPO 2715/2720 (condition B) PROB30 2720/2723 (condition C). Only one such combination per self-contained part period is allowed.

The combination FM (condition A) TEMPO (condition B) BECMG (condition C) is allowed, provided that the weather condition specified in the BECMG group is forecast to occurs after the time period specified in the TEMPO group (e.g. [condition A] TEMPO 2718/2722 [condition B] BECMG 2723/2800 [condition C]).

An exception to the above rule is allowed when a change in wind speed and/or direction is the only change expected. In such cases, a BECMG group embedded in a larger TEMPO group can be used. For example, the combination: FM(condition A) TEMPO 2714/2721(condition B) BECMG 2719/2720 30015KT …is allowed. Only one combination of TEMPO and BECMG is allowed per self-contained part period. In the TEMPO/BECMGcombination the TEMPO group shall come first. In other words, the combination … (condition B) BECMG(condition C) TEMPO (condition D) is not allowed.

Normally, only one TEMPO group should be used per self-contained part period. A maximum of two TEMPO groups may be used when neither of them are used in combination with the BECMG or PROB group and only one weather element at a time is changing. For example: FM271000 (condition A) TEMPO 2714/2717 OVC010 TEMPO 2717/2720 4SM -SHSN. In this case the visibility and the weather are considered only one element, because when the visibility changes, the weather responsible for the change shall also be stated even if it remained the same.

The triple combination of the groups TEMPOPROBBECMG is not allowed. For example, FM (condition A) TEMPO 2715/2720 (condition B) PROB30 2717/2720 (condition C) BECMG 2721/2722 (condition C) is not allowed. This type of combination confuses the end users of the product.

2.6.14 Change groups FMYYGGgg and BECMG YbYbGbGb/YeYeGeGe

These groups shall be used when, during the entire period of the forecast (Y1Y1G1G1to Y2Y2G2G2), a change in some or all of the elements forecast is expected to occur at some intermediate time YYGGgg or during the period YbYbGbGbto YeYeGeGe.

2.6.14.1 FMYYGGgg

The period of validity of a TAF may be divided into two or more self-contained part periods. Such divisions, when required, shall be done with the use of the group “FMYYGGgg” where:

  • FM is the abbreviation for “from”; and
  • YYGGgg is the date, hour and minutes in UTC at which the permanent change is expected.

Most often, the “gg” will take the form “00”, indicating a whole hour. However, the timing of changes should be as detailed as the supporting data and forecaster knowledge will allow. If a forecaster can determine a change and/or events to a finer resolution, particularly in the early hours of the forecast period, then the use of the “gg” to indicate timing to some fraction of an hour is encouraged (e.g. FM271830, FM081215).

The group FMYYGGgg is used to indicate a permanent change (typically over a period of one hour or less) of meteorological conditions expected to occur at time GGggZ on day YY.

When the group FMYYGGgg is used, all forecast conditions given before this group are superseded by the conditions indicated after the group. In other words, a complete forecast will follow and all weather elements shall be indicated.

When hours and minutes are used to begin a new self-contained part period (e.g. FM261930), any group of the form PROB/TEMPO used after the FM group and intended to start at the same time as the FM group, shall be indicated as starting on the whole hour before the fraction of the hour stated in the FM group. For example, FM081230 (condition A) TEMPO 0812/0816 (condition B)…meaning that the group TEMPOstarts at 1230Z rather than at 1200Z. Similarly, a group of the form PROB/TEMPO used before such a FM group and intended to end with the beginning of the new FMgroup shall be indicated as ending on the whole hour after the fraction of the hour stated in the new FM group. For example: ... (condition A) TEMPO 0818/0820 (condition B) FM081930 (condition C) … meaning that the group TEMPO ends at 1930Z rather than 2000Z. The same reasoning applies to the PROB group.

For the sake of clarity, each FM group will start a new forecast line with the proper indentation. This is done automatically by the national bulletin preparation software.

Note: Forecasters are encouraged to subdivide the valid period of the TAF using FMYYGGgg as often as possible rather than using the forecast change indicator BECMG. The rationale behind this is that a FMYYGGgg group is a more complete group, and therefore more effective forecast. As such, it is more useful and much preferred by users.

2.6.14.2 BECMGYbYbGbGb/YeYeGeGe

The change group “BECMGYbYbGbGb/YeYeGeGeshall indicate a gradual evolution of meteorological conditions between time YbYbGbGband time YeYeGeGe. The duration of the period from YbYbGbGbto YeYeGeGeshall normally not exceed two hours and in any case not exceed four (e.g. …BECMG 1721/1722 …or … BECMG 1722/1800…)

The permanent change group shall be followed by a description of only those weather elements for which a change is forecast to occur. In other words, if a weather element is not indicated after BECMG YbYbGbGb/YeYeGeGe, it shall be considered to be the same as it was prior to YbYbGbGb.

Note: The forecaster shallexercise judgment when to using the change group FMYYGGgg or BECMG YbYbGbGb/YeYeGeGe. As a general rule, to keep the forecast clear and unambiguous, using the change group BECMG YbYbGbGb/YeYeGeGeshould be kept at a minimum and confined to those cases where only one or at most two weather elements are expected to change, while all the others stay the same. In those cases where more than two weather elements are expected to change, the permanent change group FMYYGGgg should be used to start a new self-contained part period.

Normally, only one BECMG group should be used in the same self-contained part period. A maximum of two BECMG groups are allowed in a self-contained part period when only one weather element at a time is changing.

Example: ...(condition A) BECMG 1713/1715 1SM BR BECMG 1715/1717 4SM BR … or (condition A) BECMG 1714/1715 34015KT BECMG 1717/1718 4SM -SHSN ...

In this case the visibility and the weather are considered only one element because when the visibility changes the weather responsible for the change shall always be stated even if it remained the same.

The combination of BECMG/PROBand BECMG/TEMPO are not allowed. For example, FM(condition A) BECMG(condition B) PROB/TEMPO(condition C) are not allowed. Refer to 2.6.12 and 2.6.13 for the use of BECMG when it follows a PROB or TEMPO group respectively.

2.6.15 Remarks (RMK)

Some remarks may be included at the end of a TAF preceded by the designator “RMK”. The following remarks are authorized for use as appropriate:

  1. FCST BASED ON AUTO OBS
  2. NXT FCST BY YYGGggZ, where YY is the date and GGgg is the hour and minutes UTC of issue of the regular TAF (GG is the whole hour and gg is ‘00’ minutes). Any remarks associated with partial program notices as described in 2.5.2.1
  3. Remarks explaining possible discrepancies between AWOS observations and aerodrome forecasts when forecasters have reasons to believe that the AWOS observations are non-representative of the actual weather at the aerodrome:
    • RMK AUTO OBS REP NON-REPRESENTATIVE WND SPD
    • RMK AUTO OBS REP NON-REPRESENTATIVE WND DCTN
    • RMK AUTO OBS REP NON-REPRESENTATIVE WND
    • RMK AUTO OBS REP NON-REPRESENTATIVE VIS
    • RMK AUTO OBS REP NON-REPRESENTATIVE CLD HGT
    • RMK AUTO OBS REP NON-REPRESENTATIVE T
    • RMK AUTO OBS REP NON-REPRESENTATIVE TD
    • RMK AUTO OBS REP NON-REPRESENTATIVE PCPN TYPE
    • RMK AUTO OBS REP NON-REPRESENTATIVE PCPN INTSTY
    • RMK AUTO OBS REP NON-REPRESENTATIVE PCPN
    • RMK AUTO OBS NON-REPRESENTATIVE OF CURRENT WX
  4. When one or more sensors are inoperative but you have other means of obtaining the pertinent weather information, you can leave the TAF running by adding the proper remark from the list below:
    • WND SPD SENSOR INOP
    • WND DCTN SENSOR INOP
    • WND SENSORS INOP
    • VIS SENSOR INOP
    • CLD SENSOR INOP
    • PCPN TYPE SENSOR INOP
    • PCPN INTSTY SENSOR INOP
    • PCPN SENSORS INOP

Note: The above remarks can be included when a regularly scheduled TAF is issued or when amending an existing TAF. Remarks of this nature should only be included when forecasters are confident of the weather condition at the aerodrome and that it is safe not to cancel the TAF for that aerodrome. The last remark could be used when three of more sensors are reporting non-representative observations.

If forecasters are not confident of the actual weather condition at the aerodrome, they should cancel the TAF according to the procedure described in 2.10.2.

2.7 Aerodrome advisories

Aerodrome advisories may be issued in place of aerodrome forecasts, for the reasons outlined in the following sections.

2.7.1 Offsite (OFFSITE)

To be used when the forecast is based on observations that are not always considered to be representative of weather conditions at the airport. In normal situations, an observation shall be considered representative of the weather conditions at the aerodrome if it is taken within 1.6 NM (3 km) of the geometric centre of the runway complex.

In cases where the 1.6 NM (3 km) criteria does not apply because of local characteristics, the representativeness of an observation shall be determined and approved by Transport Canada for civilian aerodromes, or by the Director of Meteorology and Oceanography for military aerodromes.

The word “ADVISORYshall appear after the period of validity group. The word “OFFSITEshall be added, followed by one space, after the word “ADVISORY” (e.g. TAF  CCCC 151040Z 1511/1523 ADVISORY OFFSITE …). This is intended to indicate to the users that the observations do not necessarily reflect the actual conditions at the aerodrome.

2.7.2 Observation incomplete (OBS INCOMPLETE)

The term “OBS INCOMPLETE” is to be used when the forecast is based on observations with missing or incomplete data on a regular basis (e.g. MSL pressure not reported).

The word “ADVISORYshall appear after the period of validity group. The words “OBS INCOMPLETEshall be added immediately after the word “ADVISORY” (e.g. TAF CCCC 201640Z 2017/2105 ADVISORY OBS INCOMPLETE …).

2.7.3 No specials (NO SPECI)

The term “NO SPECI” is to be used when the forecast is based on observations from a station with a limited observing program that does not issue special weather observations.

The word “ADVISORYshall appear after the period of validity group. The words “NO SPECIshall be added immediately after the word “ADVISORY” (e.g. TAF CCCC 252240Z 2523/2612 ADVISORY NO SPECI…).

2.8 Updated forecasts

An updated forecast is a forecast issued on a scheduled basis which replaces the previous TAF without extending its period of validity. Updated forecasts are issued at the request of NAV CANADA usually for major, high traffic aerodromes. Normally, updated TAFs are issued for aerodromes whose regular TAFs have a period of validity of 24 hours or 30 hours; however, this is not a necessary condition. Updated TAFs, since they are more recent forecasts, are intended to provide more accurate information than their predecessors, especially in the short range, and will normally be scheduled for issuance three hours following each regular TAF issue. In some cases NAV CANADA may decide that at certain times of the day, due to low traffic volume, the issuance of updated TAFs is not required (e.g. overnight).

An updated TAF does not extend the period of validity of the TAF that it supersedes. For example, if a 24 hour TAF covering a period from 041100Z to 051100Z is updated at 041340Z, the updated TAF shall indicate a period of validity as 0414/0511 (i.e. 041400-051100Z). As with other TAFs, an updated TAF is valid from the time it is issued until it is amended or until it is superseded by the next issue of the regular TAF for the same aerodrome.

In preparing both regular and updated TAFs, forecasters should attempt to provide as much precision as supporting data will allow to the first three hours of its period of validity. In particular, forecasters are encouraged to time the start of new part-periods as precisely as possible (i.e. using hours and minutes as described in 2.6.14.1).

2.9 Amended TAFs

A TAF shall be amended when the forecast conditions are no longer representative of the actual or expected conditions according to the criteria listed in 2.9.5.

TAF Amendment response time and performance thresholds shall be agreed upon between the meteorological service provider and its client. The meteorological service provider shall demonstrate performance against the established thresholds. TAF Performance relative to these targets is to be reviewed, deviations investigated and continuous improvement implemented.

As in a regular TAF, the time in the telecommunication header of an amended TAF shall indicate the whole hour (UTC) that precedes the time of entry to the collection circuit (refer to 2.5.1).

The date/time group YYGGggZ in the bulletin, however, shall indicate the date and time of origin of the amended TAF. For example, a first amendment of a regular TAF for CYYZ issued at 1845Z on the 21st day of the month shall be issued as:

TAF AMD CYYZ 211845Z 2118/2300...

An amended forecast covers the remaining period of the original forecast and is identified by AA(X) in the telecommunication header (refer to 2.5.1) and by the prefix TAF AMD in place of TAF on the following line.

Note: Amended TAFs with a different (X) in the AA(X) group shall be issued in separate bulletins.

2.9.1 Responsibility for issuing amended TAFs

A forecaster must use initiative, discretion, and good judgment in determining when amendments should be issued. It must be emphasized that both the responsibility and the authority for issuing amendments rest with the forecaster and the supervising meteorologist.

TAFs should be amended whenever they become, in the forecaster’s judgment, unrepresentative of existing or expected weather conditions. Furthermore, forecasters should strive to issue amended TAFs proactively rather than reactively.

2.9.2 Amendments based on PROBC2C2YbYbGbGb/YeYeGeGe

The group “PROBC2C2YbYbGbGb/YeYeGeGe” may be used to state the probability of occurrence of a weather phenomenon significant to aviation. If the condition introduced by this probability group is observed and is considered to be a short lived phenomenon that will last for less than one-half hour and is not expected to recur, it is not necessary to amend the TAF. In those cases when the same phenomenon is expected to last for a longer period or to recur, an amended TAF is required. The only exception is for TS, GR, FC, +FC, FZRA, FZDZ, PL and SG for which an amendment is always required regardless of their duration

2.9.3 Amendments based on human/machine mix

There is no requirement to amend or update a TAF solely due to a change in the type of observations. If, however, a scheduled TAF (based on manned observations) needs to be amended and the amendment is triggered by an observation generated from an automated station, that amendment shall include the phrase FCST BASED ON AUTO OBSin the remark section of the TAF. The converse is also true, i.e., if a TAF based on automated observations needs to be amended and the amendment is triggered by a manned observation, that  amendment shall not include the phrase FCST BASED ON AUTO OBS in the remark section of the TAF.

2.9.4 Amendments to aerodrome advisories

Aerodrome advisories are to be amended according to the amendment criteria set out in 2.9.5.

2.9.5 Amendment criteria

An amendment shall be issued when the actual or expected weather condition is in a different category than the forecast condition.

A weather category is defined by the lower of either the ceiling or the visibility.

2.9.5.1 Cloud and visibility

The principal weather categories are delineated by the following thresholds:

  1. Ceiling 2,500 feet and visibility six miles (generally representative of non-alternate IFR threshold)
  2. Ceiling 1,000 feet and visibility three miles (threshold between VFR and IFR)
  3. Alternate limits for the aerodrome
  4. IFR approach limits for the aerodrome
  5. Additional limits as determined by other responsible parties

Limits for categories three, four and five are provided by NAV CANADA, DND or other responsible parties in line with TC regulation. These limits are published in Appendix A of this manual.

For amendment purposes, each of the above thresholds are always part of and included in the category immediately above the threshold. This means that for deteriorating conditions, a change in category occurs when either the ceiling or the visibility crosses the threshold value leading to the next lower category. On the other hand, for improving conditions, both ceiling and visibility must cross the threshold values leading to the next higher category.

Note: A TAF does not need to be amended for changes in ceiling or visibility when both the forecast and observed values are less than the lowest published landing minima for the aerodrome. This rule is valid provided that the lowest approach limits for the aerodrome are not higher than the IFR threshold (1000 feet/3 statute miles). In such cases, an amendment is required when the weather condition crosses the IFR threshold. For example, if the lowest approach limit of an aerodrome is 1500/4 and the TAF calls for 1200/3, an amendment is needed if the weather condition lowers to below 1000/3.

2.9.5.2 Weather and visibility
  1. Thunderstorms, hail, tornado, funnel clouds, and water spouts: An amendment shall be issued when thunderstorms, hail, tornado, funnel clouds or water spouts are observed or expected to occur and were not forecast or when they were forecast and subsequently no longer expected to occur. These rules apply with no exception, regardless of the value of the visibility and the duration of the phenomenon.
  2. Freezing precipitation, ice pellets, and snow grains: An amendment shall be issued when freezing precipitation, ice pellets, or snow grains at the surface are observed or expected to occur and are not forecast or when they are forecast and subsequently no longer expected to occur. These rules apply with no exception, regardless of the value of the visibility and the duration of the phenomenon.
  3. Rain, snow, snow pellets, and moderate or heavy drizzle: An amendment shall be issued when rain, snow, snow pellets, or moderate or heavy drizzle, lowering visibility to less than six statute miles, is observed or is expected to occur and is not forecast. Similarly, an amendment shall also be issued when the same type of precipitation, lowering visibility to less than six statute miles, is forecast and subsequently is no longer expected to occur. When the visibility remains six statute miles or greater before and after the onset of precipitation, there is no need to amend a TAF for the onset or cessation of rain, snow, snow pellets, or moderate or heavy drizzle.
  4. Precipitation state change: When precipitation, listed in 3 above, is observed and the state of the precipitation (liquid or solid) is not the same as forecast, then an amended forecast is required, provided that the visibility due to precipitation is reduced to less than six statute miles. Similarly, when precipitation of the forecast state is not occurring and is no longer expected to occur, then an amended forecast is required provided that the visibility was originally forecast to be less than six statute miles.
  5. Obscuration, light drizzle and ice crystals: When a change in visibility due to an obscuring phenomenon, to light drizzle, or to ice crystals is observed or is expected to occur and is not forecast, a TAF amendment is required only if the visibility lowers to less than six statute miles. Similarly, when such a change is forecast and subsequently is no longer expected to occur, an amended TAF is required provided that the visibility was originally forecast to be less than six statute miles. The only exception is volcanic ash which requires an amendment, when not forecast, regardless of the value of the visibility.
2.9.5.3 Winds

An amendment shall be issued when:

  1. Speed
    • the observed speed is double or more, or half or less of the forecast value and either the observed or forecast value is greater than 15 KT; or
    • the difference between the observed and forecast speed is 20 KT or more.
  2. Direction
    • the observed direction differs by at least 45 degrees from the forecast value for all observed speeds greater than 15 KT.
2.9.5.4 Low level wind shear

An amendment shall be issued when a strong wind shear (as defined in 2.6.7) was not forecast but subsequently occurs or is expected to occur or when it was forecast but subsequently is no longer expected to occur. In addition, if the forecast wind speed, direction, or height of the wind shear layer is not representative of the actual condition, the forecaster can amend the TAF at their discretion.

2.10 Cancellation of TAFs

When a TAF must be cancelled, it shall be done by the issuance of an amended TAF. The format and reasons for cancelling TAFs are stated in 2.10.1 and 2.10.2.

2.10.1 Cancellation for missing observations

In the event that scheduled hourly observations are not received, the forecaster shall determine the reason for the missing observations.

If the observations were not received because of a telecommunication problem, the forecaster shallmake every reasonable effort to obtain them, whenever possible, by alternative means. If two consecutive hourly observations from an aerodrome cannot be obtained, the TAF for that aerodrome shall be cancelled by sending an amended TAF.

  1. An existing TAF must be cancelled:
    When an existing TAF must be cancelled because of a lack of weather observations for that aerodrome, an amended TAF to cancel the existing one must be issued according to the following format:
    TAF  AMD CYQG 051935Z 0518/0618 FCST CNCLD DUE NO OBS
    RMK NXT FCST BY 060000Z=
  2. The first TAF of the day for an aerodrome with a partial program is not issued because weather observations are not available:
    When weather observations are not available or are insufficient for issuing the first TAF of the day for an aerodrome operating under a partial program (less than 24 hours per day), the first regular TAF of the day indicating that the forecast is not available must be issued according to the following format:
    TAF CYPQ 101140Z 1012/1020 FCST NOT AVBL DUE INSUFFICIENT OBS
    RMK NXT FCST BY 101800Z=

Once a TAF for an aerodrome has been cancelled due to missing observations, the TAF for that aerodrome shall notbe reissued until a minimum of one hourly observation (in VFR situations) is received or a maximum of two consecutive hourly observations (in below VFR situations) representative of the aerodrome are received.

The two observations must be consecutive and must be not more than one hour apart (e.g. two regular hourly observations). In both cases, the forecast shall be issued within 20 minutes after the appropriate observation is received.

2.10.2 Cancellation due to unreliable or missing AWOS observation element(s)

A forecaster may cancel a TAF for an aerodrome equipped with a stand-alone AWOS site when:

Note : One of the last two statements in the above list shall be used when more than one of the sensors is inoperative.

Example:
TAF AMD CYTL 202030Z 2018/2102 FCST CNCLD DUE CLD HGT SENSOR INOP
RMK NXT FCST BY 210000Z=
Note: One of the last two phrases in the above list shall be used when more than one of the sensors are malfunctioning or more than one of the elements are deemed to be incorrect.

Example:
TAF AMD CYTL 281515Z 2812/2902 FCST CNCLD DUE VIS SENSOR MALFUNCTION
RMK NXT FCST BY 281800Z=
  1. The observation of any critical element (e.g. ceiling) is missing or believed to be incorrect or affected by a mechanical malfunction for two hours; and
  2. All attempts have failed to determine a reasonable inferred value(s) based on sound meteorological knowledge and techniques. In the above circumstances, the TAF shall contain a brief explanation for its cancellation according to the following:
    1. Forecast cancelled due to missing element(s) and forecaster’s inability to infer a reasonable value. When this occurs, the TAF shall be cancelled using one of the following phrases:
      • FCST CNCLD DUE CLD HGT SENSOR INOP
      • FCST CNCLD DUE VIS SENSOR INOP
      • FCST CNCLD DUE PCPN TYPE SENSOR INOP
      • FCST CNCLD DUE PCPN INTSTY SENSOR INOP
      • FCST CNCLD DUE PCPN SENSORS INOP
      • FCST CNCLD DUE T SENSOR INOP
      • FCST CNCLD DUE TD SENSOR INOP
      • FCST CNCLD DUE WND SPD SENSOR INOP
      • FCST CNCLD DUE WND DCTN SENSOR INOP
      • FCST CNCLD DUE WND SENSORS INOP
      • FCST CNCLD DUE SENSORS INOP
      • FCST CNCLD DUE INSUFFICIENT DATA
    2. Forecast cancelled due to sensor malfunction or incorrect observation element(s) and forecaster inability to infer a reasonable value. When this occurs, the TAF shallbe cancelled using one of the following phrases:
      • FCST CNCLD DUE CLD HGT SENSOR MALFUNCTION
      • FCST CNCLD DUE VIS SENSOR MALFUNCTION
      • FCST CNCLD DUE PCPN TYPE SENSOR MALFUNCTION
      • FCST CNCLD DUE PCPN INTSTY SENSOR MALFUNCTION
      • FCST CNCLD DUE PCPN SENSORS MALFUNCTION
      • FCST CNCLD DUE T SENSOR MALFUNCTION
      • FCST CNCLD DUE TD SENSOR MALFUNCTION
      • FCST CNCLD DUE WND SPD SENSOR MALFUNCTION
      • FCST CNCLD DUE WND DCTN SENSOR MALFUNCTION
      • FCST CNCLD DUE WND SENSORS MALFUNCTION
      • FCST CNCLD DUE SENSORS MALFUNCTION
      • FCST CNCLD DUE INSUFFICIENT DATA

If a forecaster is capable of inferring a reasonable value for the current weather, and this is in agreement with the current TAF but differs from the AWOS observation, the forecaster does not have to cancel the TAF. In such a situation, the forecaster shall amend the TAF including an explanatory note in the remark section justifying the difference between the AWOS observation and the TAF as indicated in 2.6.15.

2.11 Corrections

A TAF shall be corrected when typographical errors and/or forecast text omissions are made in the original TAF and are such that the information content on the TAF is unclear or subject to misinterpretation.

The abbreviation “AMDshall be used in the body of a forecast when a corrected TAF is issued. In the bulletin header, however, the term CC(X), indicating a correction, shall be placed by the national bulletin preparation software. The issue time added to the body of the TAF indicates which TAF is the latest.

Note: Corrected TAFs must not be transmitted in the same bulletin as amended TAFs because the bulletin header for corrected TAFs is CC(X), while the header for amended TAFs is AA(X).

2.12 Examples of TAFS

Example (1):

FTCN34 CWAO 101100
TAF CYTL 101140Z 1012/1100 24010KT P6SM BKN030 TEMPO 1018/1020 5SM -SHRA BR
FM102000 24005KT P6SM SKC
RMK FCST BASED ON AUTO OBS. NXT FCST BY 101800Z=

Example (2):

FTCN34 CWAO 101530 AAA
TAF AMD CYTL 101530Z 1012/1100 FCST CNCLD DUE VIS SENSOR MALFUNCTION
RMK FCST BASED ON AUTO OBS. NXT FCST BY 101800Z=

Example (3):

FTCN34 CWAO 011700
TAF CYWG 011740Z 0118/0218 28015KT P6SM -SNRA SCT015 OVC040 TEMPO
0118/0200 2SM -SNRA BR OVC015
FM020000 28015KT P6SM BKN030 BKN250 TEMPO 0200/0203 P6SM -SHRA
FM021000 30015KT P6SM SKC
RMK NXT FCST BY 020000Z=

Note: If an updated TAF were scheduled for CYWG, then the remark in the above example would say: NXT FCST BY 012100Z.

Example (4):

FTCN35 CWAO 021700
TAF CYYZ 021740Z 0218/0400 30015G25KT P6SM BKN015 OVC025CB PROB30
0218/0223 3SM -TSRA
FM022300 30010G20KT P6SM SKC
FM031000 VRB03KT 4SM BR SKC TEMPO 0310/0313 3/4SM BR BECMG
0314/0316 P6SM NSW
RMK NXT FCST BY 022100Z=

Example (5):

FTCN38 CWAO 021100
TAF CYYR 021140Z 0212/0312 VRB03KT 2SM BR BKN025
FM021230 27015KT P6SM SKC
RMK NXT FCST BY 021800Z=

Example (6):

FTCN38 CWAO 020500Z
TAF CYQX 020540Z 0206/0306 13005KT 1SM -DZ BR OVC005 TEMPO 0206/0210
1/4SM DZ FG VV002
FM021300 31005KT 4SM BR SKC TEMPO 0302/0305 3/4SM BR SCT005
RMK NXT FCST BY 021200Z=

Example (7):

FTCN32 CWAO 151900 AAA
TAF AMD CYEG 151912Z 1519/1618 27015G35KT 3SM -TSRA SCT008 OVC015CB
TEMPO 1520/1523 VRB25G45KT 1SM TSRAGS OVC008CB PROB30
1520/1523 1/2SM +TSRAGR
FM160000 33015G25KT P6SM SKC BECMG 1609/1610 33005KT
RMK NXT FCST BY 152100Z=

Example (8):

FTCN36 CWAO 231500 AAA
TAF AMD CYUL 231515Z 2315/2412 30015KT 2SM -SHSN OVC010
FM231615 27015KT P6SM BKN030 OVC060 TEMPO 2321/2405 4SM -SHSN
FM240500 27015KT P6SM SKC
BECMG 2409/2411 00000KT
RMK NXT FCST BY 231800Z=

Note: Example (8) represents the first amendment since the last regular forecast update.

Example (9):

FTCN36 CWAO 051100
TAF CYFB 051140Z 0512/0612 04025G45KT WS015/12060KT P6SM SKC
FM051330 36010G20KT P6SM SCT030 TEMPO 0518/0522 3SM -SHSN BKN030
FM060000 36005KT P6SM SKC
RMK NXT FCST BY 051800Z=

Example (10):

FTCN36 CWAO 090500
TAF CYOW 090535Z 0906/1006 04015KT 1SM -FZRA -SN OVC010
FM090730 27015G25KT P6SM SCT010 OVC025 TEMPO 0913/1005 P6SM -SHRA
RMK NXT FCST BY 090900Z=

Example (11):

FTCN37 CWAO 171100 CCA
TAF AMD CYZX 171152Z 1712/1812 00000KT 1SM BR SKC
FM171315 VRB03KT 3SM BR SKC BECMG 1713/1715 P6SM NSW SCT020
FM180600 VRB03KT 2SM BR SKC TEMPO 1809/1811 1/4SM FG VV001
RMK NXT FCST BY 171800Z=

Note: Example (11) represents the first correction issued since the last regular issue of the forecast. “AMD” is used in the body of the forecast.

Example (12):

FTCN38 CWAO 151100
TAF CYCA 151140Z 1512/1521 ADVISORY OFFSITE 32010KT P6SM SKC
FM151721 32015G25KT P6SM FEW025 TEMPO 1517/1522 BKN025
RMK NXT FCST BY 151800Z=

Example (13):

FTCN35 CWAO 181700
TAF CYYZ 181740Z 1818/2000 24010KT 6SM HZ BKN250
FM181930 32015G25KT P6SM SKC BECMG 1822/1823 30005KT BKN025
RMK NXT FCST BY 182100Z=

Example (14):

FTCN35 CWAO 101200 CCA
TAF AMD CYHM 101215Z 1012/1100 20010KT 6SM HZ BKN250
FM101700 22015KT 6SM HZ BKN030 TEMPO 1017/1020 5SM -SHRA HZ
FM102100 23005KT P6SM SKC
RMK NXT FCST BY 101800Z=

Example (15):

FTCN31 CWAO 192000
TAF CYVR 192040Z 1921/2100 26015KT 3SM BR BKN020 TEMPO 1921/1923 1SM
-SHRA BR
FM200300 24005KT 3SM BR SCT020
FM200700 VRB03KT 1SM BR SCT004 TEMPO 2009/2011 1/2SM FG BKN004
BECMG 2013/2015 5SM BR
FM201600 24010KT P6SM SKC
RMK NXT FCST BY 200000Z=

Note: The TAF in example (15) is an updated forecast.

Example (16):

FTCN35 CWAO 151700
TAF CYVV 151740Z 1518/1606 24045G70KT P6SM SQ BKN030
FM151930 32020G40KT P6SM BKN030 TEMPO 1520/1523 P6SM -SHRA
RMK NXT FCST BY 160000Z=

Example (17):

FTCN35 CWAO 171100
TAF CYQG 171140Z 1712/1812 26015KT 6SM -FZRA BR SCT010CB OVC020 TEMPO
1712/1715 4SM TS -FZRA BR
FM171500 34010G20KT P6SM OVC015 TEMPO 1717/1722 4SM -SHSN OVC025
FM172300 34005KT P6SM SKC
RMK NXT FCST BY 171800Z=

Example (18):

FTCN35 CWAO 202300
TAF CYHM 202340Z 2000/2003 26010KT 5SM HZ FEW030
BECMG 2001/2002 P6SM NSW SKC
RMK NO FCST COVERAGE 200300-201100Z=

Example (19):

FTCN35 CWAO 201135 AAA
TAF AMD CYPQ 201135Z 2012/2020 FCST NOT AVBL DUE
INSUFFICIENT OBS
RMK NXT FCST BY 201800Z=

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